Programmable full-frame hyperspectral imaging device

Abstract

An imaging system (10, 100) comprising: a first optical subsystem (24, 32, 54, 104, 122, 152) that receives incident light and produces an image comprising a spatially invariant light propagation defining a substantially spatially invariant angle of incidence at a first predetermined location with an image plane; a dispersive element (26, 34, 56, 106) positioned at the predetermined location to receive the image at the spatially invariant angle of incidence and to produce wavelength-dispersed light; a second optical subsystem (108, 126, 156) that receives the wavelength-dispersed light and produces a spatially dispersed spectrum at a second predetermined location such that, in at least one dimension of a spectral plane, light of a predetermined wavelength is directed to a substantially identical position within the spectral plane;a selective element located at the second predetermined position and optically coupled to the dispersive element (26, 34, 56, 106), wherein the selective element is configured to programmably modulate the amplitude of the light at one or more positions in the spectral plane to generate spectrally modified light; and a detector receiving the spectrally modified light from the selective element, wherein the spectrally modified light comprises an image representation of the incident light in which the spectral content has been modified by the selective element in one or more spectral bands, and wherein such spectral modification is substantially identical for nearly all positions in the image representation.

Description

AREA OF INVENTION The present invention relates in general to spectral and spatial imaging and, more specifically, to programmable multiband spectral filtering and hyperspectral imaging, in which each pixel contains a high-resolution spectrum. BACKGROUND AND SUMMARY Spectral imaging, particularly hyperspectral imaging where each pixel contains a high-resolution spectrum, has proven valuable for the remote detection, identification, and quantification of chemical species, with applications ranging from proliferation detection and environmental monitoring to planetary science and medical imaging. The spectral content of each pixel is typically analyzed for the presence of various chemicals or materials that possess characteristic spectral signatures within the spectral range under consideration. As currently implemented, hyperspectral imaging is inherently slow, typically requiring several seconds to acquire a single image.Monitoring phenomena occurring on shorter timescales has necessitated either sacrificing spectral specificity—that is, reducing the spectral component to a simple bandpass filter in front of a fast camera—or sacrificing spatial information and simply pointing a spectrometer at a single point (or at most a line). Both options are unsatisfactory for various reasons. An existing programmable HSI system is described in “Programmable Matched Filter And Hadamard Transform Hyperspectral Imagers Based On Micro-Mirror Arrays” by Steven P. Love, Proc. SPIE, Vol. 7210, 721007 (2009), the entire content and teaching of which are hereby expressly incorporated by reference. Unfortunately, according to Love, the programmable HSI must either scan the spectral patterns across the micro-mirror array or scan the instrument's orientation to produce a complete two-dimensional spatial image with the desired spectral processing. The development of a programmable spectral filter has therefore been hampered by the entanglement of spatial and spectral direction, a feature of nearly all imaging spectrometer designs. Accordingly, there is a need for an improved full-frame hyperspectral imaging device and / or system.As described in detail below, embodiments of the present invention overcome the disadvantages and limitations of the prior art by providing a device that obtains a full-frame hyperspectral image without requiring an external push-broom scan or scanning of the image on the micromirror array to generate spectral and spatial information, where the spectral data are correctly linked with the spatial data. A preferred embodiment of the present invention may comprise a first optical subsystem that receives incident light and generates an image comprising a spatially invariant light propagation that defines a substantially spatially invariant angle of incidence at a first predetermined location with respect to an image plane. A dispersive element is arranged at the predetermined location to receive the image at the substantially spatially invariant angle of incidence and to generate wavelength-dispersed light. A second optical subsystem receives the wavelength-dispersed light and generates a spatially dispersed spectrum at a second predetermined location, such that in at least one dimension of a spectral plane, virtually all the light of a predetermined wavelength is directed to a substantially identical position within the spectral plane.The preferred device may further comprise a selective element located at the second predetermined position and optically coupled to the dispersive element, wherein the selective element is configured to programmably modulate the amplitude of the light at one or more locations in the spectral plane to generate spectrally modified light, and a detector that receives the spectrally modified light from the selective element. As described below, the spectrally modified light preferably comprises an image of the incident light in which the spectral content has been modified by the selective element in one or more spectral bands, and furthermore, this spectral modification is preferably substantially identical for nearly all locations in the image. Advantages and benefits of embodiments of the