Off-axis illumination direct-to-digital holography

[0003] 1. Field of the Invention

[0004] The invention relates generally to the field of direct-to-digital holography (interferometry). More particularly, the invention relates to off-axis illumination for improved resolution in direct-to-digital holography.

[0005] 2. Discussion of the Related Art

[0006] Prior art direct-to-digital holography (DDH), sometimes called direct-to-digital interferometry, is known to those skilled in the art. For instance, FIG. 1 illustrates one simplified embodiment of a DDH system. Light from a laser source 105 is expanded by a beam expander/spatial filter 110 and then travels through a lens 115. Subsequently, the expanded filtered light travels to a beamsplitter 120. The beamsplitter 120 may be partially reflective. The portion of light reflected from the beamsplitter 120 constitutes an object beam 125 which travels to the object 130. The portion of the object beam 125 is that is reflected by the object 130 then passes through the beamsplitter 120 and travels to a focusing lens 145. This light then passes through the focusing lens 145 and travels to a charge coupled device (CCD) camera (not shown).

[0007] The portion of the light from the lens 115 that passes through the beamsplitter 120 constitutes a reference beam 135. The reference beam 135 is reflected from a mirror 140 at a small angle. The reflected reference beam 135 from the mirror then travels toward the beamsplitter 120. The portion of the reference beam 135 that is reflected from the beamsplitter 120 then travels through the focusing lens 145 and toward the CCD camera (not shown). The object beam 125 from the focusing lens 145 and the reference beam 135 from the focusing lens 145 constitute a plurality of object and reference waves 150 and will interfere at the CCD to produce the interference pattern characteristic of a hologram as noted in U.S. Pat. No. 6,078,392.

[0008] In FIG. 1, the object beam 125 is parallel to, and coincident with, the optical axis 127. This type of DDH set-up can be referred to as on-axis illumination.

[0009] A limitation of this technology has been that the imaging resolution of the DDH system is limited by the optics of the system. The most notable limitation of the optics is the aperture stop, which is required to prevent degradation of the image quality due to aberrations. With regard to a two-dimensional Fourier plane, only object spatial frequencies within a circle of radius q0 can be transmitted. In the case of on-axis illumination, the aperture with radius q0 appears centered on a zero spatial frequency (q=0). What is needed, therefore, is an approach that permits spatial frequencies outside the circle of radius q0 to be transmitted.

[0010] There is a need for the following aspects of the invention. Of course, the invention is not limited to these aspects.

[0011] According to an aspect of the invention, a process of recording an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises, reflecting a reference beam from a reference mirror at a non-normal angle; reflecting an object beam from an object at an angle with respect to an optical axis defined by a focusing lens; focusing the reference beam and the object beam at a focal plane of a digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; digitally recording the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; Fourier analyzing the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes by transforming axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined as an angle between the reference beam and the object beam; applying a digital filter to cut off signals around an original origin; and then performing an inverse Fourier transform.

[0012] According to another aspect of the invention, a machine operable to digitally record an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises: a laser; a beamsplitter optically coupled to the laser; a reference beam mirror optically coupled to the beamsplitter; a focusing lens optically coupled to the reference beam mirror; a digital recorder optically coupled to the focusing lens; and a computer that performs a Fourier transform, applies a digital filter, and performs an inverse Fourier transform, wherein a reference beam is incident upon the reference beam mirror at a non-normal angle, an object beam is incident upon an object at an angle with respect to an optical axis defined by the focusing lens, the reference beam and the object beam are focused by the focusing lens at a focal plane of the digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis which is recorded by the digital recorder, and the computer transforms axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam and cuts off signals around an original origin before performing the inverse Fourier transform.

[0013] These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.

[0014] The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore nonlimiting, embodiments illustrated in the drawings, wherein identical reference numerals designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale.

[0015] FIG. 1 illustrates a schematic view of a conventional direct-to-digital holography apparatus, appropriately labeled “PRIOR ART.”

[0016] FIG. 2 illustrates a schematic view of an off-axis illumination direct-to-digital holography apparatus (interferometer) in an on-axis position, representing an embodiment of the invention.

[0017] FIG. 3 illustrates a schematic view of the off-axis illumination direct-to-digital holography apparatus (interferometer) of FIG. 2 in an off-axis position.

