Hadamard spectrometer

[0002] The invention relates to a Hadamard spectrometer which enables the detection of a number of polyatomic gases or liquids using, for example, infrared absorption. Such devices are utilized, in particular, for emission control, workplace inspection, and environmental monitoring.

[0003] With respect to prior art related to the invention, reference is made to a spectrophotometer disclosed in U.S. Pat. No. 5,257,086 which enables a mathematical evaluation of detected spectral data and, hence, a spectral analysis under use of the Hadamard transformation. The spectrophotometer described therein is provided with a radiation source array, which is typically an essential unit for spectrometers. The individual radiation sources of the radiation source array provide different wavelengths which can be controlled in accordance with the Hadamard method. The spectrophotometer includes an imaging diffraction grating with a detector arranged behind a slit for detecting spectral signals, and electronic units which ensure a very fast spectral analysis of a sample to be examined. The essential advantage resulting from the Hadamard transformation is a considerable improvement of the signal-to-noise ratio. A disadvantage of the arrangement described in U.S. Pat. No. 5,257,086 is that at first LEDs have to be available as radiation sources which correspond to the frequency spectrum to be examined, and this at present limits the arrangement to a wavelength spectrum of from 250 nm to about 2.5 μm. Another significant disadvantage of this device is that considerable expenditures have to be spent in order to stabilize the intensities of the individual LEDs in order to obtain spectra which can be analyzed quantitatively.

[0004] Furthermore, spectrometers are known, namely the monolithic miniature spectrometer designated MMS 1 manufactured by Carl Zeiss, Business Field for Special Engineering, Sales Department for Optical Components, D-73446 Oberkochen, Germany, in which diode rows are employed as detecting elements. In this case, it is required to image a spectral line onto at least three adjacent detector elements. Only the three measurement signals obtained permit a precise detection of the wavelength. Hence, the resolution power of such spectrometers with respect to a wavelength interval is limited by the number of the detector elements employed and their mutual distance.

[0005] It is an object of the present invention to provide a Hadamard spectrometer which, under application of conventional detector rows, ensures an increased resolution power, an improved wavelength accuracy, and an enhanced signal-to-noise ratio, and has a small and compact construction.

[0006] The object is realized by providing a Hadamard spectrometer with a detector row having individual detector elements spaced from one another by a set distance and a unit for the light emitting and reflecting having three or more adjacent identical luminous sources. The luminous sources are individually controllable in accordance with the Hadamard principle and arranged apart from one another at a set distance related to the distance between the detector elements.

[0007] An important feature of the invention is that row-shaped units, i.e., units having a row of constituent elements, are employed both, for a detector unit for registering spectra and for a light emitting and reflecting unit. The detecting unit includes a plurality of individual detecting ranges or elements and the light emitting and reflecting unit includes a plurality of individual luminous sources. Therefore the designation “double array” is used hereinafter, and the Hadamard principle, known per se, finds application herein.

[0008] When within the scope of the invention the Hadamard principle is referred to, then multiplexing in the spectroscopy including entry switching matrices after Hadamard is intended to be encompassed within the meaning of the term.

[0009] The invention will be explained in the following in more detail with reference to the drawings wherein

[0010] FIG. 1 shows a diode row according to the prior art with a scanning of a spectral line, at least necessary for three times, to obtain a precise detection of wavelength,

[0011] FIG. 2 shows a setup of a Hadamard spectrometer according to the present invention,

[0012] FIG. 2ashows an illumination row as a part of the Hadamard spectrometer according to FIG. 2,

[0013] FIG. 2bshows a detector row as a part of the Hadamard spectrometer according to FIG. 2,

[0014] FIGS. 3a, 3b, 3cand 3dshow as an example, the control of an illumination row including four reflecting elements of a unit according to FIG. 2a.

[0015] FIGS. 4a, 4b, 4cand 4dshow as an example, the spectral distributions obtained within the scope of the invention from four measurements with differently controlled individual luminous sources,

[0016] FIG. 4eshows a spectral signal curve prepared in accordance with the present invention,

[0017] FIG. 5 shows a micro-slit row of individually closeable illuminated slit elements which may be used as the luminous sources in accordance with an embodiment of the invention, and

[0018] FIG. 6 shows a fiber optical row of individually controllable luminous light conducting fibers which may be used as the luminous sources in accordance with an embodiment of the invention.

[0019] FIG. 1 schematically shows a part of a conventional diode row spectrometer comprising a plurality of adjacently arranged detector elements 20. In order to be able to carry out a precise wavelength determination it is, however, required to image a spectral line upon at least three adjacent detector elements 20n1, 20n2, 20n3. Thus, as can be seen in FIG. 1, the resolution power of such a spectrometer with respect to a wavelength interval is limited by the number and the distance x of the individual detectors 20.

[0020] FIG. 2 shows a first embodiment of the invention including essential components which are required in a Hadamard spectrometer. A first unit 1 reflects light, represented by three parallel arrows and which has been emitted from a light source 8, and is formed by a line 10 which is provided with micro mirrors spaced apart equidistant from one another and each being separately tiltable. In the example, there are four micro mirrors 11,12,13,14 shown in FIG. 2ain more detail. The width of each mirror is 20 μm and the length of each mirror is 183.5 μm. Furthermore, there is an imaging grating 2 provided having a diameter of 41 mm, an inside radius of 138 mm, an object-side back focus of 156.9 mm and an image-side back focus of 128 mm with respect to a mean wavelength of 350 nm. A CCD-line 30, also referred to as a detector row herein, has 1024 pixels with a center-to-center distance of 25 μm each and, accordingly, a length of 25.6 mm, and is employed as a detector unit 3. A flattening lens 7 is arranged, in the direction of light propagation, in front of the CCD line 30 in order to ensure an improved resolution over an entire preset spectral range. For the sake of clarity only the principal beams 81 are represented in FIG. 2.