present invention include, among others, the provision of a device for performing micromirror array-based spectral processing in full spatial format without wavelength shifts across the entire two-dimensional image, wherein the spectral imaging can be performed without sampling and at speeds limited only by the detector used. Compared to current hyperspectral imaging devices, the imaging time for a given area can potentially be reduced by several hundred in embodiments of the present invention. US 2006 / 0017924A1 mentions an imaging system with a programmable spectral transfer function, comprising an input image plane for light coupling, a dispersive optical system for spectral splitting of the light, a spatial light modulator for spectral component selection, a dedispersive optical system for recombination of the selected spectral components, and means for generating an output surface image. US 2010 / 0 309 467 A1 describes a single-image spectral imager that captures spatial and spectral data in a single snapshot with high optical efficiency, limited by the readout time of the detector circuit, using dispersive optics and spatial light modulators to encode a mathematical transformation, thereby simultaneously capturing multiple coded images on one focal plane and decoding a spectral / spatial hypercube. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of a full-frame, multi-band programmable spectral imaging device according to a preferred embodiment of the present invention. Fig. 2 is a schematic representation of a full-frame, multi-band programmable spectral imaging device according to a further preferred embodiment of the present invention. Fig. 3 is a schematic representation of a full-frame, multi-band programmable spectral imaging device according to a further preferred embodiment of the present invention. Fig. 4A and Fig. 4B are schematic representations of a full-frame, multi-band programmable spectral imaging device according to a further preferred embodiment of the present invention.Figure 5 is a schematic representation of a full-frame, multiband programmable spectral imaging device according to a further preferred embodiment of the present invention. Figure 6 is a schematic representation of a full-frame, multiband programmable spectral imaging device according to a further preferred embodiment of the present invention. Figure 7 is a photograph of a working prototype of an exemplary implementation of the full-frame, multiband programmable spectral imaging device according to the preferred embodiments of the present invention. Figure 8 is a panchromatic image and spectra of an incandescent lamp, a low-pressure sodium vapor lamp, and a compact fluorescent lamp from a hyperspectral image data cube obtained using the Hadamard transformation technique with the example device from Figure 7.Figure 9 is a series of multiband spectrally filtered images of an incandescent lamp and a compact fluorescent lamp, obtained with the example apparatus from Figure 7. The images show how the example apparatus can select spectral bands so that one lamp type is emphasized in the image and the other is suppressed. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS As described in detail below with reference to Figures 1, 2, 3, 4, 5 to 6, the device of the preferred embodiment can comprise a first optical subsystem that receives incident light and generates an image comprising a spatially invariant light propagation that defines a substantially spatially invariant angle of incidence at a first predetermined location with an image plane. A dispersive element is arranged at this predetermined location to receive the image at the spatially invariant angle of incidence and generate wavelength-dispersed light. A second optical subsystem receives the wavelength-dispersed light and generates a spatially dispersed spectrum at a second predetermined location, such that in at least one dimension of a spectral plane, virtually all the light of a predetermined wavelength is directed to a substantially identical position within the spectral plane.The preferred device may further comprise a selective element located at the second predetermined position and optically coupled to the dispersive element. The selective element is configured to programmatically modulate the amplitude of the light at one or more points in the spectral plane to generate spectrally modified light, and a detector that receives the spectrally modified light. Preferably, the spectrally modified light comprises an image of the incident light in which the spectral content has been modified by the selective element in one or more spectral bands, and preferably this spectral modification is substantially identical for nearly all points in the image. Those skilled in the art will readily recognize that the preferred device and its variations can be easily coupled with one or more image processing and / or visualization components, including, for example, a display, a user interface, image processing hardware, firmware, or software, and any suitable combination thereof. Such a combination of image processing and / or visualization components may be an integral part of the preferred device or be spatially distributed. Further variations of the preferred device may include one or more communication modules, enabling the preferred device to be used in a remote sensing application and / or on board a remote sensing vehicle such as a reconnaissance aircraft, an unmanned drone, a satellite, or the like. Aspects of the device of the preferred embodiment can be varied according to the strategy used to ensure that light originating from each pixel strikes the spectrally dispersive element in the dispersion region at substantially the same angle of incidence. First variations