[0018] FIG. 4 illustrates a two-dimensional Fourier plane for an object showing one instance of on-axis illumination and one instance of off-axis illumination, representing an embodiment of the invention.

[0019] FIG. 5 illustrates a two-dimensional Fourier plane for an object showing one instance of on-axis illumination and four instances of off-axis illumination, representing an embodiment of the invention.

[0020] FIG. 6 illustrates a two-dimensional Fourier plane for an object showing all spatial frequencies that contribute to a fused spectrum (merged image), representing an embodiment of the invention.

[0021] FIGS. 7A-7F illustrate the discrete Fourier spectra (7A-7E) obtained from five different holograms and a fused spectrum (7F), representing embodiments of the invention.

[0022] FIG. 8 illustrates an object amplitude reconstructed from the on-axis illuminated hologram, representing an embodiment of the invention.

[0023] FIG. 9 illustrates an object amplitude reconstructed from a fused result, representing an embodiment of the invention.

[0024] FIG. 10 illustrates the intersection of the two apertures W₀(q) and W_k(q), representing an embodiment of the invention.

[0025] The invention and the various features and advantageous details thereof are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well known starting materials, processing techniques, components and equipment are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.

[0026] Within this application several publications are referenced by Arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims after the section heading References. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference herein for the purpose of indicating the background of the invention and illustrating the state of the art.

[0027] The below-referenced U.S. Patents, and allowed U.S. patent application, disclose embodiments that were satisfactory for the purposes for which they are intended. The entire contents of U.S. Pat. No. 6,078,392, issued Jun. 20, 2000 to C. E. Thomas, L. R. Baylor, G. R. Hanson, D. A. Rasmussen, E. Voelkl, J. Castracane, M. Simkulet and L. Clow, entitled “Direct-to-Digital Holography and Holovision” are hereby expressly incorporated by reference herein for all purposes. The entire contents of allowed U.S. patent application Ser. No. 09/477,267, filed Jan. 4, 2000 by C. E. Thomas and G. R. Hanson, entitled “Improvements To Acquisition and Replay Systems” are hereby expressly incorporated by reference herein for all purposes.

[0028] This application contains disclosure that also contained in copending U.S. Ser. Nos. 10/234,043, filed Sep. 3, 2002; and 10/234,042 , filed Sep. 3, 2002, the entire contents of all of which are hereby expressly incorporated by reference for all purposes.

[0029] In general, the context of the invention can include obtaining, storing and/or replaying digital data. The context of the invention can include processing digital data that represents an image. The context of the invention can also include transforming data from multiple images into a merged image.

[0030] The invention can include a method of acquiring improved resolution holographic imagery from a direct-to-digital holography system using off-axis illumination. The invention can also include an apparatus for acquiring improved resolution holographic imagery with a direct-to-digital holography (DDH) system that uses off-axis illumination.

[0031] In general, the object to be observed (imaged) is optically coupled to an illumination source via one or more optical components. As discussed with regard to FIG. 1, the illumination beam is typically passed through the center of the target objective (i.e., lens system) along, and thus parallel to, the optical axis. This type of DDH configuration can be referred to as “on-axis illumination” and allows spatial frequencies (q) of the object to be acquired up to a certain limit (q0), which is determined by the objective aperture.

[0032] The invention can include an “off-axis illumination” scenario, where the illumination source is displaced laterally so that the beam will pass through the object objective off-center yet still parallel to the optical axis. The illumination will, due to the focusing effect of the objective, be incident upon the object at some angle to the optical axis. Due to this off-axis illumination, higher spatial frequencies (q>q0) of the object can pass through the objective aperture, and thus be observed, than can with on-axis illumination. This is an important advantage of the invention.

[0033] The invention can include an extended DDH system (apparatus) adapted to digitally capture the on-axis- and one, or more, off-axis-illuminated holograms of the same object. The invention can also include analyzing and/or processing (fusing) the digitally captured data. The resulting, fused image will contain a wider range of spatial frequencies than in any of the original holograms, thus providing a significant increase in the nominal imaging resolution of the system compared to the case where no off-axis-illuminated data is available.

