Abstract:
An imaging system having a programmable spectral transmission function, that includes an input image plane for passing input imaging light into the imaging system; a dispersive optical system for separating the input imaging light into its corresponding spectral components, thus creating spectrally-dispersed image components along a spectrally-dispersed direction. Also included is a spatial light modulator, having a plurality of operational states, for selecting spectral components for imaging; and having a width along the spectrally-dispersed direction; a de-dispersive optical system for re-combining the selected spectral components for imaging onto a detector array; and means for scanning the input imaging light from an object of interest to generate an output area image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Reference is made to commonly assigned copending provisional application Ser. No. 60/590,131, entitled “Programmable Spectral Imaging System” and filed on 23 Jul. 2004 in the names of Marek W. Kowarz, James G. Phalen, and J. Daniel Newman, which are assigned to the assignee of this application. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to an imaging system with a spectral transmission function that is electronically controllable. More specifically, this invention relates to a programmable spectral imaging system that contains a spatial light modulator for selecting spectral components for imaging.  
       BACKGROUND OF THE INVENTION  
       [0003]     Multi-spectral and hyper-spectral imaging systems, where more than 3 colors are captured by an imager, are of interest for a variety of remote sensing, health, and industrial applications. Multi-spectral imaging (MSI) systems are typically designed with thin-film spectral filters situated in front of detector arrays. MSI systems provide excellent image quality with short detector integration times and modest data sets. They are used in both line-scanned configurations, with linear detector arrays, and full-field configurations, with area detector arrays. The MSI approach can only image a relatively small number of spectral bands, typically 4 to at most 8, and these spectral bands are fixed once the system is built. Hyper-spectral imaging (HSI) systems, on the other hand, use a grating or prism to disperse the various image wavelengths onto an area detector array, providing one spatial axis and one spectral axis. HSI systems are therefore flexible and capable of capturing a vast amount of spectral information, in many very narrow spectral bands. However, compared to MSI systems, the integration times are much longer, the hyper-spectral data sets are extremely large and the spatial resolution is lower. Furthermore, in practice, only a small subset of the captured hyper-spectral data cube is of interest.  
         [0004]     Programmable spectrometers and programmable spectral imaging systems, where the spectral transmission function can be modified, have been proposed and implemented using electrically-controlled light modulators in two fundamentally different system configurations: 1) tunable transmissive filters based on liquid crystal devices, acousto-optic devices, or tunable Fabry-Perot cavities and 2) programmable dispersion-based systems with a spatial light modulator consisting of an array of individually addressable devices.  
         [0005]     Liquid crystal tunable filters have been disclosed by Miller in U.S. Pat. No. 5,689,317, issued on Nov. 18, 1997, entitled “TUNABLE COLOR FILTER” and by Miller et al. in U.S. Pat. No. 5,892,612, issued Apr. 6, 1999, entitled “TUNABLE OPTICAL FILTER WITH WHITE STATE.” However, liquid crystals are relatively slow (typically, 10-100 msec), have low efficiency and are not space compatible. A programmable spectral imaging system with an acousto-optic tunable filter has been disclosed by Bellus et al. in U.S. Pat. No. 5,828,451, issued Oct. 27, 1998, entitled “SPECTRAL IMAGING SYSTEM AND METHOD EMPLOYING AN ACOUSTO-OPTIC TUNABLE FILTER FOR WAVELENGTH SELECTION WITH INCREASED FIELD OF VIEW BRIGHTNESS.” In general, the tunable filter approach can only pass a single spectral band at a time to the detector array and the spectral bandwidth is predetermined and not tunable.  
         [0006]     Programmable dispersion-based systems have been demonstrated using a variety of spatial light modulators, including liquid crystal display panels and, more recently, micro-electromechanical mirror arrays such as the Texas Instruments DMD. In the dispersion-based approach, the input light is sent through a prism or grating to separate the various wavelengths onto the spatial light modulator. The modulator then selects the wavelengths of interest, which are sent to the detector. Most of these dispersion-based systems have been non-imaging or point imaging, utilizing only a single detector element. In a single detector configuration, a 2D image can be generated by raster scanning an object of interest by using, for example, a pair of scanning mirrors.  