of the preferred device can utilize microscopic array optics, for example, microlens arrays, aperture arrays, microlamella arrays, and capillary arrays, arranged in combination to direct the light in the required direction and suppress rays propagating in other directions. Second variations of the preferred device can utilize traditional macroscopic optics and achieve the desired condition by arranging the initial imaging elements and the entrance pupil in a configuration that is telecentric on the image space side for the initial image on the dispersive element.Experts in the field of optics recognize that numerous combinations and permutations of optical elements, both microscopic and traditional, can satisfy the desired optical conditions, as explained below in the specifically illustrated preferred embodiments of the device. As shown in Fig. 1, a first preferred device 10 comprises light from an initial imaging optic 12, which is focused to produce an image on a first optical arrangement 14. The first optical arrangement 14 may preferably comprise a stack of thin, parallel plates 14, the surfaces of which are preferably blackened to absorb light and which are spaced apart to allow light to pass between them. The first optical arrangement 14 may include a microlamella array 24, which preferably serves to select and transmit only those rays that propagate in the desired direction and to absorb all other rays. The desired direction for each ray thus selected is such that the component of its propagation vector in the spectral dispersion plane is identical for all rays, regardless of their position within the image.The orthogonal component of the propagation vector from the microlamella array 24 may assume any value, since this component does not influence the subsequent spectral dispersion. The light rays selected in this way from the microlamella array 24 preferably strike a transmission grating 26, which disperses the light into its spectral components. As shown in Fig. 1, a focusing optic 16 preferably collects the spectrally dispersed light and focuses it into a spatially dispersed spectrum in a well-defined spectral plane. Due to the action of the microlamella array 24, light of each wavelength is focused onto a uniquely positioned line in the spectral plane, regardless of its original source in the image. An addressable spatial light modulator 18, in this preferred device 10 a micromirror array 18, is arranged in this spectral plane.The micromirror array 18 preferably serves to switch any desired wavelength band or any set of bands on or off, and to do so for the same wavelengths across the entire two-dimensional image field. As shown in Fig. 1, a final imaging optic 20 preferably collects the light in those wavelength bands transmitted by the micromirror array 18, recombines these wavelength components, and focuses the light onto a detector array 22 to form a final spectrally filtered image. The detector array 22 can comprise a CCD array or another suitable array or system of photodetectors, depending on what is desirable or optimal for the respective light characteristics and conditions. In the first preferred device 10 shown in Fig. 1, an image is positioned in front of the microlamella array 24. The initial grating plane is neither a pupil nor an image, but due to the action of the microlamella array 24, it possesses some properties of both. Preferably, a micromirror array 18 is arranged at a pupil, as far as the spatial image is concerned, such that there is no spatial image on the micromirror array 18, but each wavelength in this plane is focused onto a line, with contributions from all spatial image points for each specific wavelength converging on the same line. This means that each spatial image point is represented as a spectrum on the micromirror array, and these spectra superimpose at suitable wavelengths for all spatial points.The final imaging optics 20 preferably reverse the effect of the optics 14 downstream of the grating by recombining the wavelengths of the individual pixels to form a spectrally manipulated white light image on the detector array 20. The optics 14, 20 can be simple lenses, mirrors, multi-element lens systems, or any suitable combination thereof. As mentioned above, the detector array 20 can include a CCD or another suitable photodetector. In a variation of the first preferred device 10, the microlamella array 24 can be replaced by a blackened capillary array, which preferably suppresses non-collimated rays in both the spectral dispersion region and in the orthogonal direction. In this variation, an image of the scene exists in the transmission grating 26, but the light field incident on the transmission grating 26 is pupil-like, since it consists of parallel beams of rays emerging from the capillary tubes. If the angle of incidence on the grating is the same for all image points, the diffraction angle is solely a function of the wavelength. A lens or other optical element 16, arranged directly behind the transmission grating 26, thus maps each wavelength onto a single line in the spectral plane, regardless of its original spatial position.A micromirror array 18 on this plane then preferentially selects the wavelengths that are transmitted or rejected, and does so in the same way for all pixels. As shown in Fig. 2, a second preferred embodiment of the device 10 comprises a microlens array 30 and a capillary array 32, which direct all incident light at the same angle onto a transmission grating 34. The second preferred device 10 may further comprise a liquid crystal array (LCD array) 36 arranged between two or more crossed polarizers 38 to analyze the spectral information contained therein. At least one final optical system 40 preferably recombines the wavelengths and images