Systems and methods for determining polarization properties with high temporal bandwidth

[0001] This application is a continuation-in-part of U.S. application Ser. No. 16/298,637, filed Mar. 11, 2019, which in turn is a continuation-in-part of U.S. application Ser. No. 15/921,501 filed Mar. 14, 2018, now U.S. Pat. No. 10,228,323, which claims the benefit of U.S. Provisional Application No. 62/470,979, entitled, “Systems and Methods for Optically Determining an Acoustic Signature of an Object” filed Mar. 14, 2017, the entire disclosure of which is incorporated herein by reference.

[0002] The present disclosure is generally related to remote sensing using light, and in particular is directed to remote sensing using polarization of light to detect and determine high temporal bandwidth polarization properties of a remote object.

[0003] In the modern age of space technology, reliance on satellites is significant and growing. The United Nations estimates that there are currently more than 4,000 satellites orbiting the Earth, with more than 1,000 predicted to launch over the next several years. These satellite devices have a broad span of technical uses, ranging from commercial, to civil, to military applications. Thousands of additional space debris objects are in orbit as well.

[0004] Objects in geosynchronous Earth orbits (GEO) are located about 36,000 kilometers away from Earth. At that distance, they represent a special challenge to monitor using imaging methods. Traditional means, using Earth-based remote sensing, have generally been ineffective, even with large-scale apertures and expensive equipment. However, identification and monitoring of GEO objects remains an important issue. Satellites have limited life-spans given the harsh environment of space, where weathering degrades craft thermal surfaces, and the wearing of mechanical features can lead to critical component failure. Improved characterization and prediction may lead to mitigation strategies in the event of imminent on-orbit failure. Furthermore, satellite identification and cross-tagging, as well as the recognition of changes in the behavior of a satellite, whether driven by phenomena internal or external to the satellite itself, is important in defending against military and national security threats.

[0005] Remote sensing, detection, and characterization may have numerous additional applications as well. For example, sensing and detection may be used in aerial intelligence, surveillance, and reconnaissance operations performed by civilian or military groups. Such operations may include the detection of hidden objects, characterization of buildings, supplies, and equipment, and discovery of geological features and materials. Detection may be performed from aircraft, spacecraft, or satellites.

[0006] Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.

[0007] Embodiments of the present disclosure provide a system and apparatus for remotely sensing an object using photo-acoustic polarization characteristics. Briefly described, in architecture, one embodiment of the apparatus, among others, can be implemented as follows. A photo-acoustic polarimetric remote sensing apparatus includes a telescope that directs visible light photons from an object. A polarizing beam splitter is in optical alignment with the telescope. The polarizing beam splitter has first and second pathways corresponding to first and second polarization states, respectively. The first and second pathways are substantially perpendicular. A first photodetector is in optical alignment with the first pathway, and a second photodetector is in optical alignment with the second pathway. At least one processor is in communication with the first and second photodetectors. The at least one processor generates a signal corresponding to a degree of linear polarization of the photons over time, and the signal is indicative of an acoustic signature of the object.

[0008] The present disclosure also provides an apparatus for remotely sensing an object using photo-acoustic polarization characteristics. Briefly described, in architecture, one embodiment of the apparatus can be implemented as follows. A photo-acoustic polarimetric remote sensing apparatus includes collection optics that direct photons. A polarizing beam splitter has first and second pathways, where photons within the first and second pathways have first and second polarization states, respectively. A first optical detector is located in the first pathway, and a second optical detector is located in the second pathway. At least one processor is in communication with the first and second optical detectors. The at least one processor: receives first and second signals from the first and second optical detectors, respectively; calculates, for a segment of the first and second signals, a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum; and generates, for a plurality of segments a signal corresponding to a sequential output of the ratio for each segment. The signal is indicative of an acoustic signature of an object being sensed.

[0009] The present disclosure can also be viewed as providing methods of remotely sensing an object using photo-acoustic polarization characteristics. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: collecting photons from the object; directing the photons down first and second pathways, wherein photons within the first pathway have a first polarization state and photons within the second pathway have a second polarization state; detecting photons in the first and second pathways using at least one optical detector, wherein the photons in the first polarization state produce a first signal and the photons in the second polarization state produce a second signal; receiving, with at least one processor in communication with the first and second optical detectors, a first signal and a second signal; determining, with the at least one processor and for a segment of the first and second signals, a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum; and generating, for a plurality of segments, a signal corresponding to a sequential output of the ratio for each segment.

[0010] The present disclosure can also be viewed as providing a system and apparatus for remotely sensing an object using photo-acoustic polarization characteristics. Briefly described, in architecture, one embodiment of the apparatus can be implemented as follows. A photo-acoustic polarimetric remote sensing apparatus includes collection optics that direct photons. An optical detector includes a micro-polarizer array located on the optical detector. The micro-polarizer array separates the photons into at least a first polarization state and a second polarization state. A first portion of the optical detector receives the photons in the first polarization state, and a second portion of the optical detector receives the photons in the at least second polarization state. At least one processor is in communication with the optical detector. The at least one processor: receives a signal from the optical detector, wherein a first portion of the signal corresponds to the photons in the first polarization state, and a second portion of the signal corresponds to the photons in the at least second polarization state; calculates, for a segment of the signal, a difference of the first and at least second portions of the signals, a sum of the first and at least second portions of the signals, and a ratio of the difference to the sum; and generates, for a plurality of segments, a signal corresponding to a sequential output of the ratio for each segment, wherein the signal is indicative of an acoustic signature of an object being sensed.

[0011] The present disclosure can also be viewed as providing methods of remotely sensing an object using photo-acoustic polarization characteristics. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: collecting photons from the object; directing the photons down a plurality of pathways, wherein photons in each of the plurality of pathways have a different polarization state; detecting photons in each of the plurality of pathways using at least one optical detector, wherein the photons in each of the polarization states produce a signal; receiving, with at least one processor in communication with the at least one optical detector, a plurality of signals corresponding to the photons in the plurality of pathways; determining, with the at least one processor and for a segment of the plurality of signals, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the plurality of signals; and generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment.

[0012] The present disclosure can also be viewed as providing methods of remotely sensing an object using photo-acoustic polarization characteristics. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: collecting photons from the object; directing the photons into a polarimeter, whereby at least one polarization state of the photons is measured; separating an optical wavelength spectrum of the photons; detecting the optical wavelength spectrum of the photons using at least one optical detector, wherein the optical wavelength spectrum of the photons produces a signal; receiving, with at least one processor in communication with the at least one optical detector, the signal corresponding to the optical wavelength spectrum; determining, with the at least one processor and for a segment of the signal, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal; and generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment.

[0013] The present disclosure can also be viewed as providing methods of remotely sensing an object using photo-acoustic polarization characteristics. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: collecting photons from the object; dividing the photons into a plurality of polarized states along a single pathway; dividing the plurality of polarized states along the single pathway into a plurality of focal plane pathways; separating an optical wavelength spectrum of each of the plurality of focal plane pathways; detecting the optical wavelength spectra using at least one optical detector, wherein the optical wavelength spectra produce a signal; receiving, with at least one processor in communication with the at least one optical detector, the signal corresponding to the optical wavelength spectra; determining, with the at least one processor and for a segment of the signal, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal; and generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment.

[0014] The present disclosure can also be viewed as providing methods of remotely sensing an object using photo-acoustic polarization characteristics. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: collecting photons from the object; directing the photons down a single pathway; inserting into the pathway, in sequence, a plurality of polarizing elements, whereby photons propagating through each of the plurality of polarizing elements has a different polarization state; detecting, in sequence, photons in each of the plurality of polarization states using at least one optical detector, wherein the photons in each of the polarization states produce a signal; receiving, with at least one processor in communication with the at least one optical detector, a plurality of signals corresponding to the photons in the plurality of polarization states; determining, with the at least one processor and for a segment of the plurality of signals, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the plurality of signals; and generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment.

[0015] Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.

[0039] Aspects of the invention will be further illustrated with reference to the following specific examples. It is understood that these examples are given by way of illustration and are not meant to limit the disclosure or the claims to follow.

[0040] FIG. 1A is a schematic block diagram of a remote sensing apparatus 100 in accordance with a first exemplary aspect of the present invention. The apparatus 100 includes collection optics 110 directing photons from an object 1. A polarizing beam splitter 120 has first and second pathways 122a, 122b. Photons within the first and second pathways 122a, 122b have first and second polarization states, respectively. The apparatus 100 also includes a first optical detector 130a located in the first pathway 122a and a second optical detector 130b located in the second pathway 122b. At least one processor 140 is in communication with the first and second optical detectors 130a, 130b. The at least one processor 140 is configured to: receive first and second signals from the first and second optical detectors 130a, 130b, respectively; calculate, for a segment of the first and second signals, a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum; and generate, for a plurality of segments, a signal corresponding to the sequential output of the ratio for each segment, wherein the signal is indicative of an acoustic signature of the object 1 being sensed.

[0041] The object 1 may be any remote object. For example, the object 1 may be an object 1 in space, such as a satellite, spacecraft, space station, and the like, in geosynchronous orbit. The object 1 may be an object 1 within the Earth's atmosphere, such as an airplane, weather balloon, missile, and the like. The object 1 may be a remote object 1 on Earth, commonly referred to as a terrestrial object, such as a structure, building, vehicle, and the like. Multiple objects 1 may also be viewed simultaneously or in succession, for instance, several satellites or a combination of space objects and other objects. In one example when viewing multiple objects 1, the apparatus 100 may focus on the objects 1 individually, one at a time. In another example, the apparatus 100 may detect all objects 1 within its field of view and may separate or resolve each object 1 during analysis. In another example, the apparatus 100 may focus on an atmospheric layer using conjugate planes, instead of focusing on the object 1. Focusing on an atmospheric layer may improve the overall signal-to-noise ratio by reducing noise introduced by scintillation. In another example, the systems and methods disclosed herein may be useful in upward, downward, slanted, or lateral-looking applications. Such applications may include monitoring for geo-prospecting, non-line of sight activity such as tunneling, wake identification, and the like. Such applications may additionally be used to measure audible or infrasound acoustic information content from nearby surfaces, natural or otherwise.

[0042] The photons may be from any suitable band of the electromagnetic spectrum, for example, the infrared, visible, ultraviolet, or x-ray bands. In use, the suitable band may be different depending on the object 1 being sensed. For instance, if a portion or entirety of the object 1 is optically transparent to some bands, but optically reflective to others, the reflective bands may be more useful in detection. As another example, if an object 1 is located in an area where there is no incident light from a portion of the spectrum, it may not be useful to detect photons from that portion of the spectrum. In one example, the apparatus 100 may include additional optical detectors or optical detectors 130a, 130b capable of detecting photons in multiple bands. Results from each band may be used to provide increased contrast, resolution, or detail in the final results. Depending on the implementation of the collection optics 110 and sensors 130a, 130b, the apparatus 100 may receive a broad or narrow band of photons.

[0043] Collection optics 110 operate to collect photons from object 1 in sufficient quantity to achieve a desired signal-to-noise performance. The collection optics 110 may be any collectors suitable to collect light from the desired band. For example, collection optics 110 may constitute any known or yet to be developed telescope design, including reflective and/or refractive optical elements. As shown in FIG. 1A, and by way of example, the collection optics 110 may be configured as a Cassegrain telescope. As another example, Gregorian, modified Dahl-Kirkham, Keplerian, or Ritchey-Chretien telescopes may be used. The collection optics 110 may include one or more steering optics to direct the photons to the beam splitter 120. The apparatus 100 may additionally include other common optical elements, such as collimators, filters, prisms, beam sizers, and the like.

[0044] Polarizing beam splitter 120 may be any suitable polarizing beam splitter for use with the intended spectral band. This may include cube beam splitters, plate beam splitters, high energy beam splitters, wire grid beam splitters, lateral displacement beam splitters, and the like. In one example, the polarizing beam splitter 120 may be a 50/50 cube beam splitter positioned to split outgoing light by a lateral angle of 90°. Light transmitted through the beam splitter 120 travels along a first pathway 122a to a first optical detector 130a. Light that reflects off the beam splitter 120 travels along a second pathway 122b to a second optical detector 130b. The optical detectors 130a, 130b may be aligned with the pathways 122a, 122b such that incident light strikes the optical detectors 130a, 130b normal to the surface of the optical detectors 130a, 130b. Light traveling along the first pathway 122a may have a first polarization state, while light traveling along the second pathway may have a second polarization state. For example, transmitted light may be P-polarized and reflected light may be S-polarized.

[0045] Separating the photons into two polarized portions provides two benefits. First, as described in greater detail below, the presence of two portions allows the apparatus 100, through processing, to achieve common mode rejection for signals subject to atmospheric scintillation or other noise-inducing phenomena. This is most common for non-terrestrial objects, such as satellites and spacecraft. An incident beam that has not been split will indicate the total intensity when read by an optical detector 130a, 130b. However, the total intensity is subject to scintillation, the variation in the apparent brightness or position of the object 1, which may mask or be mistaken for the acoustic signature. Therefore, rather than relying on total intensity, the apparatus 100 synchronously measures the intensities in the two orthogonal polarization states separately. In this way, variations due to apparent brightness or position of the object 1 can be compared across the first and second polarization states. Variations common to both states may indicate scintillation, and may be discarded or otherwise not considered. Additionally, measurements using total intensity may be highly sensitive to local vibrations in the sensing apparatus. The common mode rejection achieved by separating the beams may overcome this issue by allowing the apparatus 100 to discard variations induced by vibration that are common to both states.

[0046] Second, the presence of two signals with different polarization states allows the apparatus 100 to identify differences between the two signals, as well as changes in the differences between the signals, to determine characteristics of object 1, or a portion of object 1, over time, as described in greater detail below. It will be appreciated that changes in the differences over time may vary, for example, in response to vibrations of the object 1, causing a ratio of the quantities of light in the pathways to vary. The ratio is indicative of an acoustic signature of the object 1. Such variations may be particularly present where the collected photons are reflected from a man-made object. Man-made objects tend to have flat or partially flat surface structures made of certain materials, such that the polarization content of the light specularly reflected from the object is indicative of the nature of the object. For example, metallic surfaces may reflect polarized light differently relative to the polarization state of the light. Light in a first polarization state may be strongly reflected at certain times, angles, or under other conditions. Light in a second polarization state may be strongly reflected under different times, angles, or other conditions. Non-metallic surfaces may behave in a similar way. For example, solar panels, which are common to most satellites and spacecraft, present flat surfaces that may reflect polarized light differently. It may be expected that any acoustic oscillations from the solar panels will induce change in both the angle and degree of polarized light reflected. The construction of beam splitter 120 is selected so that the beam splitter operates in the selected band of the electromagnetic spectrum. For example, a polarizing beam splitter may be glass, calcite or be of a wire grid construction, although any other suitable beam splitter may be used.