         [0007]     A non-imaging DMD-based spectrometer for sample analysis is described by R. A. DeVerse et al., “Realization of the Hadamard Multiplex Advantage Using a Programmable Optical Mask in a Dispersive Flat-Field Near-Infrared Spectrometer,” Appl. Spectrosc. 54, pp. 1751-1758 (2000). They show that, when many narrow spectral bands are of interest, signal-to-noise performance can be improved by simultaneously measuring multiple bands using the Hadamard transform approach, rather than measuring the spectral bands sequentially.  
         [0008]     An area-imaging programmable spectral imager that does not contain any moving components has been described by C. M. Wehlburg et al., “Optimization and Characterization of an Imaging Hadamard Spectrometer,” Proc. SPIE 4381, pp. 506-515 (2001). Their Hadamard transform spectral imager (HTSI) is a double Offner system with an area detector array. It uses one Offner relay with a curved grating to disperse and reimage the input light onto a DMD. A second Offner relay then de-disperses and reimages the selected components onto the detector array. There are a number of issues with this configuration. 1) The DMD design is not well suited for an Offner system because the DMD is designed for off-axis illumination. With proper illumination and on-axis collection optics, the DMD produces high contrast modulation. However, in the double Offner configuration, the illumination and collection conditions are not satisfied and the DMD contrast would be low. The imaging performance of the double Offner with a DMD is also questionable because of the tilted image planes associated with the tilt angles of the micro-electromechanical mirrors. 2) The curved grating in the Offner is difficult to fabricate and very difficult to blaze for high diffraction efficiency. Therefore, Offner spectrometers have low efficiency and a double Offner spectrometer would have very low efficiency. 3) The HTSI system is designed for imaging of relatively small objects, i.e., objects that are small compared to the size of the detector array. If the HTSI system were used for extended objects, the spectral cross-talk between various parts of the object would be severe. For example, if the object fills (or overfills) the HTSI detector array, a single wavelength image of the object would fill approximately 1 of the DMD. Under this condition, spectral cross-talk in the HTSI would prevent capture of a simple 3-color image.  
         [0009]     Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued on Oct. 23, 2001, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.” Display systems based on a linear array of GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,” issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,678,085, entitled “HIGH-CONTRAST DISPLAY SYSTEM WITH SCANNED CONFORMAL GRATING DEVICE,” issued Jan. 13, 2004. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of &#39;663 has more recently become known as the conformal GEMS device or, more simply, GEMS device, with GEMS standing for grating electromechanical system. The GEMS device possesses a number of attractive features. It has a large active region that provides high-speed digital light modulation with high contrast and good efficiency for on-axis illumination. In addition, the device can be fabricated at low cost with relatively few masks in a CMOS or CCD foundry.  
         [0010]     There is a need therefore for a high-resolution imaging system that has a programmable spectral transmission function and can rapidly image large extended objects. There is also a need for a programmable spectral imaging system that provides good spectral selectivity with high efficiency over a large spectral range. There is a further need for a programmable spectral imaging system that can employ electromechanical grating devices.  
       SUMMARY OF THE INVENTION  
       [0011]     The aforementioned need is met according to the present invention by providing an imaging system having a programmable spectral transmission function, that includes an imaging system having a programmable spectral transmission function, that includes an input image plane for passing input imaging light into the imaging system; a dispersive optical system for separating the input imaging light into its corresponding spectral components, thus creating spectrally-dispersed image components along a spectrally-dispersed direction. Also included is a spatial light modulator, having a plurality of operational states, for selecting spectral components for imaging; and having a width along the spectrally-dispersed direction; a de-dispersive optical system for re-combining the selected spectral components for imaging onto a detector array; and means for scanning the input imaging light from an object of interest to generate an output area image.  
         [0012]     Another aspect of the present invention is a method for employing a spatial light modulator to select spectral components for line-scan imaging, including the steps of: 
        a) passing input imaging light into an imaging system;     b) separating the input imaging light into its corresponding spectral components, thus creating spectrally-dispersed image components;     c) selecting spectral components for imaging using the spatial light modulator;     d) directing the selected spectral components for imaging onto a detector array; and     e) scanning the input imaging light from an object of interest to generate an output area image.        