a manipulated white light image onto a detector array 42, after which the image can be further analyzed and displayed by a signal processor as described above. In a variation of the second preferred device 10, a microshutter array (not shown) can be used instead of, or in addition to, the liquid crystal array 36 and the associated polarizers 38. As shown in Fig.As shown in Figure 2, the second preferred device can comprise 10 additional optical elements 44, for example lenses, mirrors or mirror arrays, to direct and / or focus the light leaving the transmission grating 34. As shown in Fig. 3, a third preferred embodiment of the device may comprise an aperture array 52, a microlens array 54, and a transmission grating 56. The aperture array 52 and the microlens array 54 preferably function like the capillary array or the microlamella array 24 in the previously described preferred embodiments. The microlens array 54 is preferably coupled to the matching aperture array 52, which is arranged upstream of the microlens array 54 at the focal distance of the microlenses. Preferably, a direction-defining aperture and a small lens can be placed at each pixel position in the initial image plane, directing the emerging light in the required direction, which, as described above, is identical for all pixels. The transmission grating 56 preferably generates a parallel array of independent microspectrometers, one for each pixel.The microlens array 54 can be of any suitable type or configuration and can be made of materials for both the visible and IR ranges. Suitable aperture arrays 52 of virtually any configuration can be fabricated using standard technologies, such as photolithography, laser structuring, or film coating techniques, the aperture spacing preferably corresponding to that of the microlenses. The microapertures in the aperture array can have different shapes, from circular to slit-shaped. For example, a one-dimensional array of slit-shaped apertures combined with microlenses in a rectangular grid offers a desirable combination of tight directional control in the spectral dispersion direction and high transmittance due to the lack of directional restriction in the orthogonal direction, where directional control is not required. As shown in Fig. 3, the third preferred device 10 may further comprise an addressable spatial light modulator as a spectral selector 60, as well as one or more additional optical elements 58, 62 and a detector array 64. Preferably, the addressable spatial light modulator as a spectral selector may comprise a micromirror array or a combination of micromirror arrays, as described elsewhere herein. The additional optical elements 58, 62 can, for example, comprise any suitable combination of lenses, mirrors, mirror arrays, or the like to direct the light in the desired direction. As mentioned above, a suitable detector array 64 can comprise a CCD or other photodetector designed to detect light within the desired spectrum and / or with the desired accuracy. Each of the preferred embodiments of the present invention, described in Figures 1, 2, and 3, comprises one or more micro-optical arrays. As mentioned above, these micro-optical arrays serve, in part, to present light to the spectrally dispersive element at the same angle of incidence for all pixels and to suppress light propagating in other directions.This function can also be achieved with more conventional macroscopic optics by arranging the macroscopic optical elements of the initial imaging segment of the device in a telecentric configuration. Preferred embodiments of such configurations are described below with reference to Figures 4A, 4B, 5, and 6. As shown in Figures 4A (top view) and 4B (side view), a further preferred embodiment of the full-frame, multiband programmable spectral imaging device 100 of the present invention may comprise one or more telecentric optics to achieve the desired image-position-independent constant angle of incidence onto the dispersive element 106, for example, a transmission grating. The fourth preferred embodiment may include a slit-shaped aperture 102, which serves as the entrance pupil of the optical system. Preferably, the aperture 102 may be arranged at a distance of one focal length in front of the first imaging optic 104, a configuration that ensures that the subsequent image is telecentric, i.e., that the principal rays emanating from each field point are parallel to each other.The slit-shaped aperture 102 preferably ensures that only a narrow range of angles around the main beams reaches the transmission grating 106, thus enabling high spectral resolution. Preferably, a large angular range in the orthogonal direction is permitted to allow as much light as possible into the preferred device 100. The scene is preferably imaged in this telecentric manner onto the dispersive element 106, which spectrally disperses the light. At least one second optical element 108 preferably collects this spectrally dispersed light and focuses the spectrum onto a programmable array 110. The programmable array can, for example, comprise a micro-shutter array or an LCD array arranged between crossed polarizers, as described above.Preferably, a spectrally dispersed image of the slit-shaped entrance pupil 102 is focused onto the programmable array 110, with light from all field positions for each wavelength being focused onto a uniquely positioned slit-shaped image of the pupil. As in the previously described preferred embodiments, the programmable array preferably allows desired wavelengths to pass through and blocks others. The fourth preferred device 100 can optionally include at least one final optic 112 that collects the spectrally manipulated light and focuses it onto a detector array 114 to form a final image of the