Abstract:
A wide field catadioptric imaging spectrometer with an immersive diffraction grating that compensates optical distortions. The catadioptric design has zero Petzval field curvature. The imaging spectrometer comprises an entrance slit for transmitting light, a system with a catadioptric lens and a dioptric lens for receiving the light and directing the light, an immersion grating, and a detector array. The entrance slit, the system for receiving the light, the immersion grating, and the detector array are positioned wherein the entrance slit transmits light to the system for receiving the light and the system for receiving the light directs the light to the immersion grating and the immersion grating receives the light and directs the light through the system for receiving the light to the detector array.

Description:
[0001]     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]     This application has new improved designs, smaller sizes with wider fields of view compared with those previously given in Patent Application Publication US 2005/0073680 A1, “Imaging Spectrometer Utilizing Immersed Gratings with Accessible Entrance Slit,” Chrisp et al publication date Apr. 7, 2005.  
       BACKGROUND  
       [0003]     1. Field of Endeavor  
         [0004]     The present invention relates to a spectrometer and more particularly to a compact catadioptric imaging spectrometer designed for a wide field of view with larger format detectors.  
         [0005]     2. State of Technology  
         [0006]     U.S. Pat. No. 5,717,487 provides the following state of technology information: “A spectrometer is a known instrument for examining the spectral characteristics of light. Light emitted from or reflected by an object is received within the spectrometer and separated into its spectral components, such as the red, green and blue colored spectra as occurs in equal intensity when standard white light is so analyzed. The intensity of each such spectral component of that received light may be readily observed and measured. Each element of nature, molecular components, organic and inorganic compounds, living plants, man, animal and other substances is known to emit a unique spectrum that may be used as an indicium to identify the emitter.” 
         [0007]     United States Patent Application Serial No. 2002/0135770 published Sep. 26, 2003 provides the following state of technology information: “Imaging spectrometers have been applied to a variety of disciplines, such as the detection of defects in industrial processes, satellite imaging, and laboratory research. These instruments detect radiation from a sample and process the resulting signal to obtain and present an image of the sample that includes spectral and chemical information about the sample.” 
         [0008]     U.S. Pat. No. 6,078,048 provides the following state of technology information: “In recent years substantial effort has been directed to the problem of detection of airborne chemicals. The remote detection of airborne chemicals issuing from exhaust stacks, vehicle exhaust, and various exhaust flumes or plumes, offers a non-intrusive means for detecting, monitoring, and attributing pollution source terms. To detect, identify, and quantify a chemical effluent, it is highly desirable to operate at the limiting spectral resolution set by atmospheric pressure broadening at approximately 0.1 cm −1  This provides for maximum sensitivity to simple molecules with the narrowest spectral features, allows for corrections for the presence of atmospheric constituents, maximizing species selectivity, and provides greater opportunity to detect unanticipated species.” 
         [0009]     U.S. Pat. No. 5,880,834 provides the following state of technology information: “There are three problems in designing an imaging spectrometer where light in a slice of an image field passing through an entrance slit is to be diffracted by a grating parallel to the slit and imaged onto a focal plane for display or recording with good spatial resolution parallel to the slit and good spectral resolution perpendicular to the slit: 1. Eliminating astigmatism over the spectrum on the image plane. 2. Removing field curvature from the spectrum focused onto the image plane. 3. Obtaining good spatial resolution of the entrance slit which involves eliminating astigmatism at different field angles from points on the entrance slit.” 
       SUMMARY  
       [0010]     Features and advantages of the present invention will become apparent from the following description. Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.  
         [0011]     The present invention provides a compact imaging spectrometer with an immersive or reflective diffraction grating that compensates optical distortions. The imaging spectrometer comprises an entrance slit for transmitting light, a system with a catadioptric lens and a dioptric lens for receiving the light and directing the light, an immersion grating, and a detector array. The entrance slit, the system for receiving the light, the immersion grating, and the detector array are positioned wherein the entrance slit transmits light to the system for receiving the light and the system for receiving the light directs the light to the immersion grating and the immersion grating receives the light and directs the light through the system for receiving the light to the detector array.  