 
         [0018]     A third aspect of the present invention is a two-arm imaging system having a programmable spectral transmission function, that includes an input image plane for passing input imaging light into the imaging system; a first arm, comprising: a first dispersive optical system for separating the input imaging light into its corresponding spectral components, thus creating spectrally-dispersed image components; a first spatial light modulator, having a plurality of operational states, for selecting first spectral components for imaging; a first de-dispersive optical system for re-combining the selected first spectral components for imaging onto at least one detector array. A second arm, comprises: a second dispersive optical system for separating the input imaging light into its corresponding spectral components, thus creating spectrally-dispersed image components; a second spatial light modulator, having a plurality of operational states, for selecting second spectral components for imaging; a second de-dispersive optical system for re-combining the second selected spectral components for imaging onto the at least one detector array; and a light separating element for directing the input imaging light toward the first and second arms. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1   a  illustrates the basic architecture of a programmable spectral imaging system in a transmissive configuration.  
         [0020]      FIG. 1   b  depicts a portion of the programmable spectral imaging system of  FIG. 1   a , with the projected monochromatic image of the detector array shown on the spatial light modulator.  
         [0021]      FIG. 1   c  illustrates the subsystem that scans input imaging light from an extended object.  
         [0022]      FIG. 2  is an embodiment with a diffractive spatial light modulator.  
         [0023]      FIG. 3   a  is an embodiment in a double-pass reflective configuration, based on an Ebert imaging spectrometer.  
         [0024]      FIG. 3   b  depicts spectrally dispersed images on a spatial light modulator that is a micro-electromechanical grating device.  
         [0025]      FIG. 4   a  is an embodiment with transmissive optical components that combines good imaging performance with high efficiency.  
         [0026]      FIG. 4   b  illustrates the patterned mirror of the embodiment in  FIG. 4   a.    
         [0027]      FIG. 5   a  illustrates a two-arm programmable spectral imaging system with a single detector array.  
         [0028]      FIG. 5   b  illustrates the patterned dichroic of the embodiment in  FIG. 5   a.    
         [0029]      FIG. 5   c  illustrates a two-arm programmable spectral imaging system with four detector arrays.  
         [0030]      FIGS. 6   a - 6   c  show examples of the spectral transmission function when the programmable spectral imaging system is used to captures a single spectral band at a time.  
         [0031]      FIG. 7  shows an example of the spectral transmission function when the programmable spectral imaging system captures a set of spectral bands simultaneously.  
         [0032]      FIG. 8  illustrates a single arm embodiment with two separated input images and two corresponding detector arrays. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]      FIGS. 1   a ,  1   b  and  l  c illustrate the basic principles of the programmable spectral imaging system  50 , shown here in a transmissive configuration. A dispersive imaging subsystem  42   a  reimages the spectral components of an input multi-wavelength image  40 , producing spectrally-dispersed images  52  in an intermediate image plane. In  FIG. 1   a , five distinct spectrally-dispersed images  52  are shown with wavelengths λ 1 , λ 2 , λ 3 , λ 4  and λ 5 . More typically, a continuous distribution of spectrally-dispersed images  52  would be formed. The intermediate image plane contains a spatial light modulator  48  that is controlled electronically to pass desired spectral components through the system and block undesired ones. Unblocked spectral components are reimaged by a de-dispersive imaging subsystem  42   b , providing a spectrally-filtered output image  54  on the detector array  56 . In  FIG. 1   a , shaded regions on the spatial light modulator  48  indicate a blocking operating state  51   a  that prevents unwanted spectral components from reaching the detector array  56 . Regions that are not shaded indicate an unblocking operating state  51   b  that allows the unblocked spectral components to be imaged.  