scene. The respective optics 104, 108, 112 can, for example, comprise any suitable combination of lenses, mirrors, or mirror arrays to direct the light in the desired direction. As mentioned above, the detector array 114 can, for example, comprise a CCD array or another type of photodetector. As shown in the figures...As shown in Figures 4A and 4B, each stage of the fourth preferred device 100—the slot 102, the first optic 104, the dispersive element 106, the second optic 108, the programmable array 110, the third optic 112, and the detector array 114—is arranged in a substantially telecentric configuration, with the elements spaced apart from each other by the focal length of the respective optical element. However, those skilled in the art recognize that numerous suitable variations of the fourth preferred device 100 exist in which only parts or segments of the device 100 are arranged in a substantially telecentric manner. For example, only the first section up to the dispersive element 106 can be substantially telecentric, while the remaining optical path can be arranged in any suitable way or configuration that meets the application and / or usage requirements of the fourth preferred device 100. As shown in Fig. 5, a further preferred embodiment of the full-frame, multiband programmable spectral imaging device 100 may include one or more telecentric optics to achieve the desired image-position-independent constant angle of incidence onto the dispersive element 124. The fifth preferred device 100 may further include a micromirror array with diagonally inclined micromirrors (for example, the Texas Instruments DLP® array) as a programmable spectral selector 128. The fifth preferred device 100 may also include two orthogonally dispersive diffraction gratings 130, 132 to compensate for the two-dimensional grating-like spectral dispersion inherent in micromirror-based devices that have diagonally inclined micromirrors as their only selectable positions. A first stage of the fifth preferred device 100 projects a telecentric image of the scene onto the spectrally dispersive element 124, here a transmission grating, using a slit-shaped aperture 120 as the entrance pupil, which is arranged at a distance of approximately one focal length in front of a first optic 122, as in the fourth preferred embodiment. The spectrally dispersed light exiting the dispersive element 124 is preferably collected by at least one second optic 126 and focused onto the micromirror array 128, producing a spectrally dispersed image of the slit-shaped entrance pupil. The second optic 126 is preferably arranged such that this dispersed slit image is telecentrically positioned on the micromirror array 128, so that the central rays, that is, the rays emanating from the center of the field of view, are parallel for each wavelength when they strike the micromirror array 128.The micromirror array 128 reflects and diffractes the selected wavelengths preferably in a direction determined by the micromirror tilt angle and the allowed diffraction orders, which in turn are determined by the microgrid spacing. Thus, the array effectively behaves like a two-dimensional diffraction grating with blaze angle, generating a two-dimensional grid of diffraction orders. To generate a final spectrally manipulated white light image without spectral dispersion, the different diffraction orders are preferably recombined in the fifth preferred device 100, and their spectral dispersion is reversed. Accordingly, the fifth preferred device 100 may comprise two reflection gratings 130, 132, whose grooves are oriented approximately orthogonally to each other and whose groove spacing and blaze angle substantially correspond to the two corresponding dimensions of the micromirror array 128. Preferably, the fifth preferred device 100 may comprise at least one final optical system 134 that collects the recombined and recombined diffraction orders and focuses them onto a final, non-dispersed image on a detector array 136. The optical systems 122, 126, 134 may, for example, comprise any suitable combination of lenses, mirrors, or mirror arrays to direct the light in the desired direction.As mentioned above, the detector array 136 can include a CCD or another suitable photodetector designed for the specific properties of the light to be generated. As shown in Fig. 6, a further preferred embodiment of the full-frame, multiband programmable spectral imaging device 100 of the present invention comprises a first micromirror array 150 for pre-compensating the diffraction characteristics of a second spectral selection micromirror array 158. As described in the preferred embodiments with reference to Figs. 4 and 5, the optics in the sixth preferred device 100 can be arranged in a telecentric configuration. Preferably, a slit-shaped aperture (not shown), which acts as the entrance pupil, can be arranged essentially together with the first micromirror array 150, which is essentially identical in its physical structure to the second micromirror array 158.The combined arrangement of the entrance pupil and the first micromirror array 150 can be achieved either by programming a slit-shaped pattern of "switched-on" micromirrors on the first micromirror array 150, or by placing a physical slit or aperture immediately in front of the first micromirror array 150, with all its mirrors in the "switched-on" position. Alternatively, a fixed, non-programmable two-dimensional diffraction grating (not shown) with the same grid spacing and blaze angle as the first micromirror array 150 can be placed at this location instead of the first micromirror array 150. As shown in Fig. 6, a first optical element 152 is preferably located at a distance of approximately one focal length behind the first micromirror array 150. The