         [0012]     The compact imaging spectrometer uses smaller cryogenic coolers facilitating its using in portable (man carried) gas detection systems and in small unmanned aerial vehicles for remote gas detection. These instruments have application for Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. These instruments can be used for pollution detection, and remote sensing of agricultural crops, and geological identification. They can also be used for the remote monitoring of industrial processes. The wider field of view of this design enables larger swath widths for the remote sensing of larger areas with single pass overflights and is extensible to take advantage of larger format or mosaiced infrared detector arrays.  
         [0013]     The invention is susceptible to modifications and alternative forms. Specific embodiments are shown by way of example. It is to be understood that the invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. This invention can be adapted to different spectral regions by suitable choice of refractive and reflective materials, and with different detector arrays. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate specific embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the specific embodiments, serve to explain the principles of the invention.  
         [0015]      FIG. 1  illustrates an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and dioptric lens to provide the correction over a detector array with a format of 512 spatial pixels by 256 spectral pixels.  
         [0016]      FIG. 2  is an isometric view of an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and dioptric lens to provide the correction over a detector array with a format of 512 spatial pixels by 256 spectral pixels.  
         [0017]      FIG. 3  illustrates an embodiment of a very wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and a dioptric lens to provide the correction over a detector array with a format of 1024 spatial pixels by 256 spectral pixels.  
         [0018]      FIG. 4  is a plan view of an embodiment of a very wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and dioptric lens to provide the correction over a detector array with a format of 1024 spatial pixels by 256 spectral pixels.  
         [0019]      FIG. 5  illustrates an embodiment of an imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and a dioptric lens to provide the correction over a detector array with a format of 256 spatial pixels by 256 spectral pixels.  
         [0020]      FIG. 6  is an isometric view of an embodiment of an imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and dioptric lens to provide the correction over a detector array with a format of 256 spatial pixels by 256 spectral pixels.  
         [0021]      FIG. 7  illustrates an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and a dioptric lens to provide the correction over a detector array with a format of 1024 spatial pixels by 256 spectral pixels. In this case the optical design has been reversed to reflect from the catadioptric lens immediately before the detector.  
         [0022]      FIG. 8  is a plan view of an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention using a catadioptric lens and a dioptric lens to provide the correction over a detector array with a format of 1024 spatial pixels by 256 spectral pixels. In this case the optical design has been reversed to reflect off the catadioptric lens immediately before the detector. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Referring now to the drawings, to the following detailed description, and to incorporated materials, detailed information about the invention is provided including the description of specific embodiments. The detailed description serves to explain the principles of the invention. The invention is susceptible to modifications and alternative forms. The invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.  
         [0024]     Referring now to  FIG. 1  of the drawings, an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention is illustrated. This embodiment of the present invention is designated generally by the reference numeral  100 .  FIG. 1  is a raytrace of the imaging spectrometer  100 . The structural elements of the compact imaging spectrometer  100  include an entrance slit  101 , a catadioptric lens  105 - 106 , an immersive grating  103 - 104 , a dioptric lens  107 - 108 , and a detector  109 .  
         [0025]     The light passes from the entrance slit  101  to the catadioptric lens  105 - 106  refracting through the first surface  105  and reflecting off the back  106  of the lens, which has a continuous convex surface but has a reflective coating on the upper half. The light refracts through the front of the lens  105  and proceeds to the immersive grating  103 - 104 . The light refracts through the front of grating  103  and is then dispersed by the reflective grating on the back surface  104  and refracts out through the front  103  of the grating to catadioptric lens  105 - 106 . This time the light travels through  105 - 106 , and then is focused through lens  107 - 108  on to the detector array  109 . For detector array  109  the spatial dimension is in the X dimension and the spectral dispersion is in the Y dimension. The aperture cold stop is close to surface  103  with the beam from the entrance slit and the exit beam to the detector approximately telecentric. This provides good distortion control for focusing the detector.  