[0034] As noted above, the imaging resolution of fundamental DDH systems is limited by the optics, most notably the aperture stop, which is required to prevent degradation of the image quality due to aberrations. The aperture stop is required to prevent aliasing of higher frequencies and subsequent degradation of imaging quality. This means the optics of the DDH system are such that only object spatial frequencies within a circle of radius q0 can be transmitted. In the case of on-axis illumination, the aperture with radius q0 appears centered on a zero spatial frequency (q=0). In the case of off-axis illumination, the aperture with radius q0 appears shifted (e.g., to the left) in the frequency domain. This implies that in the direction in which the aperture is shifted, spatial frequencies with q>q0 are transmitted. On the downside, some spatial frequencies with q close to q0 are “lost” in the opposite direction. By acquiring a second image with the illumination shifted in the opposite direction, the aperture appears shifted (e.g., to the right) and thus the spatial frequencies “lost” from the first image are regained with additional frequencies beyond q0. Fusing the information from the two images results in one image with better resolution. Since DDH records the phase information on the complex image wave, the information from both (or more) images can be fused with surprisingly advantageous results. The invention improves the resolution of generic object structures regardless of orientation.

[0035] The invention can include an extension of the fundamental DDH system to automatically capture both on-axis and off-axis illuminated holograms. The invention can also include methods to analyze and fuse the results of these holograms to produce a representation of the observed object with more spatial resolution than available in the prior DDH art.

[0036] As evident in FIG. 1, the object beam 125 is parallel to the optical axis 127. As noted above, this set-up can be referred to as on-axis illumination. Off-axis illumination, on the other hand, refers to the case where the object beam 125 is incident upon the object 130 at some angle with respect to the optical axis 127 (an example is illustrated by the object team 215, 305 shown in FIG. 3). There are many methods to achieve off-axis illumination; the approach presented hereafter is intended to serve only as a representative, and therefore non-limiting example.

[0037] Referring to FIGS. 2 and 3, an embodiment of an off-axis illumination DDH apparatus is illustrated. In FIGS. 2 and 3, there are two primary modifications from FIG. 1. A first modification is that the laser source 105, the beam expander/spatial filter 110, and the lens 115 are grouped into a computer-controlled, moveable enclosure 205. The enclosure 205 can be movable along an axis that is substantially parallel to the optical axis 127. In more detail, the enclosure 205 can be movable along an axis that is substantially coplanar with a normal to the beamsplitter 120.

[0038] Still referring to FIGS. 2 and 3, a second modification is the addition of the object objective 210. In FIG. 2, the laser source enclosure 205 is positioned so that the object beam 125 reflects off of the beamsplitter 120 to pass through the center of the object objective 210. The object beam 125 then leaves the object objective 210 and is incident upon the object 130, centered around the optical axis 127. In this configuration, on-axis illumination is achieved and the system of FIG. 2 is effectively the same as that in FIG. 1.

[0039] In FIG. 3, however, the laser source enclosure 205 is shifted (up in this particular configuration) so that the object beam 125 passes through the object objective 210 off-center. Of course, the laser source enclosure 205 can alternatively be shifted down. Because of the focusing properties of the object objective 210, the object beam 215 leaving the object objective 210 is incident upon the object 130 at some angle with respect to the optical axis 127, thereby achieving off-axis illumination. Thus, the object beam 215 can be incident upon the object 130 substantially non-parallel to the optic axis 127. The object beam 305 reflected from the object passes back through the object objective 210 off-axis, but due to the optical properties of the object objective 210 and the focusing lens 150 is still focused on the CCD (not shown). In the off-axis illumination case, the properties of diffraction⁽¹⁾imply that the hologram formed at the CCD by the interference of the object beam 305 and the reference beam 135 will contain some spatial frequencies of the object that are not observed using on-axis illumination.