[0047] As described above, polarizing beam splitter 120 may separate the light into two pathways 122a, 122b, where the light in the first pathway 122a is linearly polarized in a first direction and the light in the second pathway 122b is linearly polarized in a second direction that is orthogonal to the first direction. In one example the light in each pathway 122a, 122b is linearly polarized, the light being P and S-polarized, respectively. In another example, the light may be separated according to polarization such that first pathway is polarized to have light of a first Stokes parameter and the second pathway is polarized to have light of a second Stokes parameter. For instance, the first pathway 122a may be linearly polarized with its electric field aligned vertically, and the second pathway 122b may be linearly polarized with its electric field aligned horizontally. Alternatively, the first pathway 122a may be linear polarized with its electric field aligned at 45 degrees to vertical, and the second pathway 122b may be linearly polarized with its electric field at 45 degrees to vertical and orthogonal to the first pathway 122a. In another example, the first pathway 122a may be circularly polarized in a first direction and the second pathway 122b may be circularly polarized in a direction opposite to the first direction. In one example, the propagation of the first pathway 122a may be substantially perpendicular to the direction of propagation of the second pathway 122b. For example, many polarizing cube beam splitters direct the reflected beam at about a 90° angle relative to the transmitted beam. Plate beam splitters and other reflective polarizers may be positioned at an appropriate angle, usually about 45°, to achieve a 90° beam separation as well. First optical detector 130a is located in the first pathway 122a and receives photons in the first polarization state. Second optical detector 130b is located in the second pathway 122b and receives photons in the second polarization state. In one example, the optical detectors 130a, 130b may be optically aligned to receive the photons at a normal incident angle and at an optimal focal point. In another example, first and/or second pathways 122a, 122b may also include filter wheels, focal elements such as lenses, and other optical components. The optical detectors 130a, 130b may each produce a signal indicative of the quantity of photons received.

[0048] In one example, the collection optics 110 produce an image of the object 1 on the first and the second optical detectors 130a, 130b. The optical detectors 130a, 130b may be pixelated to receive spatial information about pathways of the image incident thereon. Pixelated detectors may indicate the location of the source of polarized photons relative to the object 1 as a whole. For example, pixelated detectors may indicate that a particular acoustic frequency is emanating from the top of an object 1, which may assist in identification or verification of operation. The optical detectors 130a, 130b may be any detectors suitable to operate in the selected band or bands of the electromagnetic spectrum, including CCD and CMOS devices. For instance, the optical detectors 130a, 130b may be a camera, such as a FLIR Flea® 3 CCD camera or FLIR Grasshopper® 3 CMOS camera. In another example, the detectors 130a, 130b may be other photodetectors, such as photodiodes, amplified detectors, integrating spheres, biased detectors, and the like. In yet another example, the detectors 130a, 130b may be different types of detectors, depending on implementation. In another example, the detectors 130a, 130b may be identical.

[0049] In some examples, collection optics 110 collect photons from the object 1 without forming an image. In those cases where an image is not generated or where image information is not relevant, the optical detectors 130a, 130b may operate merely to determine the quantity of light incident on the optical detectors 130a, 130b. In such examples, the optical detectors 130a, 130b may be area detectors such as single pixel detectors, single photon counters, or CCD/CMOS cameras that provide an aggregate signal rather than a pixelated signal. The pixelated detectors may be pixelated with the light from all pixels or a portion of the pixels being integrated to determine a quantity of light incident on the detectors.

[0050] It is to be appreciated that aspects of the present invention are valuable even when the object 1 is not resolvable by the optical system, i.e., collection optics 110 and the optical detectors 130a, 130b. In other words, the acoustic signature of the object 1 can still be obtained and characteristics of the object 1 can be determined, even if the optical portion of the apparatus 100 is unable to sufficiently resolve the object 1. For example, a remote satellite or vehicle that would not result in a suitable visual image may still provide polarization data sufficient to resolve the acoustic signature. Furthermore, in some examples, the optical detectors 130a, 130b may operate as a photon counter, outputting a stream of numerical values indicative of the time of arrival of each photon incident on the optical detector. This may be useful in determining the acoustic properties of the object 1.

[0051] The first optical detector 130a may generate a first signal, and the second optical detector 130b may generate a second signal. The signals generated by each optical detector 130a, 130b contain information regarding the amount of light of a given polarization that is received from the object 1 at a given point in time. The signals may be generated continuously at the maximum sampling rate allowed by the optical detectors 130a, 130b. The processor 140 may group signal data according to time segments, or periods of duration. This may allow the apparatus 100 to easily synchronize and compare the signals. By calculating a difference between the first signal corresponding to a selected time segment, and the second signal corresponding to the same selected time segment, and observing the change in the differences over time, it is possible to gain insights into the object 1, e.g., to identify the object, determine the state of operation of the object and/or determine functions of the object. Furthermore, in one example, the difference is divided by the sum of the first signal over the selected time segment and the second signal over the same selected time segment to provide a normalization factor. It will be appreciated that, in some examples, normalization may make the data based on the difference more reliable by accounting for variations in the total amount of light reflected from the object.

[0052] Although the above example was described using a difference between the first and second signals over a selected time segment, other operators comparing the first and second signals over a selected time segment may be used to gain information regarding the quantity of light in the first pathway and the quantity of light in the second pathway. For example, a ratio of the first and second signals over a selected time segment may be used.

[0053] The rate at which the signals are generated (i.e., the rate at which outputs from the first optical detector and the second optical detector are created) may be determined at least in part by the frequency of the vibration of the object 1 to be measured. It will be appreciated that it is desirable that the rate be at least at the Nyquist rate, or the minimum rate at which a signal can be sampled without introducing errors, which is generally twice the highest frequency present in the signal. For example, the rate may be in the range 1 per second to 10,000 per second for frequencies between 0.5 Hz and 5,000 Hz, although any suitable rate may be used that current technology permits or that yet-to-be produced technology may permit. For example, the rate may be 1500 per second. In some embodiments, the rate is set at the photon limit (i.e., based on the ability of the collection optics 110 to collect photons and the ability of the optical apparatus to transmit and detect the photons).

[0054] At least one processor 140 is in communication with the first and second optical detectors. In the illustrated embodiment, processor 140 is coupled in communication with the optical detectors 130a, 130b. The processor 140 may be any suitable computer processor for receiving and manipulating signal data from the detectors 130a, 130b. For instance, the processor 140 may be commonly used computer processors such as those having x86, x64, or ARM, PIC, or Arduino architecture. The apparatus 100 may comprise multiple local processors 140 for additional data processing or speed capabilities. The apparatus 100 may comprise a distributed network of processors 140, depending on implementation. Many aspects of the invention may take the form of computer-executable instructions, including algorithms executed by a programmable computer. Those skilled in the relevant art will appreciate that the invention can be practiced with other computer system configurations as well. Certain aspects of the invention can be embodied in a special-purpose computer or data processor that is specifically programmed, configured or constructed to perform one or more of the computer-executable algorithms described below. Accordingly, the term “computer” as generally used herein refers to any data processor and includes Internet appliances, hand-held devices (including palm-top computers, wearable computers, cellular or mobile phones, multi-processor systems, processor-based or programmable consumer electronics, network computers, minicomputers) and the like. A processor 140 may also include any non-programmable hard-wired electronic circuits used alone or in combination with other programmable circuits.

[0055] Some aspects of the invention may also be practiced in distributed computing environments, where tasks or modules are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules or subroutines may be located in both local and remote memory storage devices. Aspects of the invention described below may be stored or distributed on computer-readable media, including magnetic and optically readable and removable computer disks, fixed magnetic disks, floppy disk drive, optical disk drive, magneto-optical disk drive, magnetic tape, hard-disk drive (HDD), solid state drive (SSD), compact flash or non-volatile memory, as well as distributed electronically over networks. Data structures and transmissions of data particular to aspects of the invention are also encompassed within the scope of the invention.

[0056] The processor 140 may additionally include or be in communication with computer-readable memory 142 and a power supply 144. The memory 142 may be any memory suitable for signal processing, and may include both RAM and permanent storage. The power supply 144 may be any suitable power supply, including alternating or direct current, and may be fed by a wall outlet, generator, battery, photovoltaic cell, or any combination thereof. The processor 140 may include any other necessary components commonly used with computers, including motherboards, housing, input/output panels, display modules, analog-to-digital converters, network connections, and the like.

[0057] The processor 140 receives the first and second signals from the first and second optical detectors 130a, 130b, respectively. The processor 140 is then programmed to calculate a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum. These calculations may be performed for each data point generated by the detectors 130a, 130b in one example, or over a selected time segment in another example. Calculating the difference of the signals may give a difference in the quantity of light incident on the first and second detectors 130a, 130b over that time segment. Calculating the sum of the signals may give a total of the quantity of incident light. Calculating the ratio of the difference to the sum may indicate the degree of linear polarization (DOLP) of the incident light. This is because the apparatus 100 has measured, independently and simultaneously, the orthogonal polarization states as separated by the linear polarizer. In this modality, none of the Stokes parameters are being directly measured. However, the change in the DOLP as a function of time is being shown.

[0058] The first and second resultant orthogonal signals created by the optical detectors may be subject to a number of factors, including atmospheric transparency, inherent detector noise, the alignment angle of the polarizer, and the maximum and minimum observed intensities.

[0059] The processor may generate a signal corresponding to a sequential output of the ratios. Accordingly, it will be appreciated that the signal is indicative of an acoustic signature of the object. Although in the example described above a signal is generated using a difference, other operators comparing the first and second signals over a selected time segment may be used, and sequential outputs of the results of using said operators may be used to generate signals corresponding to an acoustic signature of the object 1.

[0060] The processor 140 may perform signal processing operations in order to prepare detected signals for further analysis. For example, common signal processing operations may include noise reduction, normalization, sharpening, and the like. Additional signal processing operations may include processing according to Mie theory to address atmospheric scattering, and Fourier transformation to modify the domain of the captured signals. Mie scattering may occur as the incident light travels through the atmosphere and interacts with particles with diameters similar to its wavelength. Mie theory may make the scattered signals more accurate and usable. Additional scattering treatments may be applied, according to Rayleigh scattering theory, discrete dipole approximation, and other computational techniques. Fourier transformation may transform the signals into the frequency domain, allowing the processor 140 to analyze the signals based on the frequencies present rather than the intensity of the signals. It will be appreciated that such frequency information may be highly indicative of the nature of the vibrations of object 1, as the frequencies of the vibrations may correspond to acoustic frequencies of or propagating through the object 1 itself. It will especially be appreciated that peaks in the frequency values may correspond to resonance frequencies of the object 1.

[0061] Additional techniques for time-series acoustic sampling may be employed instead of, or in combination with, any of the above techniques. For example, the signals may be autocorrelated in order to discover periodic signals, or beats, in the time domain. Time transient signals may rely on different implementations of looking at frequency distributions. For example, a Wigner distribution may be used to find chirp tones. Additionally, it may be useful to view played back high-speed footage at full or decimated rates where the apparatus 1 may process and store only subsections of the data to conserve memory.

[0062] In one example, the signals are divided into temporal segments and a transform is performed separately on each segment, and the variations in the frequency content of each segment are analyzed to determine changes in the acoustic signature over time. The length and number of each segment may be the same in one example, or may be different from segment to segment. The length and number of each segment may depend on the signal intensity, capture time, object size, number of frequencies present, and other factors. For example, a segment may have a length of about 256 samples or data points, and a measurement may contain about 100 segments. In another example, a segment may have between 30 and 100,000 samples. In another example, a measurement may contain between 1 and 100 segments. More or fewer samples, and more or fewer segments may be included, depending on the construction of the apparatus 100. A bandpass, high pass and/or low pass filter may be used to filter the signal and eliminate or reduce frequencies known to contain excessive noise. After the first and second signals have been compared, and the processor has calculated a difference, a sum, and a ratio of the difference to the sum, the resultant data may indicate the DOLP over the selected period of time. The data may be normalized for further analysis. The processor 140 may next generate a signal corresponding to a sequential output of the ratios. This signal is indicative of an acoustic signature of the object being sensed. To derive acoustic signatures from this resultant data, the processor 140 may calculate power spectra for the resultant data stream to reveal any acoustic signal present. The power spectrum describes the distribution of power over time into frequency components composing the signal. Peaks in the spectra may represent the frequencies of physical resonant vibrations on or from the object 1. A periodogram may be calculated for the entire duration of the signals and normalized. In one example, the periodograms of each segment may be averaged using overlapping windows. This may improve the accuracy of the power determination while limiting the frequency resolution.

[0063] Additionally, the power spectra data may be used to calculate a spectrogram of the frequencies detected as a function of time for the measured object 1. The spectrogram may be calculated from the power spectra of successive short sequences of the DOLP time series. In one example, each sequence may overlap. The spectrogram may allow a user to observe changes in the spectral content of the DOLP signal.

[0064] Acoustic detection may be performed empirically; however, it may also be useful to detect the activity of transient and quasi-steady signal environments in order to determine additional spectral components and develop confidence levels. The detections may be performed by generalized maximum and standard deviation arguments. More advanced tools, such as frequency domain decomposition, may further be used. Frequency domain decomposition (FDD) works by creating a spectral matrix and applying singular value decomposition to search for resonances within the detected signals. An exemplary approach to FDD includes estimating the power spectral density matrix at discrete frequencies, performing the singular value decomposition of the power spectral intensity, then selecting any desired peaks in the power spectral density corresponding to mode shapes. This may enable the apparatus 100 to determine the input signal characteristics of the object 1.

[0065] A tool such as FDD may allow modal analysis in regimes of low SNR with better performance than standard peak picking algorithms. The practical applications of this technique may be, in one example, that the application consists of measuring resolved satellites and identifying such features as modal shapes of the solar panels. This information may later be used to monitor the satellite's structural health for dangerously large resonances that could either cause structural damage or unintended orbit and attitude perturbations. The generated signal may be output by the processor 140. In one example, the generated signal may be stored in computer-readable memory 142 on the apparatus 100 or in a connected database. In another example, the generated signal may be output digitally to a display screen, visual file, numerical entry, or text data. In yet another example, the generated signal may be printed or otherwise indicated in hard copy format. The apparatus 100 may output the generated signal in any combination of formats. In another example, the apparatus 100 may compile a number of generated signals and other relevant data into a file, report, or other format.