         [0026]     The grating can be generated by a ruling engine, replication, holographically, or by e-beam lithography. Grating  103 - 104  is tilted and wedged such that the ghost reflections from the front  103  of the grating do not fall on the detector  109 . The stray light is also controlled by adjusting the curvature of surface  105  so that the ghost reflection of the light from the entrance slit off surface  105  misses the grating  103 - 104 . The ghost reflection may also be utilized by an additional detector.  FIG. 2  is an isometric view of the spectrometer shown in  FIG. 1 .  
         [0027]     The imaging spectrometer  100  has been designed to the requirements in Table 1 and is diffraction limited over the wavelength range with excellent spatial and spectral resolutions.  
                   TABLE 1                           Spectral Range   7.5-13.5 microns       F-number (round or square)   3.5       Detector array   512 spatial × 256 spectral       Pixel Size   40 microns       Entrance Slit Length   20.48 mm       Spatial Distortion: (change in   &lt;0.1 pixel (&lt;±2 microns)       Spatial Mapping with Wavelength)       Spectral Distortion: Spectral Smile   &lt;0.1 pixel (&lt;±2 microns)       Optical Performance   Diffraction Limited       Ghosting   &lt;0.1% of the primary image                  
 
         [0028]     The spectral slit curvature has been corrected to less than one tenth of a pixel over the detector array. This is the curvature of slit image on the detector  109  at a single wavelength, which is a common problem with imaging spectrometer designs. The spatial mapping distortion has also been corrected to less than one tenth of a pixel over the full wavelength range, so the spectrum from a single point in the entrance slit will not wander from the center of a row of pixels by less than ±2 microns. Correcting the spectral slit curvature and the spatial mapping distortion with wavelength to less than one tenth of a pixel ensures that the images do not have to be resampled to correct for these effects. The nominal design has much better distortion correction than the requirements in the Table 1.  
         [0029]     One important feature of this design, enabling it to be used over a wide field of view than previous designs (Patent Application Publication US 2005/0073680 A1, “Imaging Spectrometer Utilizing Immersed Gratings with Accessible Entrance Slit,” Chrisp et al publication date Apr. 7, 2005), is that it has zero Petzval field curvature. The field curvature from the positive mirror section is of opposite sign and cancels with the field curvature sum from the rest of the positive refractive surfaces.  
         [0030]     The optical prescription for the imaging spectrometer in  FIG. 1  is given  
                                                                                                 TABLE 2                           Optical Prescription                            X angle           reference   Surface notes   Y (mm)   Z (mm)   (degrees)   Radius (mm)                    101   slit   16.67   −32.71                   105   1st lens front surface   0   0   0   1409.222   cc       106   1st lens mirror/back surface   0   4.5   0   226.702   cx       107   2nd lens front surface   0   4.6   0   293.872   cx       108   2nd lens back surface   0   10.6   0   301.896   cx            103   grating front surface   −0.13   −28.37   0.072   aspheric       104   grating ruled surface   −0.14   −30.77   −0.616   flat       109   detector surface   −2.14   41.35   −0.429   flat                 (global surface coordinates with respect to first lens 105)             
 
 in TABLE 2, where cc stands for a concave surface and cx is a convex surface. The origin of the global coordinate system is at the center of the lens front face  105 , and positive X rotation angles are anti-clockwise about the X axis. The lenses  105 - 106  and  107 - 108 , and grating  103 - 104  are made from germanium; the grating period is 0.0194 mm. The sagittal equation of the fourth order rotationally symmetric asphere on grating surface  103  is given by: 
 
 z= 0.126 E− 5 ( x   2   +y   2 ) 2 
 
 This is an example of a typical design prescription, and the dimensions are given at an operational temperature of approximately 50K. 
 
         [0031]     Small size for an infrared imaging spectrometer is extremely important because it determines the requirements for the cryogenic cooling. For example, if the spectrometer is small it can fly in a small UAV. Also, if the spectrometer is small it is person portable. The imaging spectrometer  100  has a size envelope that is smaller than spectrometers currently in use for its wide field. The entrance slit  101 , catadioptric lens  105 - 106 , dioptric lens  107 - 108 , immersion grating  103 - 104 , and detector  109  fit within the envelope. The Y axis and the Z axis are shown in the plane of the paper in  FIG. 1 . The X axis extends perpendicular to both the Y axis and the Z axis. The envelope is a cylinder 6 cm diameter by 7.5 cm long. As shown in  FIG. 1 , the X axis is 6 cm, the Y axis is 6 cm, and the Z axis is 7.5 cm. The entrance slit  101  is located at or near the front and detector  109  is located at or near the back.  