         [0034]     As illustrated in  FIG. 1   a , the dispersive imaging subsystem  42   a  contains lenses  44   a  and  45   a  and a transmission grating  46   a . Lens  44   a  produces a Fourier transform of the input multi-wavelength image  40  near the transmission grating  46   a , which generates spectral dispersion by diffracting the various wavelengths at different angles. For a high-efficiency system, a blazed or holographic volume phase grating is preferred. As is well-known in the art, dispersion could be also be generated by other components, such as prisms or reflection gratings. Lens  45   a  takes another Fourier transform, generating the spectrally dispersed images  52 . The dispersive and de-dispersive imaging subsystem,  42   a  and  42   b , are similar and, as will be described later, could be the same physical system. In the de-dispersive imaging subsystem  42   b , which contains lenses  44   b  and  45   b , the function of transmission grating  46   b  is to remove the angular difference between the various wavelengths. Ideally, the spectrally-filtered output image  54  is then a perfect image of the multi-wavelength image  40  with a programmable spectral transmission function determined by the operating states of the various regions on the spatial light modulator  48 .  
         [0035]     The programmable spectral imaging system  50  concept of  FIG. 1   a  can be adapted for different types of spatial light modulators. For example, those skilled in the art will recognize that, with the addition of a pair of polarizers, the spatial light modulator  48  could be a transmissive LCD panel. The system can also be adapted for other modulator technologies, such as liquid crystal on silicon (LCOS), DMD or other micromirror arrays, or diffractive spatial light modulators. Embodiments of the programmable spectral imaging system  50  in  FIG. 2  through  FIG. 5   c  illustrate the invention for the case of a diffractive spatial light modulator.  
         [0036]     The programmable spectral imaging system  50  is intended to work with a variety of objects, including large extended objects such as those found in satellite remote sensing applications. In order to image and discriminate different spectral bands for such extended objects, the spectrally-dispersed images  52  need to be sufficiently separated on the spatial light modulator  48 . For example, in order to capture a simple 3 band RGB image of an extended object, assuming that the object fills or nearly fills the detector array, the width w D  of the projected monochromatic image of the detector array  56  on the spatial light modulator  48  should be less than approximately ⅓ of the width w S  of the spatial light modulator  48 , in the spectrally-dispersed direction.  FIG. 1   b  depicts the projected monochromatic image  57  on the spatial light modulator  48  for the system of  FIG. 1   a . In most cases, it is desirable to have a system capable of capturing and clearly resolving significantly more than 3 distinct spectral bands, which requires w D &lt;&lt;w S .  
         [0037]     In order to selectively capture many spectral bands without significant spectral crosstalk, the programmable spectral imaging system  50  is preferably configured in a line-scanned mode, as depicted in  FIG. 1   c . An area output image of an extended object  72  is then captured one line at a time, or a few lines at a time, with some means for scanning the input imaging light from the extended object  72 . In  FIG. 1   c , line-scanning is performed by a scanning subsystem  70 , consisting of a lens  74 , which collects the input imaging light and creates the input multi-wavelength image  40 , and a scanning mirror  77 . It should be noted that the view in  FIG. 1   c  is rotated 90 degrees with respect to the view in  FIG. 1   a . As is well known in the art, there are many different means for line scanning. The scanning mirror  77  could be an oscillating galvanometer, a rotating polygon or a MEMS-based mirror. Alternatively, a moving lens or other moving optical component could be used. In a system with a moving image capture platform, such as a remote sensing satellite or aircraft, the physical motion of the platform could simply perform the scanning. For systems such as microscopes, the object could also be physically translated.  
         [0038]     The detector array  56  in  FIG. 1   a  can be a linear array, a multi-linear array, a time-delayed-integration (TDI) linear array or an area array. For visible and near-IR wavelengths, the preferred detector arrays are CCD or CMOS image sensors similar to those commonly found in digital cameras and document scanners because of their high performance and low cost. When the programmable spectral imaging system  50  is configured in a line-scanned mode for use in a low signal environment, the detector array is preferably a time-delayed integration linear array, or an area array with off-chip processing used to perform time-delayed integration. The time-delayed integration is then temporally synchronized with the line scanning.  
         [0039]      FIG. 2  depicts an embodiment of the programmable spectral imaging system  50  with a spatial light modulator  48  that modulates light by switching between a diffractive transmissive state with grating period Λ and a non-diffractive transmissive state. The grating period Λ is oriented parallel to the dispersion direction. In this configuration, the spatial light modulator  48  could be, for example, an electro-optic or acousto-optic device. In the Fourier plane of the de-dispersive imaging subsystem  42   b , the 0 th  order undiffracted light is physically separated from the diffracted light. A stop  47  is used to block the undiffracted light. Therefore, in  FIG. 2 , the blocking operating state  51   a  corresponds to the non-dffractive transmissive state and the unblocking operating state  51   b  to the diffractive transmissive state.  