[0066] In one example, where an optical detector comprises an array of pixels, the photons from object 1 or a relevant portion of object 1 may be directed only onto a limited portion of the array. For example, such an occurrence may result where collection optics 110 are configured to receive photons from a set field of view, such as when the collection optics 110 are staring, and the object 1 moves within the field of view. In such examples, it may be advantageous if a determination of the quantity of light in a given time segment of a series is calculated using an output of the optical detector from only the selected segment of the array, particularly the portion of the array receiving photons from the object 1. In such examples, the segment for which the quantity of light is calculated may be movable in a manner to track the object within the field of view. In other words, the object may be electronically tracked within the field of view. In one example, electronic tracking may be used in combination with mechanical tracking using movement of the collection optics 110. It will be appreciated that such tracking may be effective in increasing the signal-to-noise of the signal output from the processor 140.

[0067] In one example, light from a passive or natural source is incident on object 1, which results in reflected photons from object 1 that are collected by collection optics 110 to generate the incident light on the apparatus 100. For example, the light may be sun light, moon light, star light or ambient light present indoors. Alternatively, remote sensing apparatus 100 may comprise one or more light sources 150 to produce light in a desired band of electromagnetic radiation that can be operated to direct light onto object 1 to generate photons to be collected by the collection optics 110. In one example, it is advantageous if the light source 150 is (or light sources are) point sources; however, in one example, the light sources may be non-point sources.

[0068] In one example, it is advantageous if a signal from processor 140 (generated as described above) is output from the processor to an output sound transducer 160 such that the remotely sensed vibrations are used to generate a corresponding audible output. For example, such an output transducer may be useful where a human ear is particularly sensitive to a particular portion of the sound output or is particularly adept at identifying a relevant sound pattern. In some cases, the frequencies detected may be outside of human hearing range. In one example, the signal may be modulated—for instance, by single side-band modulation—to bring these infrasounds into the audible range by shifting the frequencies to those suitable for human hearing.

[0069] The apparatus 100 may be partially or entirely contained in a housing (not shown). The housing may be any container, enclosure, or holding form factor suitable to hold the apparatus 100. The collection optics 110 may be located outside the housing, and may direct photons from object 1 into the housing and the optical system within. The components within the housing may be mounted using any suitable materials and techniques, including optical rails, mounts, tubes, epoxy, adhesive, and the like.

[0070] FIG. 1B is a flowchart 10 illustrating signal processing flow for signals detected using the remote sensing apparatus of FIG. 1A, in accordance with the first exemplary embodiment of the present invention. In box 20, the first and second signals are received by the at least one processor, as described relative to FIG. 1A. A number of operations may be performed on the first and second signals to distinguish the noise in the signals. In box 22, before substantial signal processing occurs, the signal may be calibrated to remove bias, ensure alignment, ensure proper sensitivity of the detector, and the like.

[0071] In box 30, one or more signal conditioning processes 31-37 may be applied to the signal.

[0072] In box 31, in instances where the sample rate of the signal is well below the Nyquist limit, the signal may be resampled spatially or temporally to act as an averaging operation. This resampling may be performed at the pixel level or in the 1-D signal. A first and second signal corresponding to the resampled photons may be averaged with the first and second signals corresponding to the photons that were first sampled below the Nyquist limit.

[0073] In box 32, signal filtering may further improve the signal to noise ratio of the signal. Such filters may be, for example, moving averages, Savitsky-Golay filters, or Weiner deconvolution with an a priori system transfer function or a blind approach. These methods may, generally, reshape the signal according to the operation of the filter.

[0074] In box 33, other methods may be used to improve human perceptibility, which may include techniques such as Bayesian estimation.

[0075] In box 34, a bandpass, notch, high pass and/or low pass filter may be used to filter the signal and eliminate or reduce frequencies known to contain excessive noise. Some exemplary applications for these operations may include correcting the local vibration environment of the collection optics. If the sensor is attached to something like a drone, that has a definitive vibratory environment that is either or both quasi-steady such as the propeller or random such as the shake from atmospheric turbulence and if the field is moving across the sensor, the signal will be measured differently as it moves. This will blend the local signal with the desired measurement from the distant vibrating source where it may be appreciated that the separation of the measured object signal and the local sensor movement signal is considerably difficult.

[0076] In box 35, a DoLP offset may be performed.

[0077] In box 36, the first and second signals may be processed using beam forming techniques. Measurements that provide spatial information from separate sensing apparatuses or from a pixelated focal plane may include coherently summing or nulling separate measurements to improve the signal estimation. Spatially separated signals will have phase differences, which can be measured from the timeseries modulation of the degree of linear polarization. The phase component may be revealed by taking the Fourier transform of the signals, which will provide a unique measurement of the signal phase for spatial components in the image plane. In beam forming applications, each of these signals may be averaged, coherently summed to improve the signal to noise ratio, or coherently nulled to remove unwanted acoustic sources. For stationary applications, the time lags from separate spatial locations computed by cross-correlation will yield scene geometry.

[0078] In box 37, feed forward, or feed back filtering operations may be performed.

[0079] After the signal has been conditioned, vibrations may be removed. In box 40, one or more vibration signal removal processes 41-45 may be performed on the conditioned signal to counter local vibrations, either from pixelated data corresponding to the separate measurements or from their respective 1-D timeseries signals. Image jitter from local vibrations to the sensor may result in a corruption of the captured signal.

[0080] In box 41, principal component analysis may be applied to the conditioned signals to remove spurious focal plane motion. Variable signal components may be orthogonally transformed to locate and determine the values of linearly uncorrelated variables. Noise discovered through this process may be removed from the affected signals to yield denoised signals that may be further processed.

[0081] In box 42, measurement exclusion may be applied to the conditioned signals.

[0082] In box 43, Kalman filtering may operate by measuring signals from an external accelerometer and gyroscope whose measurement axis aligns with that of the sensor. A series of measurements observed over time may contain image jitter noise. The resulting signal may be processed using a Kalman filter to produce a 1-D signal estimate. The 1-D signal estimate may be subtracted from the total variational signal.

[0083] In box 44, Bayesian estimation may be applied to the signals. A Bayes estimator or decision rule may be created based on a prior distribution of one or more of the received signals. The Bayes estimator may estimate the amount and location of noise and may cause the processor to filter the noise based on the estimation and the received signals.

[0084] In box 45, notch filtering may be applied to remove residual vibrational frequencies. Other operations may include using common image field sources or the object source itself in a stationary application as references to measure quasi-steady source signals. The quasi-steady source signals may be used to inform the frequency bands of high pass, low pass, notch, or other filters.

[0085] In box 50, the first and second signals may be further processed as described relative to FIG. 1A and FIG. 4 to determine a difference, sum, and ratio of the difference to the sum of the processed first and second signals. Thereafter, the ratio of the signals may be processed in one or more signal applications 61, 62, 63 according to box 60. In box 61 the ratio of the signals may be directed to a technical signal output, discussed further relative to FIG. 1C. In box 62, the ratio of the signals may be directed to an audio output as discussed relative to FIG. 1A, above. In box 63, the ratio of the signals may be directed to a visual scene output as an image or video.

[0086] FIG. 1C is a flowchart 12 illustrating the technical signal flow for acoustic signals processed according to FIG. 1B, in accordance with the first exemplary embodiment of the present invention. FIG. 1C may be considered a continuation of the signal processing flow described in FIG. 1B. In box 61, signals are directed to a technical signal output. From there, one or more processes may be applied in order to manipulate the signal data to provide technical information about the imaged object.

[0087] In box 70, one or more frequency domain decomposition processes 71-74 may be applied.

[0088] In box 71, the signal output may be expressed as a spectrogram or waterfall plot.

[0089] In box 72, a cross-spectral density calculation may allow a user to identify particular features from temporally separated measurements or to compare a detected signal to a known object-specific acoustic spectrum. A cross-spectrum may be calculated to measure the similarity between pairs of acoustic spectra measured at different points in time. The amount of time difference may depend on the intended application, but in one example may be about one month. Shorter or longer time differences may be included within the scope of this disclosure. When frequency domain signals processed by standard Fourier methods are expressed in polar coordinates, they provide a magnitude and a phase as two separate 1-Dimensional (1-D) vectors. The 1-D magnitude vectors may be used to view spectral components of the frequency domain signals.

[0090] Signals may be received at a first point in time and a second point in time. The first point in time may be an earlier point, and the second point may be a later point relative to the first. A difference, sum, and ratio of the difference to the sum of the received signals may be calculated for signals received at the earlier point and the later point. The cross-spectrum may be calculated as the earlier-determined and later-determined frequency domain signals multiplied to make one of the signals the complex conjugate. The result may show any self-similarity as spectral peaks, making an effective cross-correlation in the frequency domain. This may be compared to the result achieved when using the power spectrum, which may be calculated as the complex conjugate of the signal multiplied by itself. The power spectrum may be used instead, as the units of measure may be related back to the physical issue surrounding the desired application.

[0091] In another example, this operation may be performed to the time-series measurement to calculate a cross correlation. This may indicate if separate sensors are not recording at the same rates. Additionally, if the signal is cross-correlated with itself, it will show any beat frequencies.

[0092] The cross spectrum is a method to take frequency spectra acquired at separate times and determine self-similarity between those acquired and temporally separated spectra. The difference between the acquired signals can be seconds, minutes, months, or years and depends on the desired application. One example, in the context of observing satellites, may include measuring the spin rate of a spin-stabilized bus. Spin-stabilized buses are a common architecture for broadcast satellites manufactured by the Hughes Aircraft Company and the Boeing Company. These architecture satellites maintain their orientation to earth by maintaining a steady angular velocity particular to a specified axis while keeping the broadcasting antennae dish on a bearing which allows a near constant earthward pointing. The torque from that spin maintains that stable pointing position. These architecture satellites maintain their angular velocity by releasing propellant from a side jet. These described station keeping maneuvers occur at the discretion of the satellite ground operators and subsequently the satellite angular velocity and associated orbital parameters change with each station keeping burn.

[0093] In this example, the cross spectrum may amplify peaks in the monitored frequencies from temporally separated measurements. The first interpretation may be that if the separate spectra indicate acoustic content and the cross spectrum lacks content, either the spacecraft vibratory environment has significantly changed or this null detection may indicate to the ground operator that the observations are not of the same object. The second interpretation may be that if the separate spectra indicate acoustic content and the cross spectrum contains spectral information in the same frequency locations as the separate spectra, the ground operator may be indicated that the observations are of the same spacecraft and that the angular velocity has not changed from the previous measurement. The third interpretation may be that if the separate spectra indicate acoustic content and the cross spectrum contains acoustic content but that the content either has significant size or offset, the object may have performed a station keeping maneuver in the elapsed time from the previous set of measurements.

[0094] In box 73, the power spectral density of the signal may be calculated.

[0095] In box 74, the magnitude and phase spectra may be determined.

[0096] After the one or more frequency domain decomposition processes 71-74 have been performed, the processed signal may optionally be tested for data quality and SNR metrics according to box 75. In another example, one or more detection processes 81-83 may be applied to the processed signal according to box 80. In box 81, acoustic activity may be detected using the processed signal. This may be performed empirically, according to box 82, or in an automated detection process, according to box 83.

[0097] After detection, the signal may be used in one or more technical signal applications 91-96, according to box 90.

[0098] In box 91, the signal may be used to identify object maneuvers.

[0099] In box 92, modal imaging operations may be performed. Modes are an inherent physical property of structures determined by mass, damping, and stiffness. A mode is characterized where all parts or a subsection for a complex structure move sinusoidally with the same frequency and phase relation. The modes are excited by being physically driven by either instantaneous forces or steady-source forces. Physical structures such as buildings, bridges, or vehicles have sets of normal modes and natural resonant frequencies that depend on structure, material, boundary conditions, and excitation force. In practice these resonant modes and their amplitudes may present significant design flaws that can lead to shortened mechanical lifespan or even structural collapse given the correct input conditions.

[0100] Modes are commonly measured by in-situ accelerometers, velocimeters, or for smaller apparatuses, scanning laser Doppler vibrometers. These measurements may be interpreted using visual tools such as waterfall plots, spectrograms, periodograms, power spectra, or magnitude spectra. In one example, structural modes may be represented as a color-coded overlay onto processed high-speed imaging data played at full or decimated frame rates for visual inspection. In another example, the amplitude of the movement may be multiplied by a scale factor to the magnitude component of the returned vector from the fast Fourier transform. In processed high-speed imaging, the movement is spatially amplified for visual inspection.

[0101] In the present disclosure, the processed signal may be further processed to discover the modes present within an imaged object. Modal processing techniques may measure individual spatial locations to have frequency and phase components in equal relation to separate simultaneous spatial location measurements for a given surface or structure. In practice, each pixel for each image may be processed as a timeseries vector, which may then be processed as a fast Fourier transform (FFT). From this, the output would be a 3-Dimensional array of returned FFT vectors for each pixel for each frame. A generalized peak picking algorithm based on heuristic noise thresholds or frequency domain decomposition may identify pixels with self-similarity in both time and space in the FFT array. Self-similarity may be indicated by the received data from a group of pixels existing within an acceptable threshold with both the same frequency and phase component over a period of time. Discovered modes may be indicated by generating an additional signal corresponding to the discovered modes. The additional signal may be displayed visually as false-color enhancement, motion amplification, or any combination thereof to display post processed image data at either the full or a decimated rate. In specific circumstances and with appropriate information the technique may measure out of plane displacement of structures or surfaces, material properties may be estimated such as the stiffness or damping coefficients, or input forces as approximated measurements. It may also be recognized that in specific settings with a well approximated physical model, the effective mass may also be estimated. As an example this technique may measure such instances as the normal modes of a car engine as seen from the vehicle body, a building's modes, or an aircraft wing's modes for inspection.

[0102] In the illustrative example of observing satellites it may be appreciated that many key operating parameters may be estimated from measuring vibration spectra indicative of a maneuver event if observed and identified for the full event duration. Among these parameters are mass flow rate, fuel consumption, and specific impulse. These parameters may fully be estimated with an a priori object mass estimate, a known delta-v magnitude, and a known direction as observed from the remote acoustic sensor. A unique vibration spectrum, if observed on a spacecraft during a maneuver, will provide the necessary time resolution to accurately estimate the beginning and end of a maneuver and thus provide an accurate event duration. This allows estimations of both the work performed and impulse imparted. The innovative step for accurate event duration would allow faster than the currently used angles-only method for maneuver detection.