         [0032]     Referring now to  FIG. 3  of the drawings, an embodiment of a very wide field imaging spectrometer constructed in accordance with the present invention is illustrated. This embodiment of the present invention is designated generally by the reference numeral  300 .  FIG. 3  is a raytrace of the imaging spectrometer  300 . The structural elements of the compact imaging spectrometer  300  include an entrance slit  301 , a catadioptric lens  305 - 306 , an immersive grating  303 - 304 , a dioptric lens  307 - 308 , and a detector  309 .  FIG. 4  is a plan view of the spectrometer shown in  FIG. 3  as a cross-sectional view.  
         [0033]     The light passes from the entrance slit  301  to the catadioptric lens  305 - 306  refracting through the first surface  305  and reflecting off the back  306  of the lens, which has a continuous convex surface but has a reflective coating on the upper half. The light refracts through the front of the lens  305  and proceeds to the immersive grating  303 - 304 . The light refracts through the front of grating  303  and is then dispersed by the reflective grating on the back surface  304  and refracts out through the front of the grating  303  to catadioptric lens  305 - 306 . This time the light travels through  305 - 306 , and then is focused through lens  307 - 308  and on to the detector array  309 . For detector array  309  the spatial dimension is in the X dimension and the spectral dispersion is in the Y dimension. The aperture cold stop is close to surface  303  with the beam from the entrance slit and the exit beam to the detector approximately telecentric. The grating can be generated by a ruling engine, replication, holographically, or by e-beam lithography. Grating  303 - 304  is tilted and wedged such that the ghost reflections from the front  303  of the grating do not fall on the detector  309 .  
         [0034]     The imaging spectrometer  300  has been designed to the requirements in Table 3 and is diffraction limited over the wavelength range with excellent spatial and spectral resolutions.  
                   TABLE 3                           Spectral Range   7.5-13.5 microns       F-number (square or round)   3.5       Detector array   1024 spatial × 256 spectral       Pixel Size   40 microns       Entrance Slit Length   40.96 mm       Spatial Distortion: (change in   &lt;0.1 pixel (&lt;±2 microns)       Spatial Mapping with Wavelength)       Spectral Distortion: Spectral Smile   &lt;0.1 pixel (&lt;±2 microns)       Optical Performance   Diffraction Limited       Ghosting   &lt;0.1% of the primary image                  
 
         [0035]     The spectral slit curvature has been corrected to less than one tenth of a pixel over the detector arrays. This is the curvature of slit image on the detector  309  at a single wavelength, which is a common problem with imaging spectrometer designs. The spatial mapping distortion has also been corrected to less than one tenth of a pixel over the full wavelength range. This means that the spectrum from a single point in the entrance slit will not wander from the center of a row of pixels by less than ±2 microns. Correcting the spectral slit curvature and the spatial mapping distortion with wavelength to less than one tenth of a pixel ensures that the images do not have to be resampled to correct for these effects. The nominal design has much less distortion than the numbers in Table 3.  
         [0036]     One important feature of this design enabling it to be used over a wide field of view than previous designs (Patent Application Publication US 2005/0073680 A1, “Imaging Spectrometer Utilizing Immersed Gratings with Accessible Entrance Slit,” Chrisp et al publication date Apr. 7, 2005), is that it has zero Petzval field curvature. The field curvature from the positive mirror section is of opposite sign and cancels with the field curvature sum from the rest of the positive refractive surfaces.  
         [0037]     Small size for an infrared imaging spectrometer is extremely important because it determines the requirements for the cryogenic cooling. For example, if the spectrometer is small it can fly in a small UAV. The imaging spectrometer  300  has a size envelope that is extremely efficient given the 1024 spatial pixel width. The entrance slit  301 , catadioptric lens  305 - 306 , dioptric lens  307 - 308 , immersion grating  303 - 304 , and detector  309  fit within the envelope. The Y axis and the Z axis are shown in the plane of the paper in  FIG. 3 . The X axis extends perpendicular to both the Y axis and the Z axis. As shown in  FIG. 3 , the X axis is 9 cm, the Y axis is 6 cm, and the Z axis is 10 cm. The entrance slit  301  is located at or near the front and the detector  309  is located at or near the back.  