         [0040]      FIG. 3   a  shows an embodiment of the present invention in a double-pass reflective configuration, based on an Ebert imaging spectrometer. A single primary mirror  44  together with a reflection grating  46  perform the function of both the dispersive and de-dispersive imaging subsystems. To obtain high system efficiency, the reflection grating  46  should be blazed. The rays shown in  FIG. 3   a  correspond to the case when the spatial light modulator  48  is a micro-electromechanical grating device, preferably a GEMS device. Input imaging light  62  from the input multi-wavelength image  40  is focused by the primary mirror  44 , which produces a Fourier transform of the input at the reflection grating  46 . After dispersion by the grating  46 , the primary mirror  44  takes another Fourier transform, generating the spectrally dispersed images  52  on the spatial light modulator  48 , as illustrated in  FIG. 3   b  for the case of a micro-electromechanical grating device. The various regions on the micro-electromechanical grating device generate 0 th  order undiffracted light and diffracted light depending on their operating state. Both the undiffracted and the diffracted light is then Fourier transformed by the primary mirror  44  and de-dispersed by the reflection grating  46 . The undiffracted 0 th  order light is blocked by a stop  47  near the reflection grating  46 . The diffracted light is again Fourier transformed by the primary mirror  44 , which then produces an image on the detector array  56 . A small tilt on the micro-electromechanical grating device permits the diffracted light to be picked off by a turning mirror  63  in the optical path prior to the detector array  56 . The small tilt also allows the 0 th  order stop  47  to block the undiffracted light without obstructing the input light. As illustrated in  FIG. 3   b , for simplified manufacturing, the area of the micro-electromechanical grating device can be configured in a series of parallel modulator elements  49 , each driven by its own input signal. Such a configuration eliminates the need for an active matrix backplane. For the case of a GEMS device, each parallel modulator element  49  would consist of a large number of electromechanical ribbons that are all electrically interconnected.  
         [0041]     As is well known in the art, an Offner imaging spectrometer with a convex reflection grating produces significantly better imaging performance than an Ebert imaging spectrometer with a flat reflection grating. The programmable spectral imaging system  50  of  FIG. 3   a  can be designed in an Offner configuration with a curved grating to provide higher performance imaging. However, up to now, Offner designs have had relatively low efficiency because of the fabrication difficulty associated with the blazing of a convex reflection grating. Even unblazed convex reflection gratings are difficult to fabricate. Without blazing, the efficiency of programmable spectral imaging system  50  with an Offner design would be very low, because of the double-pass configuration.  
         [0042]      FIG. 4   a  illustrates an improved embodiment with transmissive optical components that combines good imaging performance with high efficiency. The grating is preferably a holographic volume phase grating  64  that has very high diffraction efficiency over a relatively wide spectral range. Input imaging light from the input multi-wavelength image  40  is Fourier transformed by lens  44   a  and passes through an opening in patterned mirror  65 . As shown in  FIG. 4   b , the patterned mirror  65  consists of a transmissive region  65   a  surrounded by reflective regions  65   b . The input imaging light is then sent into a dispersive arm  68  that is used in a double-pass configuration to perform dispersion, de-dispersion and selection of the spectral components of interest. In the dispersive arm  68 , the input wavelengths are dispersed by the holographic volume phase grating  64  and Fourier transformed by lens  60 , producing spectrally dispersed images  52  on the spatial light modulator  48 . The operating state of the various regions on the spatial light modulator  48  determines which spectral components are re-imaged onto the detector array  56 . Light returning from the spatial light modulator  48  is Fourier transformed by lens  60  and de-dispersed by the holographic volume phase grating  64 . The spectral components selected for re-imaging are reflected by the reflective regions  65   b  on the planar patterned mirror  65  and are Fourier transformed by lens  45   b , producing an image on the detector array  56 .  