[0103] As described above, a post-processed cube may display backwards as false-colored video at full or decimated rates or as motion amplified video. Denoising and frequency filtering techniques may be applied to remove unwanted sources in the field or sharpen the signal. In controlled environments where a priori forces and scene geometry are known, standard vibratory analysis may be applied where the motion of structures or surfaces may be measured as surface displacement, the structural stiffness may be estimated, the excitation force may be estimated, or separate material properties may be estimated. In cases where the input force is known and the stiffness and dampening coefficients are well-approximated, the effective mass may be estimated by assuming that the object moves according to a set of linear equations.

[0104] In box 93, the acoustic measurements provided by a pixelated optical detector may provide information useful for imaging through obscurants such as clouds, dust, smog, and haze. For pixelated focal planes where spatial information is available and each pixel in the plane represents a unique timeseries measurement, an obscurant cloud may be vibrated by a source 1 hidden behind the obscurant. The obscurant may be removed from the imaging data by measuring a vibration signature of each pixel of the at least one optical detector. The resulting Fourier power components may be removed by using one or more differencing techniques. A signal corresponding to the acoustic signature may be generated, revealing the vibrating source behind the obscurant. The generated signal may result from taking the measured signal into the frequency space, differencing the identified peaks, and taking an inverse fast Fourier transform. In practical use, the differencing or nulling operation may be performed in the frequency domain, then an inverse Fourier transform is performed.

[0105] In box 94, beam forming techniques may be used to identify scene geometry of the detected object.

[0106] In box 95, object recognition techniques may be used to identify the object.

[0107] In box 96, property identification techniques may be used to identify properties or physical characteristics of the detected object.

[0108] Additional signal processing and technical signal output paths are contemplated within the scope of this invention. For example, any of the frequency domain operations performed using Fourier transforms may alternatively be performed using wavelet calculations.

[0109] The following operating example is given to provide additional detail of the processes of FIGS. 1B, 1C used with FIG. 1A.

[0110] A user may record the two or more separate and simultaneous polarizations from an object using the collection optics, such as a telescope. If the bandwidth of the frequency signal being sought after is unknown such as a common car hood for lateral applications or a satellite for upward looking applications, the user may use a sufficiently fast sampling rate while maintaining a sufficient photon sampling per the detector. The user may want to detect a large number of photons to overcome the fundamental errors in the measurement where an example of an error could be intrinsic shot noise. The quality of the acquired DoLP polarization signal may then depend on the angle of illumination, the driving force and surface displacement, and the material properties of the object.

[0111] A first step for processing the data may be the DC removal from the signal. This DC removal step may include either a full frame DC subtraction for pixelated detectors or an average measurement subtraction for single cell detectors. For applications relying on the use of pixelated focal planes, the user may then remove field dependent response errors. These response errors may be the result of sensor manufacturing error, optical field curvature, field dependent transmission differences, and the like. These response errors may be removed by such a process as the division of a flat field measurement from the acquired data or a like irregularity removal process.

[0112] A second step of data processing may be the resampling of the signal, the necessity of which may be determined by the acquired acoustic content. The need for resampling may be based upon the location of the frequency content in relation to the full measurement bandwidth. The act of resampling provides several advantages to the user, those being a reduction in the extent of pink noise, and an improvement in the signal by averaging temporally the acquired data. For applications with sufficiently high sampling rates, the digitization of the recorded signal may be improved beyond the limitations imposed by the sensor.

[0113] For those skilled in the art, it may be appreciated that each of the processing steps are linear operations allowing the operations to be carried out without regard for their order. The acquired signal may then if necessary, be improved by removing the local vibration signal from the total acquired signal. These removal processes may encompass the processes found in boxes 41-45 and may depend on the quality of the acquired data.

[0114] The signal may then be improved in terms of noise content. In the ideal measurement environment, a system transfer function may be known prior or be acquired from the object. From this system transfer function, deconvolution may produce a sufficiently restored signal. Other approaches to restoration may be such approaches as blind deconvolution, Savitsky-Golay filters at the expense of nulling higher frequency content, or normalizing the signal to standard metrics such as the mean. Additional processing steps may include the coherent summing of measurements from beam forming, the perceptibility improvement with Bayes methods, bandpass and notch filtering, feed filters, and delaying samples to create an effective derivative filter. The delay of samples may improve the signal to noise ratio for higher frequency content by creating an effective derivative filter without sacrificing signal bandwidth. These conditioning processes may encompass the processes found in boxes 32-37 and may depend on the quality of data acquired and a priori information available to the user.

[0115] The signal after being sufficiently processed may be used for either visual interpretation with either the full, decimated, or single instances of the acquired data. Other uses may include converting the optical measurement into audio for interpretation; the last use may also be continuing to use the signal for technical applications. These signal uses may encompass the processes found in boxes 61-63 and may depend on the user application.

[0116] The technical applications of the data may include the visualization of the acquired data using frequency space visualization tools. These tools may be those described in processes 71-74 and may depend on the application for the user. The visualization of the time domain signals may include 1-dimensional timeseries plots of the signal or the playback of imaging or video data. The later applications of the technical data may include detecting significant features from the data using libraries and correlation, a machine learning framework, or empirically. These tools may be those described in processes 81-83 and FIG. 3. The user may then use the automatically or empirically generated features of the signal to perform modal analysis, estimations of spacecraft parameters, object recognition, or material and property identification. These signal uses may encompass the processes 91-96 and may depend on the user application.

[0117] FIG. 2 is a perspective illustration of the remote sensing apparatus 200 of FIGS. 1A-1B. FIG. 2 shows an exemplary implementation of the optical system of the apparatus 200, including collection optics 210, directing optics 260, focusing elements 262, 264a, 264b, beam splitter 220, and optical detectors 230a, 230b. In this example, the collection optics 210 are of a Cassegrain design and operate to direct photons to directing optics 260, which are shown as a mirror. In another example, directing optics 260 may be a prism or other elements suitable for orienting the apparatus 200 in a desired direction. The directing optics 260 direct the photons into a first focusing element 262, which collimates the incident beam of light before it reaches the beam splitter 220. The first focusing element 262 may be any suitable lens or combination of elements suitable to collimate light in the desired band of the electromagnetic spectrum. The beam splitter 220 divides the light into two pathways where the light in each pathway is linearly polarized and the linear polarizations are orthogonal to one another (i.e., the pathways are p and s polarized, respectively). Second and third focusing elements 264a, 264b, respectively, focus the light from each pathway onto the optical detectors 230a, 230b, respectively.

[0118] FIG. 3 is a schematic illustration showing a library 300 of acoustic signatures. In one example, it may be advantageous to have a library 300 of acoustic signatures with which a signal output 310 from the processor 340 can be compared, e.g., by performing a correlation calculation. The library 300 may include acoustic signatures for known objects, such as satellites, spacecraft, vehicles, and the like. The library 300 may include acoustic signatures for known objects in known environments, such as in space, high altitude, inclement weather, and the like. The library 300 may include a collection of acoustic signatures for known activities, such as the operation of a mechanical piece or extension of solar panels. The activities may be further categorized based on whether they are running normally, i.e., the components are functioning well, or running abnormally, i.e., a component may be failing or functioning improperly. The library 300 may include only partial acoustic signatures as well. In one example, the library 300 may be comprised of entries 370, where each entry is the signal output from processor 140 of the remote sensing apparatus 100 when it is receiving light from a known object in a known state (i.e., it is the acoustic signature of a known object 1 operating in a known manner). The entries may be stored as digital files on computer-readable memory 342 which may be the on-board memory of the apparatus 100 relative to FIG. 1A, or a connected database. The database may be connected locally or remotely by a network system. The processor 340 may be programmed to compare the signal output 310 with the library entries 370. The comparison may utilize machine learning, deep learning, neural networks, or other computational techniques to find signal patterns that match. Comparisons of this nature may use the database entries 370 to train the processor 340 which factors are indicative of certain objects, environments, activities, and the like. In one example, one or more machine learning techniques, such as principal component analysis, may be used. In another example, the entries 370 may be converted into audio fingerprints, where the data-heavy digital frequency information is converted into easy-to-search numeric and hash values. These numeric and hash values may be normalized and compared against the signal output 310 to detect matching values or patterns of values. In one example, t-distributed stochastic neighbor embedding (t-SNE) may be used. It will be appreciated that information regarding an unknown object could be determined when there is a correlation between the acoustic signature in the library 300 and a signal from the processor 140 when it is observing the unknown object. The processor 140 may compare the library entries 370 directly, or it may convert the unknown data into audio fingerprints for faster searching. The comparison may be performed contemporaneously with the signal acquisition, i.e., as the apparatus 100 is collecting the photons. In another example, the comparison may be performed after the entire signal 310 has been captured and processed. Other statistical tools, such as probabilistic graphical models, may be used.

[0119] This framework may be useful in a number of instances. For example, in the application of identifying non-resolved spacecraft, ground operators may need to identify and characterize several objects or spacecraft in a limited timespan while relying on the acquired acoustic spectra. From these many measurements the implementation of probabilistic graphical models (PGMS) may allow the computer to make decisions on limited amounts of data, in contrast to other computational approaches such as neural networks. The ground operator may then rely on the output of the PGM to decide or assist in the identification and characterization of relevant aspects of the spacecraft. Those decisions may have a developed confidence interval. These decisions may provide such useful information to the ground operator as the satellite architecture, object specific ID, and subsystem activity among the other described aspects of the invention. The PGM approach is useful because the model of data that the probabilities are calculated from may be idealized calculations based upon the physical principles of the source interaction. In the continued satellite example, these realized models may be such instances as the outputs of finite element analysis models, satellite reflectance models, and the like. This proves beneficial because the data to dictate decisions may be modelled and simulated whereas neural net methods may require large amounts of acquired data to be accurate.

[0120] FIG. 4 is a flowchart 400 illustrating a photo-acoustic, polarimetric method of remotely sensing an object, in accordance with the first exemplary embodiment of the present invention. It should be noted that any process descriptions or blocks in flow charts should be understood as representing modules, segments, portions of code, or steps that include one or more instructions for implementing specific logical functions in the process, and alternate implementations are included within the scope of the present disclosure in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present disclosure.

[0121] Step 410 includes collecting photons from the object. The collection optics 110 may be those discussed relative to FIG. 1A, such as reflective or refractive telescopes, zoom lenses, or other optics.

[0122] Step 420 includes directing the photons down first and second pathways, wherein photons within the first pathway have a first polarization state and photons within the second pathway have a second polarization state. The input beam may be split by one or more polarizing beam splitters, beam splitting plates, birefringent glass, prisms, and the like. The angular deviation between the first and second pathways may be any suitable deviation necessary to separately detect the first and second pathways. For instance, when certain prisms are used, as will be discussed further in FIG. 5, below, the angular deviation may be small, up to several degrees. When a polarizing beam splitting cube is used, the angular deviation may be much greater, up to about 90° or more. The proper angular deviation may depend on the optical system design.

[0123] Step 430 includes detecting photons in the first and second pathways using at least one optical detector, wherein the photons in the first polarization state produce a first signal and the photons in the second polarization state produce a second signal. In one example, a single optical detector may be used to detect both the first and second pathways. The photons in the pathways, may strike different portions of the optical detector, allowing the detected signals to be calculated independently. In another example, multiple detectors may be used. For instance, one detector may be placed in each pathway to detect the incident light separately. In one example, more than two detectors may be used to create spatial separation in the detected data.

[0124] Step 440 includes receiving, with at least one processor in communication with the at least one optical detector, a first signal and a second signal corresponding to the photons in the first and second pathways, respectively.

[0125] Step 450 includes determining, with the at least one processor and for a segment of the first and second signals, a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum. Where the first and second signals both come from a single optical detector, the signals may need to be separated by the processor before the determinations can be made. Separation may simply include recording values in different areas or different pixels on the optical detector separately. In one example, separation may require noise reduction operations to ensure that the signal data is truly separate, especially at the part of the optical detector where the two pathways hit nearest each other. Additional signal processing, as discussed relative to FIG. 1A, above, may be performed by the processor.

[0126] Step 460 includes generating a signal for a plurality of segments corresponding to a sequential output of the ratio for each segment, wherein the signal is indicative of an acoustic signature of the object. The generated signal may be the signal discussed relative to FIG. 1A, above, and may be useful as an acoustic indicator of the nature, operation, or state of the object being sensed.

[0127] The method may further include any other features, components, or functions disclosed relative to any other figure of this disclosure.

[0128] FIG. 5 is a schematic block diagram of a remote sensing apparatus 500 in accordance with a second exemplary aspect of the present invention. The apparatus 500 includes collection optics 510 directing photons from an object 1. A polarizing prism 520 angularly separates the photons into a first polarization state 522a and a second polarization state 522b. An optical detector 530 is included. A first portion of the optical detector 530 receives the photons in the first polarization state 522a, and a second portion of the optical detector 530 receives the photons in the second polarization state 522b. At least one processor 540 is in communication with the optical detector 530. The at least one processor 540: receives a signal from the optical detector 530, wherein a first portion of the signal corresponds to the photons in the first polarization state 522a, and a second portion of the signal corresponds to the photons in the second polarization state 522b; calculates, for a segment of the first and second signals, a difference of the first and second portions of the signals, a sum of the first and second portions of the signals, and a ratio of the difference to the sum; and generates, for a plurality of segments, a signal 550 corresponding to a sequential output of the ratio for each segment. The signal is indicative of an acoustic signature of an object 1 being sensed.

[0129] Many aspects of the optical system of apparatus 500 may be the same or similar to those discussed relative to FIGS. 1 and 2, above. For instance, the collection optics 510, the optical detector 530, and the other optical elements such as directing optics and focusing optics (not shown) may be used. The collection optics may direct photons from object 1 into the rest of the apparatus 500.

[0130] The photons may be directed to a polarizing prism 520. The polarizing prism 520 may angularly separate the incident photon beam into two beams having two different polarization states. The polarizing prism 520 may be any suitable birefringent polarizers, such as Glan-Taylor polarizers, Glan-Thompson polarizers, Wollaston prisms, Rochon prisms, and the like. By way of example, Wollaston prisms may provide a symmetric deviation between the first and second beams while maintaining substantially equal intensities across both beams. In one example, multiple prisms 520 may be used to separate the beams, separate the polarization states, equalize the beam intensities, or direct the beam path.