         [0038]     Referring now to  FIG. 5  of the drawings, an embodiment of a very compact imaging spectrometer constructed in accordance with the present invention is illustrated. This embodiment of the present invention is designated generally by the reference numeral  500 .  FIG. 5  is a raytrace of the imaging spectrometer  500 . The structural elements of the compact imaging spectrometer  500  include an entrance slit  501 , a catadioptric lens  505 - 506 , a dioptric lens  507 - 508 , an immersive grating  503 - 504 , and a detector  509 .  FIG. 6 . is an isometric view of the spectrometer shown in  FIG. 5  as a cross-sectional view.  
         [0039]     The light passes from the entrance slit  501  to the catadioptric lens  505 - 506  refracting through the first surface  505  and reflecting off the back  506  of the lens, which has a continuous convex surface but has a reflective coating on the upper half. The light refracts through the front of the lens  505  and proceeds to the immersive grating  503 - 504 . The light refracts through the front of grating  503  and is then dispersed by the reflective grating on the back surface  504  and refracts out through the front of the grating  503  to catadioptric lens  505 - 506 . This time the light travels through  505 - 506 , and then is focused through lens  507 - 508  and on to the detector array  509 . For detector array  509  the spatial dimension is in the X dimension and the spectral dispersion is in the Y dimension. The aperture cold stop is close to surface  503  with the beam from the entrance slit and the exit beam to the detector approximately telecentric.  
         [0040]     The grating can be generated by a ruling engine, replication, holographically, or by e-beam lithography. Grating  503 - 504  is tilted and wedged such that the ghost reflections from the front  503  of the grating do not fall on the detector  509 . The imaging spectrometer  500  has been designed to the requirements in Table 4 and is diffraction limited over the wavelength range with excellent spatial and spectral resolutions.  
                   TABLE 4                           Spectral Range   7.5-13.5 microns       F-number (square or round)   3.5       Detector array   256 spatial × 256 spectral       Pixel Size   40 microns       Entrance Slit Length   10.24 mm       Spatial Distortion: (change in   &lt;0.1 pixel (&lt;±2 microns)       Spatial Mapping with Wavelength)       Spectral Distortion: Spectral Smile   &lt;0.1 pixel (&lt;±2 microns)       Optical Performance   Diffraction Limited       Ghosting   &lt;0.1% of the primary image                  
 
         [0041]     The spectral slit curvature has been corrected to less than one tenth of a pixel over the detector arrays. This is the curvature of slit image on the detector  509  at a single wavelength, which is a common problem with imaging spectrometer designs. The spatial mapping distortion has also been corrected to less than one tenth of a pixel over the full wavelength range. This means that the spectrum from a single point in the entrance slit will not wander from the center of a row of pixels by less than ±2 microns. Correcting the spectral slit curvature and the spatial mapping distortion with wavelength to less than one tenth of a pixel ensures that the images do not have to be resampled to correct for these effects.  
         [0042]     One important feature of this design compared with previous designs (Patent Application Publication US 2005/0073680, “Imaging Spectrometer Utilizing Immersed Gratings with Accessible Entrance Slit,” Chrisp et al publication date Apr. 7, 2005), is that it has zero Petzval field curvature. The field curvature from the positive mirror section is of opposite sign and cancels with the field curvature sum from the rest of the positive refractive surfaces.  
         [0043]     Small size for an infrared imaging spectrometer is extremely important because it determines the requirements for the cryogenic cooling. For example, if the spectrometer is small it can fly in a small UAV. The imaging spectrometer  500  has a size envelope that is extremely efficient given the 256 spatial pixel width. The entrance slit  501 , catadioptric lens  505 - 506 , dioptric lens  507 - 508 , immersion grating  503 - 504 , and detector  509  fit within the envelope. The Y axis and the Z axis are shown in the plane of the paper in  FIG. 5 . The X axis extends perpendicular to both the Y axis and the Z axis. As shown in  FIG. 5 , the X axis is 3.4 cm, the Y axis is 3.4 cm, and the Z axis is 4 cm. The entrance slit  501  is located at or near the front and the detector  509  is located at or near the back.  