[0040] Thus, the invention can include an apparatus operable to digitally record a spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprising: a laser; a beamsplitter optically coupled to the laser; a reference beam mirror optically coupled to the beamsplitter; an object optically coupled to the beamsplitter; a focusing lens optically coupled to both the reference beam mirror and the object; a digital recorder optically coupled to the focusing lens; and a computer for performing a Fourier transform, applying a digital filter, and performing an inverse Fourier transform, wherein a reference beam is incident upon the reference beam mirror at a non-normal angle, an object beam is incident upon the object at an angle with respect to an optical axis defined by the focusing lens, the reference beam and an object beam, which constitute a plurality of simultaneous reference and object waves, are focused by the focusing lens at a focal plane of the digital recorder to form a spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis which is recorded by the digital recorder, and the computer transforms axes of the recorded spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam and cuts off signals around an original origin before performing the inverse Fourier transform. The apparatus can include an object objective optically coupled between the beamsplitter and the object. The apparatus can include an aperture stop coupled between the object and the focusing lens. The beamsplitter, the reference beam mirror and the digital recorder can define a Michelson geometry. The beamsplitter, the reference beam mirror and the digital recorder can define a Mach-Zehner geometry. The apparatus can also include a digital storage medium coupled to the computer for performing a Fourier transform, applying a digital filter, and performing an inverse Fourier transform. The digital recorder can include a CCD camera 350 that defines pixels. The apparatus can include a beam expander/spatial filter 230 optically coupled between the laser and the beamsplitter. The angle between the reference beam and the object beam, and a magnification provided by the focusing lens, can be selected in order that the digital recorder may resolve features of the spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis. So that the digital recorder may resolve a feature, two fringes, each having two pixels per fringe, can be provided. The invention can include a spatially heterodyne hologram produced by the above-described apparatus, embodied on a computer-readable medium.

[0041] Accordingly, the invention can include a method of recording a spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprising: splitting a laser beam into a reference beam and an object beam; reflecting the reference beam from a reference mirror at a non-normal angle; reflecting the object beam from an object at an angle with respect to an optical axis defined by a focusing lens; focusing the reference beam and the object beam, which constitute a plurality of simultaneous reference and object waves, with the focusing lens at a focal plane of a digital recorder to form a spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; digitally recording the spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; Fourier analyzing the recorded spatially heterodyne hologram including spatially heterodyne fringes by transforming axes of the recorded spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined as an angle between the reference beam and the object beam; applying a digital filter to cut off signals around an original origin; and then performing an inverse Fourier transform. The method can include refracting the object beam with an object objective before reflecting the object beam from an object at an angle with respect to an optical axis defined by a focusing lens and after reflecting the object beam from an object at an angle with respect to an optical axis defined by a focusing lens. The step of transforming axes of the recorded spatially heterodyne hologram can include transforming with an extended Fourier transform. The step of digitally recording can include detecting the beams with a CCD camera that defines pixels. The off-axis illuminated spatially heterodyne hologram can be an off-axis illuminated spatially low-frequency heterodyne hologram; the phrase low-frequency implies that the fundamental fringe spatial frequency is below the Nyquist sampling limit. The method can also include storing the spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis as digital data. The method can also include replaying the Fourier analyzed spatially heterodyne hologram. The method can also include transmitting the Fourier analyzed spatially heterodyne hologram. The invention can include a spatially heterodyne hologram prepared by the above-described method(s), embodied on a computer-readable medium.

[0042] Referring to FIG. 4 a two-dimensional Fourier plane is depicted for a hypothetical object, where zero frequency 405 is the center. The zero frequency for the object 405 corresponds to the heterodyne or carrier frequency in the hologram. All of the spatial frequencies of the object are represented by the filled circle 410. On-axis illumination will capture only those frequencies indicated by the sold-line circle 415. One instance of off-axis illumination, however, will capture the frequencies indicated by the dash-line circle 420. Other instances of off-axis illumination can capture other regions of the object spectrum 410.

[0043] A DDH apparatus such as that shown in FIGS. 2 and 3 can be used to capture and store multiple holograms—both on-axis and off-axis illuminated—of the same object area. Realizing that each such hologram contains information about difference spatial frequencies of the object, methods to analyze these holograms and compute a single, reconstructed image will be discussed in detail below. The reconstructed image can contain all of the observed spatial frequencies and thus provide a significantly higher resolution image than any single recorded image. The methods presented hereafter are intended to serve as representative examples and are not intended to preclude extensions, modifications, or other approaches.