[0131] The photon beam may be separated into photons in a first polarization state 522a and a second polarization state 522b. The photons from the first polarization state 522a may be linearly polarized and orthogonal to the photons from the second polarization state 522b. In one example, the photons may be circularly or elliptically polarized by introduction of a quarter wave plate. The photons from both states 522a, 522b may hit the optical detector 530 along different portions of the detector surface. A first portion of the optical detector 530 may receive the photons from the first state 522a, and a second portion of the optical detector 530 may receive the photons from the second state 522b. In one example, the signals created by the first and second polarization states 522a, 522b may be individually resolvable due to the angular separation of the photons in the first and second polarization states 522a, 522b. In another example signal processing may be required to separate a portion of the signals.

[0132] The at least one processor 540 may be any of the processors and accompanying components 542, 544 as discussed relative to FIG. 1A, above. The processor 540 may receive a signal from the optical detector 530, wherein a first portion of the signal corresponds to the photons in the first polarization state 522a, and a second portion of the signal corresponds to the photons in the second polarization state 522b. The first and second portions of the signal may correspond with the number or intensity of photons incident upon the optical detector 530 relative to the first and second polarization states 522a, 522b. The processor 540 may calculate a difference of the first and second portions of the signals, a sum of the first and second portions of the signals, and a ratio of the difference to the sum. This operation may be similar to the calculation performed relative to FIG. 1A, above. The processor 540 may generate a signal 550 corresponding to a sequential output of the ratios. The signal 550 may directly express the degree of polarization of the input photons, and may vary in magnitude over time. As discussed above, the variations in magnitude over time may be transformed or otherwise used to determine oscillating frequencies coming from the object 1. These frequencies can be used to create an acoustic signature of the object 1. The signal 550 may be further processed to make it easier to hear, to filter noise, or to focus on particular frequencies present. The signal 550 may be fed to an acoustic transducer, output to a screen, report, or other file, or compared against a library of entries as discussed in FIG. 3.

[0133] In one example, the apparatus 500 may include a flip mirror 524 that can be moved into and out of the beam path before the photons reach the polarizing prism 520. The flip mirror 524 may be used to direct the incoming photons to an acquisition detector 532. The acquisition detector 532 may be an optical detector as discussed relative to FIG. 1A, including a CCD or CMOS camera. It may be in communication with the processor 540. The acquisition detector 532 may be used to locate or focus in on the object 1. This may be especially helpful when the desired imaging band is in the visible spectrum. The image provided by the acquisition detector may allow a user to align the apparatus 500 or to track the object 1. In one example, the image data may not be considered by the processor 540 in the acoustic signature calculations below. In another example, the image data may be used as a visual overlay to show, spatially, where measured frequencies originated from on the object 1. The acquisition detector may additionally be used with the other aspects of this disclosure.

[0134] In another example, the apparatus 500 may further include an accelerometer 546. The accelerometer 546 may provide 3-axis motion and directional data relative to the position and movement of the apparatus 500. The accelerometer 546 may be in communication with the processor 540, which may provide the data to a user. In one example, movement of the apparatus 500 may be controlled by a computerized motor, such as a stepper motor, capable of tracking GEO objects as they travel across the sky. The data from the accelerometer 546 may allow the motor to maintain an accurate tracking position.

[0135] FIG. 6 is a schematic block diagram of a remote sensing apparatus 600, in accordance with a third exemplary embodiment of the present invention. The apparatus 600 may remotely detect the modulation of circularly polarized light incident upon the apparatus 600. The apparatus 600 may include collection optics 610 to collect and receive photons from an object 1. The collection optics 610 may direct light from the object 1 down an optical path. In one example, the light is split by a 50/50 beam splitter 620 that divides the light into two pathways 622a, 622b of approximately equal intensity. The light may not be polarized at this time. The light in each pathway 622a, 622b may further be directed through respective quarter waveplates 630a, 630b and then through respective linear polarizers 640a, 640b. The quarter waveplates 630a, 630b and linear polarizers 640a, 640b may be selected to separate the light in the pathways 622a, 622b into left and right circularly polarized light. For example, light in the first pathway 622a may be left circularly polarized, while light in the second pathway 622b may be right circularly polarized. The 50/50 beamsplitter 620 may be any suitable plate, circular filter, tube, cube, or the like for dividing light into two distinct pathways. The quarter waveplates 630a, 630b and linear polarizers 640a, 640b may be any suitable quarter waveplates and linear polarizers commonly used, depending on the spectrum of light being detected.

[0136] Circularly polarized light from pathways 622a, 622b may hit photodetectors 650a, 650b, respectively, located in the pathways. The photodetectors 650a, 650b may be in communication with a processor 660 as described above relative to FIGS. 1 and 5. The photodetectors 650a, 650b may communicate first and second signals to the processor 660 as light is detected. The processor 660 may include computer memory, power supply, and other electronic components as shown in FIG. 1A. The processor 660 may calculate a difference of the first and second signals, a sum of the first and second signals, and a ratio of the difference to the sum. This operation may be similar to the calculation performed relative to FIG. 1A, above. The processor 660 may generate a signal corresponding to a sequential output of the ratios. The signal may directly express the degree of polarization of the input photons, and may vary in magnitude over time. As discussed above, the variations in magnitude over time may be transformed or otherwise used to determine oscillating frequencies coming from the object 1. These frequencies can be used to create an acoustic signature of the object 1. The signal may be further processed to make it easier to hear, to filter noise, or to focus on particular frequencies present. The signal may be fed to an acoustic transducer, output to a screen, report, or other file, or compared against a library of entries as discussed in FIG. 3.

[0137] FIG. 7A is a schematic block diagram of a remote sensing apparatus 700, in accordance with a fourth exemplary embodiment of the present invention. The apparatus 700 includes collection optics 110 directing photons from an object 1. A micro-polarizer array 720 is located on an optical detector 730. Photons propagating through the micro-polarizer array 720 have at least first and second pathways 722a, 722b. Photons within the at least first and second pathways 722a, 722b have at least first and second polarization states, respectively. The optical detector 730 is located in the first and second pathways. At least one processor 140 is in communication with the optical detector 730. The at least one processor 140 is configured to: receive a signal from the optical detector 730, wherein a first portion of the signal corresponds to the photons in the first polarization state, and a second portion of the signal corresponds to the photons in the at least second polarization state; calculate, for a segment of the signal, a difference of the first and at least second signals, a sum of the first and at least second signals, and a ratio of the difference to the sum; and generate, for a plurality of segments, a signal corresponding to the sequential output of the ratio for each segment, wherein the signal is indicative of an acoustic signature of the object 1 being sensed.

[0138] The micro-polarizer array 720 may be any array of polarizing elements. The micro-polarizer array 720 is located on the optical detector 730. For purposes of illustration, the micro-polarizer array 720 is shown spaced apart from the optical detector 730. This is done to illustrate that the photons that pass through different portions of the array may be divided into different states of polarization. However, in practice, the polarizing elements will be located on and part of the optical detector 730.

[0139] The processor 140 may additionally include or be in communication with computer-readable memory 142 and a power supply 144. The memory 142 may be any memory suitable for signal processing, and may include both RAM and permanent storage. The power supply 144 may be any suitable power supply, including alternating or direct current, and may be fed by a wall outlet, generator, battery, photovoltaic cell, or any combination thereof. The processor 140 may include any other necessary components commonly used with computers, including motherboards, housing, input/output panels, display modules, analog-to-digital converters, network connections, and the like.

[0140] The apparatus 700 may include an artificial light source to illuminate the object 1 as shown in FIG. 1A.

[0141] FIG. 7B is an illustration of a micro-polarizer array 720 in use with the remote sensing apparatus 700 of FIG. 7A, in accordance with the fourth exemplary embodiment of the present invention. The micro-polarizer array 720 may include a plurality of polarizers 770, 771, 772, 773 located in the path of individual pixels 724, 725, 726, 727 of the optical detector 730. One polarizer may be located in front of one pixel—for instance, polarizer 770 may be located in front of pixel 724, polarizer 771 in front of pixel 725, and so on, as illustrated in FIG. 7B. Four pixels are shown in FIG. 7B as an example only. In practice, an optical detector 730 may include millions of pixels, each with their own micro-polarizer. In the example shown, each pixel 724, 725, 726, 727 includes a linear polarizer 770, 771, 772, and 773 at 0°, 45°, 90°, and 135°. Other micro-polarizer arrays may include fewer polarizing elements, for example, at two angles of linear polarization. In one example, the micro-polarizer array 720 may be the Sony® Pregius IMX250MZR Polarization sensor. Photons propagating through the micro-polarizer array and onto the optical detector 730 are separated into their component linear polarization states at each pixel. This may allow the polarization of light incident on the optical detector 730 to be measured without the use of beamsplitters or prisms, which in turn allows the apparatus 700 to be simpler and more compact.

[0142] The micro-polarizer orientations are fixed, meaning only one state of polarization can be measured for each single pixel. But for a camera with a large number of pixels, it is possible to get many measurements and many states. For the purpose of the DoLP calculation, orthogonal pixel axes may be selected for calculations. These may be interpolated down to a DoLP image and further processed, or processed as is. In order to modulate the polarization angle with time, a polarization fiber switch implementation, described relative to FIG. 9 below, may be necessary.

[0143] FIG. 8 is a schematic block diagram showing the remote sensing apparatus 801 of FIG. 1A with adaptive optics image correction, in accordance with the first exemplary embodiment of the present invention. Adaptive optics may be effective for both upward and horizontal path imaging scenarios such as those employed by free space optical communications systems, astronomical telescopes, and directed energy systems. Passive approaches to image restoration for the improvement in measuring the intensity of a surface may be achieved by the use of image deconvolution, either through blind approaches, or as informed by additional wavefront measurements, as discussed above. Active approaches to image restoration for the improvement in measuring the intensity of a surface may include the use of an adaptive optics system. As an object 1 is imaged by collection optics 810, photons from the object 1 may be subject to atmospheric distortion 870 or other conditions that alter the appearance of the object 1. As a result, the imaged signal may be distorted proportional to the degree of atmospheric distortion 870. Adaptive optics systems may include one or more deformable mirrors 812 that are adjustable by actuators 872, 874. The actuators 872, 874 may adjust the shape of the deformable mirrors 812 to compensate for the optical aberration caused by the atmospheric distortion 870. The photons may travel to the rest of the components 890 in the apparatus 801 as shown in FIG. 1A. The apparatus 801 may further include a wavefront sensor 892, which may read the received photons, calculate the appropriate adaptive corrections, and communicate them to the actuators 872, 874. The distortion having been reduced or corrected, the resultant signals received by the optical detectors 130a, 130b may be significantly more accurate.

[0144] The adaptive optics system may rely on the light reflected by the surface of the object being measured, artificial illumination of the object, or separate guide star illumination. An artificial source 880 may be used to provide illumination. The artificial source 880 may include global illumination of the portion of the object 1 desired to be imaged. In another example, the artificial source 880 may include a guide star, which is an artificial image used to provide a reference point for the apparatus 801. In aspects where a guide star is used in operation, simultaneous field measurements from a pixelated array or multiple sensors may provide a reference source for local vibration or as a reference noise source.

[0145] Many imaging applications with a large distance between the object and the sensor may see significant degradation in the capability of the technology to acquire acoustic information because of atmospheric turbulence. For the illustrative example of observing resolved satellites, it may be appreciated that spatial information for each of the satellite subsystems will be physically blurred. These subsystems may be components such as the spacecraft solar arrays, antennae, or appendages. The application of either deconvolution or adaptive optics correction techniques either separately or combined will restore the images such that applications such as modal analysis may be implemented to measure such effects as the vibrational modes of solar panels induced by such phenomena as tracking articulation or thermal shock from earth shadowing. For non-resolved cases, the use of image correction may perform such necessary functions as sharpening the system point spread function to improve photon sampling statistics and offer improved image contrast for separating closely spaced objects.

[0146] FIG. 9 is a schematic block diagram showing a remote sensing apparatus 900 with a polarization fiber switch 920, in accordance with a fourth exemplary embodiment of the present invention. The apparatus 900 includes collection optics 110 directing photons from an object 1. The photons are directed into a fiber optic cable 910, which is in optical communication with a polarization fiber switch 920 (fiber switch 920). The fiber switch 920 is switchable between a first and second polarization state. In the first polarization state, photons having the first polarization state 922a are directed through the polarization fiber switch. In the second polarization state, photons having the second polarization state 922b are directed through the polarization fiber switch. An optical detector 130 is in optical communication with the fiber switch 920. At least one processor 140 is in communication with the optical detector 130. The at least one processor 140: receives a signal from the optical detector, wherein a first portion of the signal corresponds to the photons in the first polarization state 922a, and a second portion of the signal corresponds to the photons in the second polarization state 922b; calculates, for a segment of the signal, a difference of the first and second portions of the signals, a sum of the first and second portions of the signals, and a ratio of the difference to the sum; and generates, for a plurality of segments, a signal corresponding to a sequential output of the ratio for each segment, wherein the signal is indicative of an acoustic signature of an object being sensed.

[0147] The fiber optic cable 910 may be a single cable or a plurality of cables as required by the implementation of the apparatus 900, the spectrum being imaged, and any other relevant factors. The fiber switch 920 may be a switch that can switch the incoming signal quickly between two orthogonal polarization states, for instance, two orthogonal linear polarization states. This may allow the photons directed to the fiber switch 920 to be separated into the two orthogonal polarization states when the fiber switch 920 switches states. When the fiber switch 920 is in the first polarization state, only photons in the first polarization state 922a are allowed to propagate through. When the fiber switch 920 is in the second polarization state, only photons in the second polarization state 922b are allowed to propagate through.

[0148] The photons in the first and second polarization states 922a, 922b may arrive at the optical detector 130 at different times. For instance, if the fiber switch 920 switches rapidly back and forth between the first and second polarization states, then photons in the first polarization state 922a may arrive at the optical detector 130 first, followed by photons in the second polarization state 922b, repeating over and over. The rate at which the fiber switch 920 oscillates between the first and second polarization states may be at least twice than the desired sampling rate of the apparatus to allow both polarization states to be sampled with enough frequency. The optical detector 130 may detect the photons as they reach the optical detector 130. The processor 140 may distinguish the polarization state of the signal from the optical detector 130 based on when the photons reached the optical detector 130. This may allow the apparatus 900 to detect the DoLP of the object using only a single optical detector 130, which may allow the apparatus 900 to be more compact in design.

[0149] The apparatus 900 may include any combination of the hardware components described relative to FIGS. 1A-8, above, including the collection optics 110, sound transducer 160, an illumination source, adaptive optics corrections, computer processor components, and the like.