first optical element 152 preferably serves to image the scene onto the dispersive element 154, in this example a transmission grating. Preferably, the image is telecentric but diffracted and spectrally dispersed in a manner that is exactly reversed by the second micromirror array 158, which performs the programmable spectral selection. A second optical element 156, which has an essentially identical focal length to the first optical element 152, is preferably located one focal length behind the dispersive element 154. The second optical element 156 preferably collects the spectrally dispersed light and focuses a spectrally dispersed image of the slit-shaped entrance pupil onto the second micromirror array 158.The second optical system 156 is preferably arranged such that the rays for each wavelength arrive at the spectral image on the second micromirror array 158 in such a way that, after the spectral dispersion of the second micromirror array 158, which cancels the dispersion of the first micromirror array 150, the light exiting the second micromirror array 158 is essentially telecentric, with the central ray of each wavelength being essentially parallel to the central ray of all other wavelengths. As in the previous preferred embodiments, light from all field points is focused onto a uniquely positioned slit image for each wavelength, despite the spectral dispersion of the first micromirror array 150. The second micromirror array 158 preferably selects the desired wavelengths and simultaneously cancels the spectral dispersion of the first micromirror array 150.The sixth preferred device 100 can comprise at least one final optic 160 that collects the spectrally selected light and focuses it onto a non-dispersed image on a detector array 162, which can be a CCD or another suitable photodetector. As mentioned above, the optics 152, 156, 160 can, for example, comprise any suitable combination of lenses, mirrors, or mirror arrays to direct the light in the desired direction. An alternative embodiment of a micromirror array-based multiband programmable spectral imaging device can be implemented using a micromirror array whose micromirrors can be oriented flat in the plane of the array as one of their programmable positions, rather than only in two inclined positions. When the micromirrors are brought into the flat orientation, this region of the micromirror array behaves like a simple mirror and not like a diffraction grating, so that no spectral dispersion compensation is required and neither compensating gratings nor a second compensating micromirror array are necessary. In this alternative embodiment of preferred devices 10, 100, the flat micromirror represents the "on" state, and the array is oriented such that this micromirror position reflects the light along the optical path toward the detector.The inclined micromirror position represents the "off" state, where the light is deflected away from the optical path and towards an absorbing aperture or beam dump. This inclined position produces diffraction and associated spectral dispersion, but this is insignificant because the light reflected from this position is not used but instead absorbed. This alternative embodiment offers the advantage of eliminating compensating gratings or micromirror arrays and provides higher optical transmittance because losses due to multiple diffraction orders are eliminated. The preferred embodiments have been described with reference to specific optical components arranged in selected configurations for illustrative purposes. However, variations of the preferred devices 10, 100 may include configurations in which the initial optical element or elements comprise one or more of the following: a telecentric imaging lens, an imaging element and a microlens array, an aperture array optically coupled to the microlens array, an imaging element and a capillary array, or an imaging element and a microlamella array. Further variations include configurations in which the dispersive element comprises one or more of the following: a refractive grating, a transmission grating, a diffraction disperser, an interferometric disperser, or a refractive disperser.Further variations include configurations in which the selective element comprises one or more of the following: a micro-shutter array, an LCD array, a micro-mirror array, a mask, or a slot array. Accordingly, countless permutations of the aforementioned optical components exist, which can be arranged in countless geometric configurations, including essentially telecentric configurations. It will therefore be clear that the foregoing preferred embodiments have been described solely for illustrative purposes and that many more embodiments of the present invention exist that fall within the scope of the appended claims. Example designs Fig. 7 shows a photograph of a working prototype of an embodiment with two micromirror arrays of the full-frame, multiband programmable spectral imaging device according to the preferred embodiment described above with reference to Fig. 6. Fig. 8 shows data from a complete spectrum of a hyperspectral image of a test scene obtained with the apparatus shown in Fig. 7, in which the spectral selection micromirror array was programmed to perform a 127-band implementation of the Hadamard transform technique from 400 nm to 740 nm. On the left of Fig. 8 is a panchromatic image of the test scene, generated by summing over all 127 spectral bands of the hyperspectral data cube. The test scene consists of three light sources: a broadband standard incandescent lamp at the top, a discrete single-line low-pressure sodium vapor lamp in the middle, and a multi-line discrete compact fluorescent lamp (CFL) at the bottom.The spectra shown to the right of the image, presented here in