[0021] The mirrors of the line 10 which, in the example, form the luminous sources are adapted to be individually activated. They are, as indicated by example of FIGS. 3a, 3b, 3cand 3d, successively controlled in accordance with a preset pattern, preferably the pattern of the lines of a HADAMARD-S matrix. Bright areas in FIGS. 3a, 3b, 3cand 3dare indicative of mirrors reflecting in the direction of the grating 2, dark areas are indicative of diaphragmed out mirrors (see also nirror 14 in FIG. 2a). Each reflecting mirror produces a spectrum on the CCD line 30. The spectra of the respective reflecting mirror are superimposed on the CCD line 30.

[0022] For further explanation of the invention, the 250 nm spectral line of a Hg lamp is employed in the example, which is triply generated in each case (by three mirrors), instead of analyzing the effect of a sample which is, for example, provided in a receptacle (space) 4. The variants of the pattern are successively imaged so that four measurements are required in the example. It is feasible to control the micro mirrors 11,12,13,14 in accordance with the Hadamard principle via a control unit 9, which is controlled in turn by an evaluating and processor unit 5. It is essential in the invention that the micro mirrors 11,12,13,14,..., n, wherein n>3, are arranged relative to each other at a defined distance b such that, when the micro mirrors are imaged upon the detector row 30, distance b amounts to an integral multiple of the preselected mutual distance a provided for the detecting ranges or elements 31, plus 1/n or minus 1/n. Note that the distance a is measured from an edge of one detecting range 31 to the corresponding edge of an adjacent detecting range 31 (FIG. 2b) and the distance b is measured from an edge of each micro mirror to the corresponding edge of an adjacent micro mirror (FIG. 2a). In this manner, it is not necessary that the defined distances amount to a continuous succession of respective integral multiples of the preselected distance a plus 1/n or minus 1/n. It is important though that the individual luminous sources are arranged relative to each other at a distance b such that when imaged upon the detecting row, the distance of the light/luminous sources amounts to an integral multiple of m/n, wherein m is an integer greater than n. Thus, for example, in the case of n=4 the following sequence is feasible: ¼, {fraction (2/4)}, ¾, {fraction (4/4)} or {fraction (2/4)}, {fraction (4/4)}, ¾, ¼ or ¼, {fraction (6/4)}, {fraction (11/4)}, {fraction (16/4)} or ¾, {fraction (6/4)}, {fraction (9/4)}, {fraction (12/4)} or ¾, {fraction (9/4)}, {fraction (6/4)}, {fraction (12/4)} or ¾, {fraction (10/4)}, {fraction (17/4)}, {fraction (24/4)}. Therefore, it is only essential that all fractions of n, supplemented each by integral multiples of the distances, are contained in the images of the luminous sources.

[0023] In a subsequent electronic evaluation of the spectral signals, obtained during the individual measurements by means of the CCD line 30 and amplified by an amplifier unit 6, a “lateral” backward displacement of the spectra is carried out, in addition to a Hadamard-transformation, in the evaluation unit 5. This displacement is adapted to the respective pattern by which the micro mirrors have been controlled.

[0024] FIGS. 4a, 4b, 4cand 4dillustrate the spectral representations obtained from the four single measurements mentioned above, which are required in the example, and which correspond to the respective combinations of reflecting micro mirrors. The experimental final result is shown in FIG. 4e. The form of the spectral line is resolved by the aid of the arrangement of an input reflecting row for the double-array spectrometer with only four mirrors, described in FIG. 2. The resolution is increased remarkably. Simultaneously, the departures in the single measurements are reduced and the signal-to-noise ratio is enhanced.

[0025] In this manner, the pixel limited resolution of available detector rows can be increased by about a factor of 3 compared to conventional diode array spectrometers. When increasing the number of the entry mirrors, it is feasible to further improve the signal-to-noise ratio. For example, if 103 entry mirrors are used, the signal-to-noise ratio can at least be improved by a factor of 10, at the same total measuring time which is divided up into 103 single measurements.

[0026] Within the scope of the invention, it is feasible to utilize for the emitting and reflecting unit 1, instead of the above described embodiments in the form of micro mirrors, individually closeable illuminated slit elements of a micro-slit row (as shown in FIG. 5) and individually controllable luminous light conducting fibers of a fiber optical row (as shown in FIG. 6), whereby the same measures as for the above described micro mirrors are valid for the mutual arrangement of the individual slits and the individual light conducting fibers.

[0027] The Hadamard spectrometer in accordance with the invention can be employed over a wide spectral range of from the ultraviolet (UV) to the far infrared range (IR). By means of a specific comparative measurement task, the advantages of the invention will be outlined with respect to a compact construction of the spectrometer.

[0028] For the monitoring of a carcinogenic benzol with a concentration of 1 ppm in, for example, the IR range, a spectrometer including efficient components (CCD-rows) is required to provide for an absorption way in an order of magnitude of 60 cm, due to the low optical density and, hence, a typical prior art spectrometer cannot be used effectively. With an embodiment of the Hadamard-spectrometer as disclosed herein, an embodiment comprising, for example, 103 micro mirrors requires an absorption way of only about 6 cm. In this manner, it is feasible to provide small and compact instruments which can be adapted to different monitoring tasks at improved device parameters.

[0029] All features disclosed in the specification, in the subsequent claims, and in the drawing are substantial for the invention both, individually and in any combination with one another.