[0150] The present disclosure may also describe methods and systems to measure the full Stokes vector as the I, Q, U, and V components in addition to the components which describe the polarization ellipse as the angle of linear polarization (AoLP), the degree of polarization (DoP), the degree of circular polarization (DoCP), and additional methods for measuring hyperspectral information. Each of the separate polarization components may replace the DoLP measurement to provide the acoustic detection or may be used in conjunction to provide additional information or improve the total signal. The hyperspectral data may be used to enhance the detection of sources or provide an additional color differencing technique. The inclusion of both spectral data and the relation of the full polarization description may allow materials and absorbing media to be estimated in composition with a priori information of illumination and observation angle. In addition the surface normals can be determined from the AoLP.

[0151] The full stokes cast as I, Q, U, V, and additional descriptive measurements can be found as follows: the summed total intensity between two orthogonal polarization states I_II and I_⊥ for a given plane of reference is found as the total intensity I. In practice these intensities can be measured on two separate detectors or a single pixelated detector where the light has been split orthogonally by a polarizing beam splitter element or other similar optical process. In the Stokes cast, Q and U both describe the quantity of linear polarization in separate planes and V describes the quantity of circular polarization. The intensity I for two orthogonally measured intensities I_II and I_⊥ is found as;
I=I_II+I_⊥

[0152] The Stokes component Q describes the linear polarization for a single plane where the linearly orthogonal polarizing axes are commonly 0° and 90° respectively. The component Q is found in practice as;
Q=I_0°−I_90°

[0153] In the relationship, the I_II and I_⊥ components are found as the intensities on a detector where each component is filtered by a linearly polarizing element whose axes are mutually orthogonal. The Q component in relation to the intensity I for the axis gives the degree of linear polarization q or also called the normalized Q parameter for the measured axis relative to the source and collecting optics. This is the basis for what has previously constituted the polarized hyper temporal imaging measurement. This measurement is written as the ratio of the difference over the sum of the two measured intensities. The relation can also be equated to the total linear degree of polarization LP by the angle of rotation θ which represents the orientation of the polarizer.

[0154] $q=\frac{Q}{I}=\frac{I_{0°}-I_{90°}}{I_{0°}+I_{90°}}=\mathrm{LP}\cos 2\theta$

[0155] The DoLP quantity q measurement should yield a result of 1 for polarizations at 90 degrees, 0 for 45 degrees, −1 for 0 degrees, and 0 for 135 degrees. Circularly polarized light also yields a value of 0 and elliptically polarized light yields an intermediate value. The nuance however is that the measurement q only describes the linear polarization in one plane. This means in practice a change in the measured signal where the modulation angle is found at an unfavorable angle relative to the optical axis of the collecting telescope will be unseen. This leads to the Stokes component U which is found in the same manner with a rotated reference plane. In practice, the U parameter is found by rotating the optical axis of the linear polarizer 45 degrees to that of the Q component. This means generally for an optical instrument to capture Q and U, four separate intensity measurements are required and for the purpose of this concept they must be measured simultaneously. The stokes component U is then found similarly as;
U=I_45°−I_135°

[0156] In the same fashion, the degree of linear polarization or normalized Stokes parameter u for the rotated plane about 45° can be found as;

[0157] $u=\frac{U}{I}=\frac{I_{45°}-I_{135°}}{I_{45°}+I_{135°}}=\mathrm{LP}\sin 2\theta$

[0158] In a measurement system either u or q as the DoLP for separate planes will yield valuable information for the acoustics of an object being measured if signal-to-noise ratio (SNR) issues are of concern. The issue as noted is that an ambiguity is found for either solely measuring u or q.

[0159] The angle of linear polarization describing the ellipse or the direction of the polarization θ is found as;

[0160] $\tan \left(2\theta \right)=\frac{Q}{U}=\frac{q}{u}$

[0161] The value is important because the AoLP measurement may be used to estimate the surface normals from a single shot image.

[0162] The combination of both planes to find the total linear degree of polarization as LP is written as;

[0163] $LP=\frac{\sqrt{Q^2+U^2}}{I}=\sqrt{q^2+u^2}$

[0164] The value represents the total degree or fraction of power which is linearly polarized. The value excludes circularly and unpolarized components, and can takes the value of 0-1 for unpolarized to perfectly polarized light respectively. The LP value is independent of the plane of polarization or the orientation of the major axis describing the ellipse of polarization.

[0165] The stokes component V describing the quantity of circular polarization is found as;

[0166] $V=I_{0°,\lambda /4}-I_{90°,\lambda /4}=2I_{90°,\lambda /4}-I$

[0167] In practice the beam path may include a quarter waveplate retarder in front of the separate orthogonal linear polarizers. By using the two separate orthogonalities, the right hand and left hand polarizations are measured separately and their subtraction yields the Stokes parameter V of circular polarization. The second relation may be used to reduce the number of necessary additional measurements to 1 instead of 2.

[0168] The degree of circular polarization is then found as;

[0169] $\mathrm{CP}=\frac{V}{I}=\frac{I_{0°,\lambda /4}-I_{90°,\lambda /4}}{I_{0°,\lambda /4}+I_{90°,\lambda /4}}$

[0170] The total degree of polarization describing the complete geometry of polarization is then found as P.

[0171] $P=\frac{\sqrt{Q^2+U^2+V^2}}{I}=\sqrt{q^2+u^2+{\mathrm{CP}}^2}$

[0172] This measurement follows LP where the values may take on any value between 0-1 describing unpolarized to totally polarized light.

[0173] In the absence of light losses through beam splitting elements, the intensity component may be found as;
I=I_0°+I_90°

[0174] The Q component may be written the same as;
Q=I_0°−I_90°

[0175] The U component may be found as;
U=2I_45°−I

[0176] And the V component may be found as;
V=2I_45°,λ/4−I

[0177] The remainder of the polarization components may be found accordingly as described below. In practice this method may be difficult for a snapshot polarimetric measurement as the I component acts as the normalization parameter which changes through intensity losses by splitting the beam to separate detectors. As a result, a different I parameter may be used to normalize each of the separate components for the snapshot methods.

[0178] In addition to the separate polarization measurements, hyperspectral data may be simultaneously measured. In practice the number of hyperspectral measurements may be severely limited by the type of polarimetry required because of SNR issues. An ideal system would measure the full Stokes cast, including the descriptive polarization quantities. The system would in addition have either a single or partitioned signal or separate hyperspectral signals for each respective polarization component. The hyperspectral signal may take separate parts of the optical spectrum and difference the intensity values either as a standalone measurement or in regards to a polarization element and this too may yield the vibration of a sensing source. In addition, the inclusion of spectral data to that of polarization allows for materials identification of the object being imaged. With a priori knowledge of the pointing angle, the illumination angle, and the out of plane displacement of the modulation, a sensed source's composition may be identified from the spectral and polarimetric measurements when compared to a database of models.

[0179] FIG. 10 is a diagrammatic illustration of a remote sensing apparatus 1000 for capturing the Stokes vector and additional polarization descriptors, in accordance with a fifth exemplary embodiment of the present invention. An object 1001 may be measured by collecting optics 1002. The collecting optics 1002 may direct the photons from the object 1001 down a plurality of pathways 1009-1014. The plurality of pathways 1009-1014 may be created by beam splitters 1003-1007 and a right angle prism 1008. Photons in each of the plurality of pathways 1009-1014 may be separated into different polarization states by polarizing elements 1015-1022. The photons in the plurality of pathways 1009-1014 may be detected by at least one optical detector 1023-1028. The at least one optical detector 1023-1028 may be in communication with at least one processor 1041, which may receive a plurality of signals corresponding to the photons, determine at least one polarization property of the photons, and generate a signal corresponding to at least one of the polarization properties.

[0180] Exemplary collecting optics 102 are discussed relative to FIGS. 1-9, above. Photons from the object 1001 may be directed down a plurality of pathways 1009-1014 by beam splitters 1003-1007 and a right angle prism 1008. The beam splitters 1003-1007 may include any suitable beam splitters, such as 50/50 right angle beam splitters that divide the incident photons into two photon pathways separated by a 90° angle. The number and position of the beam splitters 1003-1007 and the right angle prism 1008 may correspond to the number of polarization states to be measured. For example, where six polarization states are to be measured, five beam splitters 1003-1007 and a right angle prism 1008 may be used. Where fewer polarization states are to be measured, fewer beam splitters may be used. Each beam splitter 1003-1007 may separate the incident photon beam into a beam to be polarized, travelling along pathways 1009-1014 and a beam to be incident upon an adjacent beam splitter or right angle prism 1008. It should be noted that these elements may take the form of any other suitable optical division mechanism. The measurement may also be increased or decreased depending on the quantity of detecting devices and pathways to fully describe the Stokes cast.

[0181] The collected light may be directed into pathways 100901014 where the light is incident upon polarizing elements 101511018. In one example, each of the polarizing elements 1015-1018 may output light in a different polarization state from the other polarizing elements. For instance, polarizing elements 1015-1018 may include linear polarizers with different orientations. The polarizing element 1015 may have a polarization axis set to 0 degrees, polarizing element 1016 may be set to 45 degrees, polarizing element 1017 may be set to 90 degrees, and polarizing element 1018 may be oriented to 135 degrees. Quarter wave plate retarders 1019 and 1020 may be located in front of linear polarizing elements 1021 and 1022, which may themselves be oriented to 0 and 90 degrees as an example for mutual orthogonality producing left and right handed circularly polarized light. In this way, the original incoming photons may be separated into orthogonally separated linear, orthogonally separated diagonal, and left and right handed circularly polarized states.

[0182] At least one optical detector 1023-1028 may detect the photons in each of the plurality of pathways 1009-1014. In one example, a plurality of detectors 1023-1028 may be used. For instance, each pathway 1009-1014 may correspond with a separate detector 1023-1028. In another example, the optical detector may be a focal plane array having elements 1023-1028. The at least one optical detector 1023-1028 is in communication with a processor 1041. The processor 1041 may include any common computing components, such as a power source, computer-readable memory, user interface components, networking and communications components, and the like.

[0183] The at least one processor 1041 may receive a plurality of signals corresponding to the photons. The at least one processor 1041 may determine at least one polarization property of the photons, and may generate a signal corresponding to at least one polarization property of the object. The at least one polarization property may be determined based on the Stokes quantities, which may be determined as follows: The parameter Q may be calculated as the signal from detector 1023 subtracted from the signal of detector 1025. The normalized quantity q or the DoLP for a single plane may be calculated as the ratio of the signal of detector 1023 subtracted from the signal of detector 1025, to the signal of detector 1023 added to the signal of detector 1025. Similarly, the parameter U may be calculated as the signal of detector 1024 subtracted from the signal of detector 1026. The normalized quantity u or the separate degree of linear polarization for the separate plane may be calculated as the ratio of the signal of detector 1024 subtracted from the signal of detector 1026 divided by the signal of detector 1024 added to the signal of detector 1026. The Stokes component V may be found as the signal of detector 1027 subtracted from the signal of detector 1028. The Stokes component V may also be found as the signal of detector 1028 multiplied by 2 subtracted from the total intensity as the sum of detectors 1027 and 1028. Similarly, the intensity for the Stokes component I can be found as the sum of the signal of detectors 1023 and 1025 or as the sum of the signal of detectors 1024 and 1026. The degree of circular polarization may be found as the ratio of signal of detector 1027 subtracted from the signal of detector 1028 divided by the sum of the signal of detectors 1027 and 1028. The linear degree of polarization may be found as the root squared sum of the Stokes components U and Q over the intensity I or alternatively the root square sum of the q and u normalized Stokes parameters. The total degree of polarization may be found as the root square sum of the Stokes components Q, U, and V over the intensity I or alternatively as the root square sum of the normalized stokes components q, u, and the degree of circular polarization. The angle of linear polarization is found as one-half multiplied by the arctangent of the ratio of the Stokes components U and Q or alternatively the normalized Stokes components u and q. Each of the detections may be used solely or in conjunction with the DoLP as described above.

[0184] FIG. 11 is a flow chart 1100 illustrating a method of remotely sensing an object, in accordance with the fifth exemplary embodiment of the present invention.

[0185] Step 1110 includes collecting photons from the object.

[0186] Step 1120 includes directing the photons down a plurality of pathways, wherein photons in each of the plurality of pathways have a different polarization state. As described above relative to FIG. 10, the photons may be directed by a plurality of beam splitters and a right angle prism, which may be oriented in a cascading fashion to split and direct the photons along the plurality of pathways. The photons in each pathway may be separated into different polarization states by a plurality of polarizing elements, including linear polarizers positioned at different angular orientations, and quarter wave plate retarders. In one example, the photons may be separated into a plurality of polarization states selected from the set of: 0 degree linear, 45 degree linear, 90 degree linear, 135 degree linear, left-hand circular, and right-hand circular polarization states.

[0187] In another example, the photons may be directed by a plurality of beam splitters and at least two right angle prisms. Each pathway may be incident through a partitioned polarizer. In another example, the partitioned polarizer may be one from the set of: a wire grid polarizer, at least one polarizing beam splitter, at least one Rochon prism, and at least one Wollaston prism. This is shown in greater detail in FIG. 12, below.

[0188] In another example, the photons may be separated into a plurality of polarization states by inserting, one after another, a plurality of polarizing elements into a propagation path of the photons. The photons may be separated into 0 degree linear, 45 degree linear, 90 degree linear, and circular polarization states by the polarizing elements. This is shown in greater detail in FIG. 13, below.

[0189] Step 1130 includes detecting photons in each of the plurality of pathways using at least one optical detector, wherein the photons in each of the polarization states produce a signal. In one example, photons in each pathway may be detected by a separate optical detector. In another example, the photons in each of the polarization states may be detected by a focal plane array or other singular detector.

[0190] Step 1140 includes receiving, with at least one processor in communication with the at least one optical detector, a plurality of signals corresponding to the photons in the plurality of pathways.

[0191] Step 1150 includes determining, with the at least one processor and for a segment of the plurality of signals, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the plurality of signals. The segment of the plurality of signals may be determined by a time length of measurement of the signal, which is discussed above. The angle of linear polarization, degree of polarization, and degree of circular polarization may be determined for the segment as discussed above relative to FIG. 10.

[0192] Step 1160 includes generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment. The signal corresponding to the sequential output of the plurality of segments may indicate the instantaneous polarization information over time.

[0193] The signal of step 1160 may be used to generate a polarization signature of an object, detect the presence of at least one object in an imaged scene, determine the direction of normal vectors to an object's surfaces, determine the shape of an object, determine the shape, amplitude, and frequency of resonant modes of an object, or identify materials of which an object is made, and other physical properties of an object.