their raw, radiometrically uncalibrated form, were obtained by averaging pixels from small regions of the hyperspectral image, each representative of a specific light source in the scene. The incandescent lamp spectrum shown above exhibits the broadband characteristics of a quasi-blackbody spectrum, but with additional spectral structure attributable to the spectral response functions of both the CCD detector array and the two micromirror arrays. The second spectrum shows the strong single-line spectrum of the sodium vapor lamp. The final spectrum shows the multi-line discrete spectrum of the compact fluorescent lamp, dominated by the typical mercury vapor lines characteristic of most fluorescent lamps. Fig. 9 shows a series of three spectrally filtered images obtained with the device shown in Fig. 7 and illustrates how the programmable multiband spectral filtering capability of the example device can be used to distinguish one target type from another based on its spectral properties and to highlight a particular target type in the real-time image. In all three images, the test scene includes two light sources: a standard broadband incandescent lamp on the left and a multiline discrete compact fluorescent lamp (CFL) on the right. The spectrum of the CFL acquired with the example device is shown in Fig. 8. Both lamps are switched on at full brightness in all three images. The brightness differences in the three images result from the effect of the programmable spectral filter, not from changes in the brightness of the lamps themselves. In the first image (a), the spectral selection micromirror array was programmed to pass through three narrowband wavelength ranges corresponding to the brightest spectral lines of the CFL spectrum. The CFL appears bright in this image because most of its light is emitted in these spectral bands. However, the incandescent lamp also appears relatively bright because its continuous spectrum also contains significant energy in these spectral bands. In the second image (b), the micromirror array was programmed to allow three narrowband spectral ranges to pass through, located in the dark areas between the main emission lines of the CFL. The CFL appears dark in this image because almost all of its discrete spectrum is blocked by the programmable filter. However, the incandescent lamp appears just as bright as in the first image because its continuous spectrum contains comparable energy in this second set of spectral bands. Remarkably, the CFL light was suppressed so effectively by the programmable filter that even with the lamps switched on, the reflection of the incandescent lamp on the surface of the CFL is visible. The third image (c) shows the difference between the first two images. The spectral band widths in the first two images were chosen so that the incandescent lamp, normalized to the sensitivity of the entire example apparatus, has the same integrated energy in both images. In the third difference image, the brightness of the incandescent lamp appears nearly zero, even though the lamp is switched on. In this case, the incandescent lamp is so effectively suppressed that the reflection of the CFL on its surface can be seen. This third image can be considered a background-suppressing, spectrally matched filter image, where the background is the incandescent lamp spectrum and the target is the CFL spectrum. A similar process can be used, for example, in the infrared range to highlight a particular chemical vapor in a scene consisting of a cloud of that chemical vapor against a spectrally complex background. As described in detail above, the principles outlined in general terms in the following claims can be achieved by employing microfabricated two-dimensional microarray optical elements such as the microlens array, the microlamella array, and / or the capillary array, and / or by using telecentric optics to generate a constant angle of incidence onto a dispersive element that is independent of the image position. As mentioned herein, the generation of the desired image allows the wavelengths selected by the micromirror array or other addressable spatial light modulators to be identical across the entire image. With such full-frame capability, the selected spectral bands can be programmed and reprogrammed across the entire image on millisecond timescales, thereby enabling the implementation of background-suppressing matched filters at real-time video rates. Applications of the preferred devices 10, 100 range from rapid broadband searches for military or intelligence targets (reducing the time required to scan a given area by two to three orders of magnitude compared to conventional hyperspectral imagers), real-time medical imaging, spectroscopic monitoring of fast transients, spectrum-based tracking of vehicles and people, and—when coupled with a suitable ultrafast detector—ultrafast spectral imaging on nanosecond timescales. In other applications, the preferred devices 10, 100 can be used as a spectrally tailored filter for any predetermined chemical substance, solid, liquid, gas, or mixture; for any predetermined biological tissue or medical abnormality; for any predetermined mineral type; or for any forensic signature.As an example, the preferred devices 10, 100 can be used as remote sensing sensors for certain types of chemicals with known spectral properties, such as methane, carbon dioxide, carbon monoxide, ozone or any other chemical or by-product of energy production, industry, manufacturing or transportation. Throughout the foregoing description of the invention, the term "light" is to be understood as encompassing electromagnetic radiation of any wavelength, including ultraviolet, visible and infrared wavelengths, and as not being limited to any particular wavelength range.