[0044] The invention can include the capture (digital acquisition) of k=(0, . . . , N) holograms. FIG. 5 illustrates the spatial frequencies contained in each of hologram of a set of holograms where N=4. Similar to FIG. 4, FIG. 5 represents the Fourier plane of the object where the zero frequency 505 is the center point. The solid circle 510 represents spatial frequencies observed with on-axis illumination while the dotted-line circles 515, 520, 525, 530 represent spatial frequencies observed by four different off-axis illuminated holograms. By appropriately fusing the information from all five holograms, the resolution (i.e., bandwidth) of the DDH system is effectively increased and all spatial frequencies indicated by the shaded region 610 in FIG. 6 contribute to the final image.

[0045] In the Fourier domain, let the true spectrum of the object be given by F(q). The portion of F(q) (region of F(q)), G_k(q), observed by each hologram is then given by

[0046] G_k(q)=μ_kexp(jγ_k)·F(q)W_k(q),

[0047] where μ_kand γ_kare the fringe contrast and phase offset, respectively, for hologram k and j is the square root of negative one.

[0048] The invention can include normalizing and/or compensating for varying fringe contrasts. One such method for fringe contrast normalization can be described as follows. First we note that G_k(q) above represents the Fourier transform of one acquired object wave (or image) g_k(x):

[0049] G_k(q)=ℑ{g_k(x)}

[0050] where ℑ{·} indicates the Fourier transform operation. We now define two additional images

[0051] g_k,0(x)=ℑ⁻¹{G_k(q)W₀(q)}=ℑ⁻¹{μ_kexp(jγ_k)F(q)W_k(q)W₀(q)}

[0052] and

[0053] g_0,k(x)=ℑ⁻¹{G₀(q)W_k(q)}=ℑ⁻¹{μ₀exp(jγ₀)F(q)W₀(q)W_k(q)}

[0054] for k=1, . . . , N where k=0 is assumed to be the reference image and where ℑ⁻¹{·} indicates the inverse Fourier transform operation. The image g_k,0(x) is constructed by keeping only the frequencies of g_k(x) common to both g_k(x) and the reference image g₀(x). Similarly, the image g_0,k(x) comprises only the frequencies of g₀(x) common to both g₀(x) and g_k(x). These common frequencies can be visualized as the intersection of the two apertures W₀(q) and W_k(q) as illustrated in FIG. 10. Once g_k,0(x) and g_0,k(x) have been constructed, a ratio image χ_k(x) is computed as ${\chi }_k\left(x\right)=\frac{g_{k,0}\left(x\right)}{g_{0,k}\left(x\right)}\approx \frac{{\mu }_k\exp \left(j{\gamma }_k\right)}{{\mu }_0\exp \left(j{\gamma }_0\right)}.$

[0055] After computing the ratio image, the relative fringe contrast, μ_k/μ₀, can be estimated as the sample mean (average) of the magnitude of the ratio image χ_k(x): $\frac{{\mu }_k}{{\mu }_o}\approx \frac{1}{P}\sum _{l=1}^P\left|{\chi }_k\left(x_i\right)\right|$

[0056] where P is the total number of pixels in the (digitized) ratio image and |χ_k(x_i)| represents the magnitude value of the ratio image at a single pixel location.

[0057] The invention can include normalizing and/or compensating for the phase offsets. One method for phase offset normalization is to compute the same ratio image described above for fringe contrast normalization. Then the relative phase offset, exp(j(γ_k−γ₀)), can computed as the (weighted) sample mean of the phase of the ratio image χ_k(x): $\exp \left(j\left({\gamma }_k-{\gamma }_0\right)\right)\approx \frac{1}{P}\sum _{i=1}^P{\alpha }_k\left(x_i\right)\angle {\chi }_k\left(x_i\right)$

[0058] where ∠χ_k(x_i) is the phase (angle) of the (digitized) ratio image at a single pixel location and where α_k(x_i) is a weighting factor. In the most straightforward embodiment, the weighting factor is fixed equal to one. In other embodiments, the weighting factor may be related to the magnitudes of the individual images used to compute the ratio image. This may be used because phase data is often inaccurate when the magnitude is very small. One such embodiment for computing the weighting factor is $\sum _{i=1}^P\frac{\left|g_k\left(x_i\right)||g_0\left(x_l\right)\right|}{\sum _{n=1}^P\left|g_k\left(x_n\right)||g_0\left(x_n\right)\right|}.$

[0059] Similarly motivated weighting factors may also be employed. Another method for phase offset estimation is described as follows. Often phase offset is due to errors in locating the carrier frequency. Under such a condition, the phase offset of the ratio image takes the form

[0060] ∠χ_k(x)=exp(j(e₁x₁+e₂x₂+γ_k))

[0061] where e₁and e₂are related to the error in finding the carrier frequency. In this situation, both e₁and e₂must be found in addition to γ_k. The above equation can also be written as

[0062] ∠χ_k(x)=cos(e₁x₁+e₂x₂+γ_k)+jsin(e₁x₁+e₂x₂+γ_k).