[0194] For example, the polarization information may indicate the acoustic signature of the object as discussed above relative to FIGS. 1-10. The acoustic signature may be compared against a database of acoustic signatures in order to determine the nature, shape, or composition of the object.

[0195] In another example, the polarization information may be used to detect the presence of at least one object in an imaged scene. There are several scenarios for the detection of multiple objects within a scene, whether for resolved or unresolved imaging, and active or inactive illumination. The cases are briefly described as follows:

[0196] Non-Resolved, Non-Active Illumination:

[0197] In the instance of unresolved imaging such as the example of monitoring a distant satellite through an optical telescope, the object may have a measured historical recorded resonance spectrum. In one scenario, the object may be continually measured and the spectrum of the recorded polarization signal may be correlated with historical data to determine whether nominal operation is being maintained. In another scenario, if new spectral features are detected, they may indicate the measurement of new activity from the object or they may instead indicate the presence of at least one separate object within the same point spread function core. The spectrum measured may be, for the case of a non-resolved source, a superposition of the temporal events from all objects.

[0198] Non-Resolved, Active Illumination:

[0199] In the instance of measuring a non-resolved source, the source intensity may be limited, making the polarization measurement difficult to perform at significant speed. In this instance, it may be helpful to actively illuminate the object to improve sampling statistics. A laser source may be linearly polarized and may be made to match the optimal angle of the input system's linear polarizer to maximize the signal to noise ratio of the measurement, which improves the minimum change observable for the measurements.

[0200] Resolved, Non-Active Illumination:

[0201] In the instance of resolved imaging, each pixel in an optical detector's image cube may be transformed into a power spectrum where the detection of object activity may be made. In the case of a resolved scene with ambient illumination, separate targets may be identified within the same scene from measuring separate single or summed sets of pixels to generate identifying spectra from the polarization signals. The pixels in the cube with matching spectra may be averaged for an improved total signal to noise ratio of the detection.

[0202] Resolved, Active Illumination:

[0203] In the instance where a scene is actively illuminated, a linearly polarized laser may be made to match an input polarizer's reference axis so as to improve the contrast ratio of the targets of interest in cluttered scenes. Alternatively, the polarization axis may be modulated to search for the peak intensity for targets of interest and reject background clutter in scenes. By changing the output or input polarization, the reflected degree of linear polarization may be increased relative to the ambient illumination.

[0204] In one example, the polarization properties may be used to determine the direction of normal vectors to an object's surfaces. Incident light from a reflected object undergoes a linear polarization which is dependent on the index of refraction of the material of the object and the angle of incidence. The degree of linear polarization may be measured, as described above. The angles of incidence and reflection may be previously known or may be estimated from prior knowledge of the index of refraction of the material. Analytically, the relationship for the value measured as the degree of linear polarization LP in relation to the angles of incidence and reflection θ, to the index of refraction of the reflecting material is written as the equation below.

[0205] $\mathrm{LP}=\frac{\sqrt{{\sin }^4{\mathrm{\theta cos}}^2\theta \left(n^2-{\sin }^2\theta \right)}}{\left[{\sin }^4\theta +{\cos }^2\theta \left(n^2-{\sin }^2\theta \right)\right]/2}$

[0206] In one dimension, the surface normal may be determined, or a global optimization may be used from a collection of measurement data. Multiple measurements may be made. In one example, the multiple measurements, made with a stable light source, may be used to derive a surface normal estimate at a multiplicity of points on the surface of the object by taking advantage of known angles of incidence and known changes in the geometry of the measurements. This may give the surface normal for each pixel of the measurement and in the case of a surface each pixel in an image measuring at least one property of the polarization. For a camera image that is moving, the pixels and corresponding object features may be tracked from frame to frame.

[0207] In another example, the polarization properties may be used to determine the two-dimensional or three-dimensional shape of an object. Each measurement may individually describe an X-Y grid point, but not the surface height of the object. The surface may be integrated in order to move from the gradient to a surface profile. The rows and columns of the surface normal map may be integrated. In one example, the surface may be assumed to be a smooth surface.

[0208] In another example, the polarization properties may be used to determine the shape, amplitude, and frequency of resonant modes of an object. A surface vibrating in a resonant mode may have all parts of the surface vibrating at a given frequency; however, the amplitude and phase for frequency features may be different for every point on a surface. In a remote sensing system, the surface may be measured with video and, for all of the pixels that correspond to an object, their frequency spectra may be identified. The frequency spectra for all of the pixels may also denote the phase and amplitude for each of the frequency peaks. The identification of frequencies may be used for linear systems approaches to vibration such as eigenmode analysis. In eigenmode analysis, certain shapes and mechanical bodies have special modes such as a struck cantilevered arm or a drum head. The eigenmode analysis may assume a shape and the identification of the location of a frequency peak in frequency-space can be used to do such tasks as estimate material stiffness and mass if certain physical assumptions are made. In a practical sense, a user of a remote sensing system may only look for the identification of modes. In this instance, a user display may show video feedback and false color pixels that have been identified to have spectral features and may color code pixels with intensity based upon the pixels' amplitude and phase for those with specific frequencies.

[0209] In another example, the polarization properties may be used to identify materials of which an object is made, and other physical properties of an object. In an absorption process, a broad spectrum of electromagnetic radiation such as that emitted by a blackbody is incident upon a medium such as a gas and selected wavelengths unique to each molecule, atom, and phase transition modulate the light's intensity. In the case of reflection, the light may be incident upon a surface and there may be a wavelength dependent return because of spectrally dependent reflectances. In addition, there may be absorption and scattering processes which also shape the observed reflectance spectrum. Each of these processes causes materials, gases, liquids, and plasmas to have specific wavelength signatures. In practice, this basic phenomenon may be captured by a hyperspectral imaging instrument or a spectrometer and the measured spectral wavelength profile may be compared to those measured in a laboratory to perform identification. The comparison may be done by standard practices such as by human comparison, cross-correlation, or any set of machine learning approaches. The measurement may be made alone or in combination with a polarization measurement.

[0210] In addition, a reflecting material may be identified through measuring at least one property of polarization. In this technique if the reflecting phase angle, or the angle that a source is incident on a surface and reflects to an observer is known and the intensity is measured, the at least one property of polarization may be used to reference to a set of known material responses to estimate the amount present of each material in the set. The same method may be used to measure a range of reflectance phase angles against measured values of the at least one property of polarization to generate a vector of measurements for comparison to known reference profiles.

[0211] FIG. 12 is a diagrammatic illustration of a remote sensing apparatus (“apparatus”) 1200 for capturing two linear polarization planes, in accordance with the fifth exemplary embodiment of the present invention. The apparatus 1200 may be an exemplary implementation of the method described in FIG. 11, above. In accordance with the method, photons from an object 1001 may be collected by collecting optics 1002 and directed down a plurality of pathways 1208-1211. The photons in each pathway 1208-1211 may have a different polarization state. In the example shown in FIG. 12, photons may be separated into a plurality of pathways by a plurality of beam splitters 1203, 1204, 1207 and at least two right angle prisms 1205, 1206. The beam splitters and right angle prisms 1203-1207 may be positioned to divide incident photons into four separate photon paths, which may correspond with four linear polarization states. This beam direction train may, alternatively, take the form of other beam directing elements such as optical windows, prisms, beams splitters and the like, as may be known.

[0212] The photons may be incident upon a segmented wire grid polarizer 1212 with partitioned sections. The wire grid polarizer 1212 may also take the form of polarizing beam splitters, Rochon prisms, Wollaston prisms, and the like. The partitioned sections of the wire grid polarizer 1212 may have linear polarization axis orientations of 0 degrees, 45 degrees, 90 degrees, and 135 degrees. Photons passing through the respective portions of the wire grid polarizer 1212 may be filtered into photons having 0 degree, 45 degree, 90 degree, and 135 degree linear polarization states. Thus, photons in the plurality of pathways 1208, 1209, 1210, and 1211 may be polarized according to those states, respectively.

[0213] Photons within the pathways 1208-1211 may be detected by at least one optical detector 1216 which may be a photodiode, a focal plane array or the like. The optical detector 1216 may be in communication with at least one processor 1041, as described relative to FIG. 10, above. The optical detector 1216 may detect photons in each of the plurality of pathways 1208-1211 to produce a signal. The processor 1041 may receive a plurality of signals corresponding to the photons in the plurality of pathways 1208-1211, and may determine, for a segment of the plurality of signals, at least one polarization property of the photons as described relative to FIGS. 10-11.

[0214] The Stokes parameter U may be found as the subtraction of the signals measured from pathways 1208 and 1210. The intensity component I with respect to the U component is found as the addition of the signals from pathways 1208 and 1210. The degree of linear polarization or the normalized U component as u is found as the ratio of U divided by I. The Stokes parameter Q may be found as the subtraction of the signals measured from pathways 1209 and 1211. The intensity component I with respect to the Q component is found as the addition of the signals generated from pathways 1209 and 1211. The degree of linear polarization or the normalized Q component as q is found as the ratio of Q divided by I. The angle of linear polarization is then measured as one-half multiplied by the arctangent of the ratio of the Stokes parameter U, divided by the Stokes parameter Q or alternatively the u and q components. The linear degree of polarization is found as the ratio of the root squared sum of the Q and U components to the total intensity I, or alternatively as the root squared sum of the u and q components.

[0215] It should be noted, with respect to FIGS. 10-18, that the Stokes parameters and other polarization properties may be determined in relation to the signal or signals generated by the at least one optical detector corresponding to the plurality of pathways. Thus, if a single optical detector 1216 is used, the signal generated by photons from a pathway, e.g. 1208 on that portion of the detector 1216 may be used to determine the Stokes parameters. If a separate detector is used for each pathway, the signal upon each detector, which is caused by photons in its corresponding pathway, may be used to determine the Stokes parameters. If a fewer number of detectors than pathways are used, the portion of the signals corresponding to photons in each pathway may be used to determine the Stokes parameters. Thus, it can be understood that the Stokes parameters may be determined with relation to either the photons in the plurality of pathways or the signals corresponding to those photons.

[0216] FIG. 13 is a diagrammatic illustration of a remote sensing apparatus (“apparatus”) 1300 for capturing polarization measurements with a reduced number of measurements, in accordance with the fifth exemplary embodiment of the present invention. In one example, the apparatus 1300 may be an exemplary implementation of the method described in FIG. 11, above. In another example, the apparatus 1300 may be an exemplary implementation of the method described in FIG. 19, below. In accordance with the method of FIG. 11, photons from an object 1001 may be collected by collecting optics 1002. The photons may be directed down a plurality of pathways 1303-1306, wherein photons in each of the plurality of pathways 1303-1306 have a different polarization state. As shown in FIG. 13, a plurality of polarizing elements 1313-1317 may be inserted into a propagation path 1302 of the photons to cause only particular and desired polarization states to propagate through to at least one optical detector 1318. In one example, the polarizing elements may be mechanically inserted, for example, with the use of a motor. In another example, the polarizing elements may be optically inserted, for example, by using a mirror or prism 1320 to alter the propagation path 1302 to align with the optical axis of each of the polarizing elements. It should be understood that, within the scope of this disclosure, the insertion of polarizing elements 1313-1317 may include insertion of the elements themselves as well as alteration of the propagation path 1302.

[0217] The propagation path 1302 may be first incident upon a polarizing element 1313 with a pathway 1303 onto optical detector 1318. Photons in pathway 1303 may have a first polarization state. Polarizing element 1314 may next be inserted into the propagation path 1302. Polarizing element 1314 may direct the photons down pathway 1304 in a second polarization state different from the first polarization state. Polarizing element 1315 to pathway 1305 may next be inserted into the propagation path 1302. Photons in pathway 1305 may have a third polarization state different from the first and second polarization states. In one example, polarizing elements 1313-1315 may be linear polarizers oriented to produce linearly polarized light at 0 degrees, 45 degrees, and 90 degrees, respectively. Polarizing elements 1316 and 1317 to pathway 1306 may next be inserted into the propagation pathway 1302. In one example, polarizing elements 1316, 1317 may be a quarter wave plate retarder and linear polarizer oriented to produce linearly polarized light at 45 degrees. The combination of polarizing elements 1316, 1317 may be oriented to produce circularly polarized light.

[0218] The optical detector 1318 may be in communication with a processor 1041. The optical detector 1318 may detect photons in each of the plurality of pathways 1303-1306. Photons in each of the polarization states may produce a signal. The at least one processor 1041 may receive a plurality of signals corresponding to the photons in the plurality of pathways 1303-1306. The at least one processor 1041 may determine, for a segment of the plurality of signals, at least one of the polarization properties discussed below, and may generate, for a plurality of segments, a signal corresponding to a sequential output of the polarization properties.

[0219] The Stokes cast may be measured where the component I is found as the sum of signals generated from the detection of optical pathways 1303 and 1305. The Stokes parameter Q may be found as the subtraction of the intensities measured by a detector from pathways 1303 from pathway 1305. The Stokes parameter U may be found as two times the intensity measured from optical pathway 1304 subtracted from the Stokes parameter I. The Stokes parameter V may be found as the detection of optical pathway 1306 subtracted from the Stokes component I. The degree of linear polarization u in regard to the U plane or the normalized Stokes U component may then be found as the ratio of U to I. The degree of linear polarization q in regard to the Q plane or the normalized Stokes Q component may then be found as the ratio of Q to I. The angle of linear polarization may then be found as one-half times the arctangent of the ratio of the Stokes components U and Q. Alternatively the angle of linear polarization may be found as one-half times the arctangent of the ratio of the normalized stokes components u and q. The linear degree of polarization may be found as the ratio of the root squared sum of the Q and U Stokes components to the Stokes intensity component I. Alternatively the linear degree of polarization may be found as the root squared sum of the normalized Stokes components q and u. The degree of circular polarization or the normalized stokes component regarding circular polarization may be found as the ratio of the Stokes component V to the Stokes intensity component I. The degree of polarization may then be found as the ratio of the root squared sum of the Stokes components Q, U, and V to the Stokes component I. Alternatively the degree of polarization may be found as the root squared sum of the normalized Stokes components q, u, and the normalized Stokes component regarding V as the degree of circular polarization CP.