         [0043]     As illustrated in  FIG. 4   a , the three lenses,  44   a ,  45   b  and  60 , have the same focal lengths, providing unity magnification between the input multi-wavelength image  40 , the spectrally dispersed images  52  and the image on the detector array  56 . However, it will be apparent to those skilled in the art that the programmable spectral imaging system  50  could have non-unity magnification or even anamorphic magnification between any of the image planes. For example, a system could be designed with anamorphic magnification in lens  45   b  to change the F/# of light incident on the spatial light modulator  48 , while still providing unity magnification between the input multi-wavelength image  40  and the image on the detector array  56 .  
         [0044]     The design of the patterned mirror  65  depends on the specific type of spatial light modulator  48  in the programmable spectral imaging system  50 .  FIG. 4   b , as drawn, corresponds to the case when the spatial light modulator  48  is a GEMS device. The operating state of the GEMS device determines the spectral components that are imaged on the detector array  56 . 0 th  order undiffracted light, which contains unwanted spectral components, passes through the transmissive region  65   a  and is not used. Any residual 0 th  order light can be eliminated with an additional stop placed between patterned mirror  65  and lens  45   b . The diffracted orders (+2 nd , +1 st , −1 st , −2 nd  and higher orders), which contain desired spectral components, are reflected by the reflective regions  65   b  and are imaged on the detector array  56 .  
         [0045]     The programmable spectral imaging system  50  can be used to capture individual spectral bands in a time sequential fashion. For example, a spectral band centered at wavelength of 500 nm could be captured, followed by one centered at 600 nm, then by one centered at a 700 nm. This sequential approach is depicted in the idealized spectral transmission graphs of  FIGS. 6   a ,  6   b  and  6   c . Alternatively, the programmable spectral imaging system  50  can simultaneously capture wavelengths from a set of spectral bands, as illustrated in  FIG. 7 . The set of spectral bands can then be dynamically modified. This second approach, in which a set of spectral bands is imaged on the detector array  56 , can be advantageous when seeking a specific spectral signature. It can also provide a higher signal-to-noise ratio, when many spectral bands are of interest, through the use of the Hadamard transform. In this approach, when images from individual spectral bands are of interest, instead of sequentially capturing the individual spectral bands on the detector array  56 , sets of spectral bands are sequentially captured. The spectral images of interest are then reconstructed from the captured data sets using the Hadamard transform. The construction and selection of the sets of spectral bands is determined by the well-known S-matrices.  
         [0046]     In above embodiments of the programmable spectral imaging system  50 , the useful spectral range is limited by the roll-off in diffraction efficiency of the grating, used for dispersion and de-dispersion, and by any roll-off in efficiency of the spatial light modulator. As illustrated in  FIG. 5   a , an alternate embodiment consisting of a two-arm programmable spectral imaging system  80  can be used to provide increased spectral range compared to a single arm system. For example, a two-arm system with good efficiency could be designed to have a visible arm  68   v , covering wavelengths between 440 nm and 660 nm, and a near-IR arm  68   i , covering wavelengths between 660 nm and 1000 nm. The components of the visible arm  68   v , i.e., spatial light modulator  48   v , lens  60   v  and holographic volume phase grating  64   v , can be optimized to operate in the visible range. The near-IR arm  68   i , containing spatial light modulator  48   i , lens  60   i  and holographic volume phase grating  64   i , can be optimized independently for near-IR operation.  
         [0047]     A patterned dichroic  66 , depicted in  FIG. 5   b  for the case of GEMS spatial light modulators, is used to separate light into the arms of the two-arm programmable spectral imaging system  80 . The patterned dichroic  66  can contain three different dichroic coatings: dichroic region  66   a , which transmits in the visible and reflects in the near-IR, dichroic region  66   b , which reflects in both the visible and near-IR, and dichroic region  66   c , which reflects in the visible and transmits in the near-IR. Clean-up filters (not shown) are used to eliminate IR light from the visible arm  68   v  and visible light form the near-IR arm  68   i . Visible input light is transmitted into the visible arm  68   v  by dichroic region  66   a , whereas near-IR input light is reflected into the near-IR arm  68   i  by both dichroic region  66   a  and dichroic region  66   b . 0 th  order undiffracted light returning from the two arms is recombined by dichroic regions  66   a  and  66   b  and is sent back towards the input multi-wavelength image  40 . Diffracted orders from the visible arm  68   v  are reflected towards the detector array  56  by dichroic regions  66   b  and  66   c . On the other hand, diffracted orders from the near-IR arm  68   i  are transmitted towards the detector array  56  through dichroic region  66   c.    