[0063] The real and imaginary parts can be considered separate elements of a two-dimensional vector $\left(\begin{array}{c}\mathrm{Re}\left\{{\mathrm{\angle \chi }}_k\left(x\right)\right\}\\ \mathrm{lm}\left\{{\mathrm{\angle \chi }}_k\left(x\right)\right\}\end{array}\right)\approx \left(\begin{array}{c}\cos \left(e_1{x}_1+e_2{x}_2+{\gamma }_k\right)\\ \sin \left(e_1{x}_1+e_2{x}_2+{\gamma }_k\right)\end{array}\right).$

[0064] The three parameters in question on the right of the above equation—e₁, e₂, and γ_k—can then be estimated from the observations (on the left of the above equation) through any standard non-linear optimization technique.

[0065] Thus, after fringe contrast and phase offset normalization, the equation for the observed spectrum in hologram k can be simplified to

[0066] G_k(q)=F(q)W_k(q)

[0067] In the above equations, W_k(q) is essentially a window function that represents the region of the spectrum observed by hologram k. As a simple example, the window functions from FIG. 5 would be equal to one inside and zero outside the respective circles 510, 515, 520, 525, 530. In a more sophisticated embodiment, the window function can be modeled using a circularly symmetric Butterworth function: $W_k\left(q\right)=\frac{1}{{\left(1+\frac{\sqrt{{\left(q_1-c_1\right)}^2+{\left(q_2-c_2\right)}^2}}{r}\right)}^{2m}}$

[0068] where q₁and q₂represent the horizontal and spatial frequency variables of the vector q, c₁and c₂represent the center point of the function in the Fourier plane, r represent the radius of the function, and m is the order of the filter.

[0069] The goal of the fusion method is to form an estimate of F(q) from the observations G_k(q). One approach to estimate F(q) is to employ a linear estimator based upon the minimum mean square error criterion; this is the well-known linear, minimum mean-square error (LMMSE) estimator. Using a linear estimator, the estimate for F(q), where q represents a sample in the discrete Fourier domain, is given by $\hat{F}\left(q\right)=\sum _{k=0}^N{\beta }_k\left(q\right)G_k\left(q\right)$

[0070] where the coefficients, as determined by the LMMSE criterion, are given by ${\beta }_k\left(q\right)=\frac{W_k\left(q\right)}{\sum _{k=0}^N{W}_k^2\left(q\right)+c\left(q\right)},$

[0071] where c(q) is a positive number that represents a regularization parameter that can be selected dependent upon the specifics of the system. This regularization parameter is used to compensate for system noise. In a perfect, zero noise case c(q) could be set to zero, in which case the estimate for F(q) reduces to a simple average. In practice, such a technique produces undesirable artifacts in the fused image. The most straightforward approach is to set c(q) to some constant number for all q based on experimental observations. In more sophisticated approaches, c(q) can be set (either to a constant or to vary with q) based on some computational analysis of the observed holograms and images. Many such approaches for computing the regularization parameter may be devised by those skilled in the art.

[0072] Simple averaging or LMMSE estimation are perhaps the most straightforward approaches for computing the fused image. A few alternative optimization criteria for computing the fused image, well-known to those skilled in the art of image processing, might include maximum likelihood (ML) estimation, maximum a posteriori (MAP) estimation, and/or total least squares (TLS) estimation. Note that these examples are not exhaustive.

[0073] FIG. 7 shows the discrete Fourier spectra obtained from five different holograms. The first image 705 corresponds to on-axis illumination while images 710, 715, 720, and 725 correspond to various off-axis illumination conditions. The fusion method described above was employed to produce the fused spectrum shown in image 730. FIG. 8 shows the object amplitude reconstructed from only the on-axis illuminated hologram. FIG. 9 shows the object amplitude reconstructed from the fused result. It is important to note the grid structure that becomes apparent in FIG. 9 (and which is absent in FIG. 8) due to the increased resolution that results from the fusion.