[0220] FIG. 14 is a diagrammatic illustration of a remote sensing apparatus (“apparatus”) 1400 for capturing a hyperspectral polarimetric measurement, in accordance with a sixth exemplary embodiment of the present invention. The apparatus 1400 may allow an object 1001 to be remotely sensed. The apparatus 1400 may include collecting optics 1002, which may collect photons from the object 1001. The collecting optics 1002 may direct the photons into a polarimeter 1403. At least one polarization state of the photons may be measured by the polarimeter 1403. An optical spectrum separator, which is shown as a prism 1405, may separate an optical wavelength spectrum 1406 of the photons. At least one optical detector 1407 may detect the optical wavelength spectrum 1406, producing a signal. At least one processor 1041 in communication with the at least one optical detector 1407 may receive the signal. The at least one processor 1041 may determine at least one polarization property for a segment of the signal, and may generate a signal corresponding to a sequential output of the at least one polarization property corresponding to at least one polarization property of the object 1001.

[0221] The collecting optics 1002 may direct the light from the object 1001 to a polarimeter 1403. The polarimeter 1403 may measure at least one polarization state of the photons from the object 1001. The at least one polarization state may include any of the Stokes vectors, namely I, Q, U, V, and any of the polarization properties which may be determined from the Stokes vectors, namely q, u, the AoLP, the LP, or the CP. The output of the polarimeter 1403 may be directed along optical pathway 1404 to an optical spectrum separator. In one example, the optical spectrum separator may be a prism 1405. In another example, any other additional optical element such as a grism, a series of prisms, a grating, a series of colored filters each isolating a distinct narrow band of light, and the like may be used to produce an optical wavelength spectrum 1406. The optical wavelength spectrum 1406 may be separated across any suitable range of wavelengths, depending on the wavelengths being collected by the collecting optics 1002. For example, if visible light is being imaged, the optical wavelength spectrum 1406 may be separated into a plurality of wavelengths across the visible spectrum, or about 400 nm-700 nm. If multiple spectrums are being measured, for example, visible and infrared, the optical wavelength spectrum 1406 may be separated into a plurality of wavelengths selected from the visible and infrared spectrums. In one example, at least a portion of the wavelengths collected by the collecting optics 1002 may be separated by the optical spectrum separator. For instance, if light from the visible spectrum is collected, at least a portion of the wavelengths between 400 nm and 700 nm may be separated by the optical spectrum separator. Depending on the type of optical spectrum separator used, some wavelengths may be filtered or otherwise prevented from reaching the optical detector 1407.

[0222] The optical wavelength spectrum 1406 may be incident upon at least one optical detector 1407 which may take the form of a focal plane array, photo diode, or other optical sensing element. The optical detector 1407 may detect the optical wavelength spectrum 1406 incident upon the surface of the optical detector 1407. In one example, the optical detector 1407 may detect a plurality of bands of wavelengths within the optical wavelength spectrum 1406. For instance, if the optical wavelength spectrum 1406 covers the visible range, the optical detector 1407 may detect wavelength bands having a range of wavelengths throughout the range. For example, a plurality of bands having a width of 10 nm may be detected across the range. The spectral resolution, or number of bands, and the bandwidth may be any suitable ranges, depending on the object 1001 being imaged and the desired application. For instance, the spectral resolution and bandwidth may differ if the application is establishing the presence or absence of an object, determining the orientation of the object, or identifying the materials on an object. In one example, the width of the plurality of bands may not be the same for each band. For instance, one band may have a width of 10 nm, while another band has a width of 50 nm.

[0223] The optical detector 1407 may be in communication with at least one processor 1041. The at least one processor 1041 may receive a signal corresponding to the optical wavelength spectrum 1406 from the optical detector 1407. The at least one processor 1041 may include any of the additional components discussed relative to FIGS. 1-13, above. In one example, the at least one processor 1041 may determine, for a segment of the signal, at least one of an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal. The at least one processor 1041 may generate, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment. The signal may correspond to a polarization property of the object 1001.

[0224] In one example, the apparatus 1400 may perform a scanning hyperspectral detection method. The optical spectrum separator 1405 may be an active optical component, which may take the form of a rotating grating, prism, or filter. As the optical spectrum separator rotates, the scanning may be performed by point scanning the object 1001, line scanning the object 1001, or spectrally scanning. A plurality of points or a plurality of lines corresponding to the object 1001 may be scanned in a sequential operation to detect and image at least a portion of the object 1001. The optical pathway 1404 may be a partitioned fraction of intensity from the received signal from the collecting optics 1002, or it may constitute separate polarized signals from the polarizer 1403.

[0225] FIG. 15 is a flow chart 1500 illustrating a method of remotely sensing an object, in accordance with the sixth exemplary embodiment of the present invention. The method may be understood with particular reference to the apparatus 1400 of FIG. 14, above, and with reference to any other figure herein.

[0226] Step 1510 includes collecting photons from the object.

[0227] Step 1520 includes directing the photons into a polarimeter, whereby at least one polarization state of the photons is measured. In one example, the at least one polarization state may include the Stokes cast, particularly the values for the vectors I, Q, U, V. In another example, the at least one polarization state may include the calculated values q and u, and the polarization properties for the AoLP, LP, and CP.

[0228] Step 1530 includes separating an optical wavelength spectrum of the photons. In one example, the optical wavelength spectrum may be separated by a prism, a grating, or a grism. In another example, the optical wavelength spectrum may be separated by a plurality of prisms, gratings, or grisms. In yet another example, the at least one prism, grating, or grism may rotate to perform scanning separation of the optical wavelength spectrum.

[0229] Step 1540 includes detecting the optical wavelength spectrum of the photons using at least one optical detector, wherein the optical wavelength spectrum of the photons produces a signal. In one example, the photons are detected by scanning a plurality of points or a plurality of lines corresponding to the object. In another example, the photons are detected by scanning across a plurality of wavelengths within a range of imaged wavelengths.

[0230] Step 1550 includes receiving, with at least one processor in communication with the at least one optical detector, the signal corresponding to the optical wavelength spectrum.

[0231] Step 1560 includes determining, with the at least one processor and for a segment of the signal, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal. In one example, the determination may be performed for each of a plurality of bands of the optical wavelength spectrum.

[0232] Step 1570 includes generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment. The signal of step 1570 may be used to generate a polarization signature of an object, detect the presence of at least one object in an imaged scene, determine the direction of normal vectors to an object's surfaces, determine the shape of an object, determine the shape, amplitude, and frequency of resonant modes of an object, or identify materials of which an object is made, and other physical properties of an object, as discussed relative to FIG. 11, above.

[0233] FIG. 16 is a diagrammatic illustration of a remote sensing apparatus (“apparatus”) 1600 for generating a snapshot hyperspectral polarimetric measurement, in accordance with a seventh exemplary embodiment of the present invention. The apparatus 1600 may allow a remote object 1001 to be remotely sensed. The apparatus 1600 may collect photons from an object 1001 using collecting optics 1002. A polarimetric division element 1603 may divide the photons into a plurality of polarized states along a single pathway 1604. Exemplary polarimetric division elements 1603 may include polarizing beam splitters, Wollaston prisms, and polarizer microgrid arrays, among others. A focal plane division mechanism 1605 may divide the plurality of polarized states along the single pathway into a plurality of focal plan pathways 1606, 1608. A plurality of optical spectrum separators 1607, 1609 may separate the plurality of focal plane pathways 1606, 1608 into optical wavelength spectrums 1610, 1612. At least one optical detector 1620 may detect the optical wavelength spectra 1610, 1612 to produce a signal. A processor 1041 may be in communication with the at least one optical detector 1620. The processor 1041 may determine, for a segment of the signal, at least one of an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal. The processor 1041 may generate, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment, wherein the signal corresponds to a polarization property of the object.

[0234] Collecting optics 1002 may direct light from an object 1001 to a polarimetric division element 1603, which is directed to pathway 1604. The pathway 1604 may be a partitioned fraction of intensity from the received signal from the collecting optics 1002 or may constitute separate polarized signals from the polarimetric division element 1603. Light within the pathway 1604 may be separated into a plurality of polarized states. In one example, each polarized state may be different from the others. For instance, the polarized states may be selected from the set of: 0 degree linear, 45 degree linear, 90 degree linear, 135 degree linear, right-hand circular, and left-hand circular polarized states The pathway 1604 may be divided into separate focal plane pathways 1606, 1608 by focal plane division mechanism 1605. The focal plane division mechanism 1605 may divide the pathway 1604 into a plurality of focal plane pathways 1606, 1608. As shown in FIG. 16, this may be at least two focal plane pathways 1606, 1608. In one example, it may be more than two focal plane pathways. The focal plane pathways 1606, 1608 may be incident upon optical spectrum separators 1607, 1609, which produce the optical wavelength spectra 1610, 1612. The optical spectrum separators 1607, 1609 may take the form of prisms, grisms, gratings or the like. In one example, there may be a plurality of optical spectrum separators 1607, 1609. For instance, the apparatus 1600 may include at least one optical spectrum separator for each focal plane pathway. In another example, the apparatus 1600 may include a plurality of optical spectrum separators 1607, 1609 for each focal plane pathway 1606, 1608. The optical spectrum separators 1607, 1609 may separate the incident light into its constituent spectral components, as described in FIG. 14, above. The optical wavelength spectra 1610, 1612 may be detected by an optical detector 1620, which may take the form of photodiodes, a focal plane array, or the like. The optical detector 1620 may be in communication with at least one processor 1041. The at least one processor 1041 may determine at least one polarization property of the signal generated by the optical detector 1620, according to the methods described above.

[0235] FIG. 17 is a flow chart 1700 illustrating a method of remotely sensing an object, in accordance with the seventh exemplary embodiment of the present invention. The method may be understood with particular reference to the apparatus 1600 of FIG. 16, above, and with reference to any other figure herein.

[0236] Step 1710 includes collecting photons from the object.

[0237] Step 1720 includes dividing the photons into a plurality of polarized states along a single pathway.

[0238] Step 1730 includes dividing the plurality of polarized states along the single pathway into a plurality of focal plane pathways. In one example, this step may be performed by a focal plane division mechanism. In another example, the plurality of polarized states may be separated into at least two focal plane pathways.

[0239] Step 1740 includes separating an optical wavelength spectrum of each of the plurality of focal plane pathways. In one example, this step may be performed by any of a prism, a grating, and a grism. In one particular example, two prisms may be used. In another example, a plurality of prisms, gratings, and/or grisms may be used.

[0240] Step 1750 includes detecting the optical wavelength spectra using at least one optical detector, wherein the optical wavelength spectra produce a signal. In one example, the at least one optical detector may be at least one photodiode or at least one focal plane array.

[0241] Step 1760 includes receiving, with at least one processor in communication with the at least one optical detector, the signal corresponding to the optical wavelength spectra.

[0242] Step 1770 includes determining, with the at least one processor and for a segment of the signal, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the signal.

[0243] Step 1780 includes generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment. The signal of step 1780 may be used to generate a polarization signature of an object, detect the presence of at least one object in an imaged scene, determine the direction of normal vectors to an object's surfaces, determine the shape of an object, determine the shape, amplitude, and frequency of resonant modes of an object, or identify materials of which an object is made, and other physical properties of an object, as discussed relative to FIG. 11, above.

[0244] FIG. 18 is a diagrammatic illustration of a data cube 1800 representing hyperspectral polarimetric data, in accordance with the seventh exemplary embodiment of the present invention. An imaged source or object 1801 may be imaged on at least one optical detector 1802. The at least one optical detector 1802 may have a two-dimensional array of pixels, which may be noted as an M×N array matrix where each element constitutes a pixel representing a portion of an image. A first layer 1803 of the data cube 1800 may be accounted as element [M, N, 1]. A second layer 1804 of the data cube 1800 may be accounted as element [M, N, 2]. An nth layer 1805 may be accounted as [M, N, n]. Each layer 1803, 1804, 1805 may represent the polarization quantities as I, Q, U, V, q, u, LP, P, CP, AoLP. In one example, the data cube 1800 may stretch to n elements. In another example, the data cube 1800 may encompass any number of dimensions to account for the distribution of optical wavelengths for a point [M, N, n] in the image plane. Any one of the components may be used to determine the modulation of the acoustics of a sensed object or the color difference from separate hyper spectral channels as standalone measurements or with respect to a polarization component may also yield acoustic detection. Comparison of the signals in the data cube to those of theoretical or empirical calculation or measurement may yield the composition of a sensed material. Additionally, with a priori information, the surface normals of a sensed source may be estimated from the AoLP which may constitute a separate layer or dimensionality in the data cube.

[0245] FIG. 19 is a flow chart 1900 illustrating a method of remotely sensing an object, in accordance with an eighth exemplary embodiment of the present invention. In one example, the method may be performed using the apparatus 1300 of FIG. 13, above, and the flow chart 1900 may be understood with particular respect to FIG. 13, as well as with respect to any other figure herein.

[0246] Step 1910 includes collecting photons from the object.

[0247] Step 1920 includes directing the photons down a single pathway.

[0248] Step 1930 includes inserting into the pathway, in sequence, a plurality of polarizing elements, whereby photons propagating through each of the plurality of polarizing elements has a different polarization state. In one example, the plurality of polarization states may be selected from the set of: 0 degree linear, 45 degree linear, 90 degree linear, 135 degree linear, left-hand circular, and right-hand circular polarization states. In another example, the plurality of polarization states may be only 0 degree linear, 45 degree linear, 90 degree linear, and circular polarization states, as shown in FIG. 13, above.

[0249] Step 1940 includes detecting, in sequence, photons in each of the plurality of polarization states using at least one optical detector, wherein the photons in each of the polarization states produce a signal.

[0250] Step 1950 includes receiving, with at least one processor in communication with the at least one optical detector, a plurality of signals corresponding to the photons in the plurality of polarization states.

[0251] Step 1960 includes determining, with the at least one processor and for a segment of the plurality of signals, at least one of: an angle of linear polarization, a degree of polarization, and a degree of circular polarization of the plurality of signals.

[0252] Step 1970 includes generating, for a plurality of segments, a signal corresponding to a sequential output of the angle of linear polarization, degree of polarization, or the degree of circular polarization for each segment. The signal of step 1570 may be used to generate a polarization signature of an object, detect the presence of at least one object in an imaged scene, determine the direction of normal vectors to an object's surfaces, determine the shape of an object, determine the shape, amplitude, and frequency of resonant modes of an object, or identify materials of which an object is made, and other physical properties of an object, as discussed relative to FIG. 11, above.

[0253] It should be emphasized that the above-described embodiments of the present disclosure, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. Particularly, any aspect of this disclosure may be used by, combined with, or added onto any other aspect of this disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present disclosure and protected by the following claims.