         [0048]     It will be apparent to those skilled in the art that the two-arm programmable spectral imaging system  80  could be designed to have both arms covering the same spectral range. Such a design can provide better overall light throughput than a single arm system. A simple patterned mirror  65 , similar to the one of  FIG. 4   b  or one with a more complex pattern, can then replace the patterned dichroic  66   
         [0049]     The above embodiments enable imaging of a single spectral band, or of a single set of spectral bands, at one time.  FIG. 5   c  depicts a two-arm programmable spectral imaging system  80  that provides simultaneous imaging of 4 spectral bands, or  4  sets of spectral bands, by 4 detector arrays,  56   a ,  56   b ,  56   c  and  56   d . A dichroic beamsplitter  67  splits the spectrum between two pairs of detector arrays. For example, for a two-arm programmable spectral imaging system  80  designed for wavelengths from 440 nm to 1000 nm, the dichroic beamsplitter  67  could pass wavelengths below 550 nm and above 830 nm and transmit those in between. In addition, the near-IR arm  68   i  can be rotated to provide the needed physical separation between the light paths to detector array  56   a  and to detector array  56   b  and between the light paths to detector array  56   c  and detector array  56   d . This specific embodiment would then provide simultaneous imaging of 4 different sets of wavelengths on 4 detector arrays: from 440 nm to 550 nm on detector array  56   a , from 830 nm to 1000 nm on detector array  56   b , from 550 nm to 660 nm on detector array  56   c  and from 660 nm to 830 nm on detector array  56   d.    
         [0050]      FIG. 8  illustrates an alternate approach for imaging onto multiple detector arrays. The programmable spectral imaging system  90  is similar to the system of  FIG. 4   a , except it contains two separated input multi-wavelength images,  40   a  and  40   b , with two corresponding detector arrays,  56   a  and  56   b . This system provides simultaneous capture of two spectrally-filtered output images,  54   a  and  54   b , that have different spectral bands. The separation (in wavelength) between the two spectral bands is fixed, but the center wavelength and bandwidth can be programmed jointly.  
         [0051]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
       Parts List  
       [0000]    
       
           40  Multi-wavelength image  
           40   a  Multi-wavelength image  
           40   b  Multi-wavelength image  
           42   a  Dispersive imaging subsystem  
           42   b  De-dispersive imaging subsystem  
           44  Primary mirror  
           44   a  Lens  
           44   b  Spectrally-filtered output image  
           45   a  Lens  
           45   b  Lens  
           46  Reflection grating  
           46   a  Transmission grating  
           46   b  Transmission grating  
           47  Stop  
           48  Spatial light modulator  
           48   i  Spatial light modulator  
           48   v  Spatial light modulator  
           49  Parallel modulator elements  
           50  Programmable spectral imaging system  
           51   a  Blocking operating state  
           51   b  Unblocking operating state  
           52  Spectrally-dispersed images  
           54  Spectrally-filtered output image  
           54   a  Spectrally-filtered output image  
           54   b  Spectrally-filtered output image  
           56  Detector array  
           56   a  Detector array  
           56   b  Detector array  
           56   c  Detector array  
           56   d  Detector array  
           57  Projected monochromatic image  
           60  Lens  
           60   i  Lens  
           60   v  Lens  
           62  Input imaging light  
           63  Turning mirror  
           64  Holographic volume phase grating  
           64   i  Holographic volume phase grating  
           64   v  Holographic volume phase grating  
           65  Patterned mirror  
           65   a  Transmissive region  
           65   b  Reflective region  
           66  Patterned dichroic  
           66   a  Dichroic region  
           66   b  Dichroic region  
           66   c  Dichroic region  
           67  Dichroic beamsplitter  
           68  Dispersive arm  
           68   i  Near-IR arm  
           68   v  Visible arm  
           70  Scanning subsystem  
           72  Extended object  
           74  Lens  
           77  Scanning mirror  
           80  Two-arm programmable spectral imaging system  
           90  Programmable spectral imaging system