Claims

An imaging system (10, 100) comprising: a first optical subsystem (24, 32, 54, 104, 122, 152) that receives incident light and produces an image comprising a spatially invariant light propagation defining a substantially spatially invariant angle of incidence at a first predetermined location with an image plane; a dispersive element (26, 34, 56, 106) positioned at the predetermined location to receive the image at the spatially invariant angle of incidence and to produce wavelength-dispersed light; a second optical subsystem (108, 126, 156) that receives the wavelength-dispersed light and produces a spatially dispersed spectrum at a second predetermined location such that, in at least one dimension of a spectral plane, light of a predetermined wavelength is directed to a substantially identical position within the spectral plane;a selective element located at the second predetermined position and optically coupled to the dispersive element (26, 34, 56, 106), wherein the selective element is configured to programmably modulate the amplitude of the light at one or more positions in the spectral plane to generate spectrally modified light; and a detector receiving the spectrally modified light from the selective element, wherein the spectrally modified light comprises an image representation of the incident light in which the spectral content has been modified by the selective element in one or more spectral bands, and wherein such spectral modification is substantially identical for nearly all positions in the image representation. The imaging system (10, 100) according to claim 1, further comprising a display coupled to the detector and configured to display at least part of the multispectral image of the incident light. The imaging system (10, 100) according to claim 1, wherein the first optical subsystem (24, 32, 54, 104, 122, 152) comprises a telecentric imaging lens. The imaging system (10, 100) according to claim 1, wherein the first optical subsystem (24, 32, 54, 104, 122, 152) comprises an imaging element and a microlens array (54). The imaging system (10, 100) according to claim 4, further comprising an aperture array (52) optically coupled to the microlens array (54). The imaging system (10, 100) according to claim 1, wherein the first optical subsystem (24, 32, 54, 104, 122, 152) comprises an imaging element and a capillary array (32). The imaging system (10, 100) according to claim 1, wherein the first optical subsystem (24, 32, 54, 104, 122, 152) comprises an imaging element and a microlamella array (24). The imaging system (10, 100) according to claim 1, wherein the dispersive element (26, 34, 56, 106) comprises a refractive grating, a transmission grating (26, 56), a diffraction disperser, an interferometric disperser or a refractive disperser. The imaging system (10, 100) according to claim 3, wherein the dispersive element (26, 34, 56, 106) comprises a refractive grating, a transmission grating (26, 56), a diffraction disperser, an interferometric disperser or a refractive disperser. The imaging system (10, 100) according to claim 4, wherein the dispersive element (26, 34, 56, 106) comprises a refractive grating, a transmission grating (26, 56), a diffraction disperser, an interferometric disperser or a refractive disperser. The imaging system (10, 100) according to claim 6, wherein the dispersive element (26, 34, 56, 106) comprises a refractive grating, a transmission grating (26, 56), a diffraction disperser, an interferometric disperser or a refractive disperser. The imaging system (10, 100) according to claim 7, wherein the dispersive element (26, 34, 56, 106) comprises a refractive grating, a transmission grating (26, 56), a diffraction disperser, an interferometric disperser or a refractive disperser. The imaging system (10, 100) according to claim 1, wherein the selective element comprises a micro-shutter array, an LCD array (36), or a micro-mirror array (150). The imaging system (10, 100) according to claim 1, further comprising an entrance pupil (102) arranged between the first optical subsystem (24, 32, 54, 104, 122, 152) and a light source of the incident light. The imaging system (10, 100) according to claim 14, wherein the entrance pupil (102) comprises a slit-shaped entrance pupil (102), a micromirror array (150) coupled to a physical slit-shaped entrance pupil (102), or a micromirror array (150) programmed to simulate a slit-shaped entrance pupil (102). The imaging system (10, 100) according to claim 1, wherein the selective element comprises a programmable micromirror array (150), and further comprising a second programmable micromirror array (150) arranged between the first optical subsystem (24, 32, 54, 104, 122, 152) and a light source of the incident light. The imaging system (10, 100) according to claim 1, further comprising a controller coupled to the detector and configured to perform hyperspectral image processing on the multispectral image representation of the incident light. The imaging system (10, 100) according to claim 17, wherein the controller is integrated in the detector. The imaging system (10, 100) according to claim 1, wherein the selective element is programmable to perform hyperspectral imaging using the Hadamard transformation technique. The imaging system (10, 100) according to claim 1, wherein the selective element is programmable to serve as a spectrally adapted filter for a predetermined chemical substance, solid, liquid, gas or mixture. The imaging system (10, 100) according to claim 1, wherein the selective element is programmable to serve as a spectrally adapted filter for a predetermined biological tissue or medical abnormality. The imaging system (10, 100) according to claim 1, wherein the selective element is programmable to serve as a spectrally adapted filter for a predetermined mineral type. The imaging system (10, 100) according to claim 1, wherein the selective element is programmable to serve as a spectrally adapted filter for a forensic signature.

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