         [0044]     Referring now to  FIG. 7  of the drawings, an embodiment of a wide field imaging spectrometer constructed in accordance with the present invention is illustrated. In this case the optical design has a reverse form compared with the previous cases. This embodiment of the present invention is designated generally by the reference numeral  700 .  FIG. 7  is a raytrace of the imaging spectrometer  700 . The structural elements of the compact imaging spectrometer  700  include an entrance slit  701 , a dioptric lens  705 - 706 , an immersive grating  703 - 704 , a catadioptric lens  707 - 708 , and a detector  709 .  
         [0045]     The light passes from the entrance slit  701  through the dioptric lens  705 - 706  and then through the non-reflecting section of the catadioptric lens  707 - 708 , proceeding to the immersive grating  703 - 704 . The light refracts through the front of grating  703  and is then dispersed by the reflective grating on the back surface  704 , and refracts out through the front of the grating  703  to catadioptric lens  707 - 708 . This time the light travels through the surface  708  and is reflected of the back surface  707 , which has a reflective coating on the lower part. The light refracts through surface  708  and focuses onto the detector array  709 . For detector array  709  the spatial dimension is in the X dimension and the spectral dispersion is in the Y dimension. The aperture cold stop is close to surface  703  with the beam from the entrance slit and the exit beam to the detector approximately telecentric.  
         [0046]     The grating can be generated by a ruling engine, replication, holographically, or by e-beam lithography. Grating  703 - 704  is tilted and wedged such that the ghost reflections from the front  703  of the grating do not fall on the detector  709 .  FIG. 8  is a plan view of the spectrometer shown in  FIG. 7  as a cross-sectional view.  
         [0047]     The imaging spectrometer  700  has been designed to the requirements in Table 3 and is diffraction limited over the wavelength range with excellent spatial and spectral resolutions.  
         [0048]     The spectral slit curvature has been corrected to less than one tenth of a pixel over the detector arrays. This is the curvature of slit image on the detector  709  at a single wavelength, which is a common problem with imaging spectrometer designs. The spatial mapping distortion has also been corrected to less than one tenth of a pixel over the full wavelength range. This means that the spectrum from a single point in the entrance slit will not wander from the center of a row of pixels by less than ±2 microns. Correcting the spectral slit curvature and the spatial mapping distortion with wavelength to less than one tenth of a pixel ensures that the images do not have to be resampled to correct for these effects.  
         [0049]     One important feature of this design enabling it to be used over a wide field of view than previous designs is that it has zero Petzval field curvature. The field curvature from the positive mirror section is of opposite sign and cancels with the field curvature sum from the rest of the positive refractive surfaces.  
         [0050]     Small size for an infrared imaging spectrometer is extremely important because it determines the requirements for the cryogenic cooling. For example, if the spectrometer is small it can fly in a small UAV. The imaging spectrometer  700  has a size envelope that is extremely efficient given the 1024 spatial pixel width. The entrance slit  701 , catadioptric lens  705 - 706 , dioptric lens  707 - 708 , immersion grating  703 - 704 , and detector  709  fit within the envelope. The Y axis and the Z axis are shown in the plane of the paper in  FIG. 7 . The X axis extends perpendicular to both the Y axis and the Z axis. As shown in  FIG. 7 , the X axis is 8 cm, the Y axis is 9 cm, and the Z axis is 12 cm. The entrance slit  701  is located at or near the front and the detector  709  is located at or near the back.  
         [0051]     The imaging spectrometer of the present invention has many uses. Examples of its use include use in Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. The imaging spectrometer also has use for commercial remote sensing where portability is important. The imaging spectrometer can be used for pollution detection and remote sensing of agricultural crops. It can be used for geological identification and for the remote monitoring of industrial processes. These are examples of the various potential applications of the imaging spectrometer of the present invention.  
         [0052]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.