[0074] The disclosed embodiments show a computer controlled, moveable enclosure as the structure for performing the function of aligning the source, beam expander/spatial filter and lens so that the object beam passes through the object objective on-center or off-center, but the structure for aligning the source, beam expander/spatial filter and lens can be any other structure capable of performing the function of aligning the object beam so that it passes through the object objective on-center or off-center, including, by way of example a moveable platform for displacing the beamsplitter, the mirror, the object objective, the object, the focusing lens and the CCD camera relative to the source, beam expander/spatial filter and lens, or as another example, a series of movable optical elements (e.g., mirrors), or as another example a flexible optical fiber and/or cable.

[0075] The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. The term approximately, as used herein, is defined as at least close to a given value (e.g., preferably within 10% of, more preferably within 1% of, and most preferably within 0.1% of). The term substantially, as used herein, is defined as largely but not necessarily wholly that which is specified. The term generally, as used herein, is defined as at least approaching a given state. The term deploying, as used herein, is defined as designing, building, shipping, installing and/or operating. The term means, as used herein, is defined as hardware, firmware and/or software for achieving a result. The term program or phrase computer program, as used herein, is defined as a sequence of instructions designed for execution on a computer system. A program, or computer program, may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer or computer system. The phrase low-frequency, as used herein, can be defined as implying that the fundamental fringe spatial frequency is below the Nyquist sampling limit.

[0076] A practical application of the invention that has value within the technological arts is metrology. The invention is useful in conjunction with microelectronic (mechanical) fabrication, such as for semiconductor inspection. The invention is also useful in conjunction with nanotechnology research, development and manufacturing, such as nanovisualization, nanomeasurement, or the like. The invention is useful in the context of an interferometer using digital processing and/or a digital data acquisition, for example, a direct-to-digital holography tool based on electron holography. There are virtually innumerable uses for the invention, all of which need not be detailed here.

[0077] A method, apparatus and/or computer program, representing an embodiment of the invention, can be cost effective and advantageous for at least the following reasons. The invention can provide computer-control of object illumination. The invention can provide fusion of results from multiple holograms. The invention can provide significantly increased imaging resolution. The invention improves quality and/or reduces costs compared to previous approaches.

[0078] All the disclosed embodiments of the invention disclosed herein can be made and used without undue experimentation in light of the disclosure. The invention is not limited by theoretical statements recited herein. Although the best mode of carrying out the invention contemplated by the inventors is disclosed, practice of the invention is not limited thereto. Accordingly, it will be appreciated by those skilled in the art that the invention may be practiced otherwise than as specifically described herein.

[0079] Further, the individual components need not be combined in the disclosed configurations, but could be combined in virtually all configurations. Further, variation may be made in the steps or in the sequence of steps composing methods described herein. Further, although the apparatus described herein can be a separate module, it will be manifest that the apparatus may be integrated into the system with which it is associated. Furthermore, all the disclosed elements and features of each disclosed embodiment can be combined with, or substituted for, the disclosed elements and features of every other disclosed embodiment except where such elements or features are mutually exclusive.

[0080] It will be manifest that various substitutions, modifications, additions and/or rearrangements of the features of the invention may be made without deviating from the spirit and/or scope of the underlying inventive concept. It is deemed that the spirit and/or scope of the underlying inventive concept as defined by the appended claims and their equivalents cover all such substitutions, modifications, additions and/or rearrangements.

[0081] The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” and/or “step for.” Subgeneric embodiments of the invention are delineated by the appended independent claims and their equivalents. Specific embodiments of the invention are differentiated by the appended dependent claims and their equivalents.

[0082] (1) Goodman, Joseph W., “Introduction to Fourier Optics,” McGraw-Hill, 1998.

[0083] (2) Voelkl, E. et al., “Introduction to Electron Holography,” Kluwer Academics/Plenum Publishers, 1999.

[0084] (3) Eugene Hecht, “Optics, Third Edition,” Addison-Wesley, 1998, page 465-469; 599-602.