Abstract:
A compact imaging spectrometer comprises an entrance slit, a catadioptric lens with a mirrored surface, a grating, and a detector array. The entrance slit directs light to the mirrored surface of the catadioptric lens; the mirrored surface reflects the light back through the lens to the grating. The grating receives the light from the catadioptric lens and diffracts the light to the lens away from the mirrored surface. The lens transmits the light and focuses it onto the detector array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/680,788 filed Oct. 6, 2003 titled, “Compact Catadioptric Imaging Spectrometer Utilizing Reflective Grating.” U.S. patent application Ser. No. 10/680,788 filed Oct. 6, 2003 titled, “Compact Catadioptric Imaging Spectrometer Utilizing Reflective Grating” is incorporated herein by this reference. 
     
    
       [[0002]]     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. 
     
    
     BACKGROUND  
       [0003]     1. Field of Endeavor  
         [0004]     The present invention relates to a spectrometer and more particularly to a compact catadioptric imaging spectrometer.  
         [0005]     2. State of Technology  
         [0006]     U.S. Pat. No. 5,717,487 issued Feb. 10, 1998 to Donald W. Davies, and assigned to TRW Inc., 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. In past scientific work, the spectral analyses of a host of known elements, molecules, materials, living plants, gases and the like, has been compiled into a library. That library enables objects and things to be identified solely by the spectrometric analysis of the light reflected therefrom. Thus, as example, by examining the spectral content of light reflected from the distant planets, astronomers identified the constituent elements, such as iron, forming those planets; by examining the spectral content of Gases emitted by factory smokestacks, scientists determine if pollutants are being emitted in violation of law or regulation; by examining the spectral content of land, the environmental engineer is able to determine the botanical fertility of a region and its mineral content, and, with subsequent observations, to determine the change in the environment with time; and by examining the spectral content of light reflected in multiple scans over a geographic region, military personnel identify camouflaged military equipment, separate from plant life, in that geographic region. The foregoing represent but a small number of the many known uses of this useful scientific tool.” 
         [0007]     United States Patent Application No. 20020135770 published Sep. 26, 2003 by E. Neil Lewis and Kenneth S. Haber for a Hybrid Imaging Spectrometer, 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 issued Jun. 20, 2000 to Charles G. Stevens and Norman L. Thomas for an immersion echelle spectrograph, assigned to The Regents of the University of California, 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.sup.−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. Fourier transform spectrometers, such as Michelson interferometers, have long been the instrument of choice for high resolution spectroscopy in the infrared spectral region. This derives from its advantage in light gathering power and spectral multiplexing over conventional dispersive spectrometers. For remote sensing applications and for those applications in hostile environments, the Fourier transform spectrometer, such as the Michelson interferometer, is ill suited for these applications due to the requirements for keeping a moving mirror aligned to better than a wavelength over the mirror surface. Furthermore, this spectrometer collects amplitude variations over time that are then transformed into frequency information for spectral generation. Consequently, this approach requires stable radiation sources and has difficulty dealing with rapidly changing reflectors or emissions as generally encountered in remote field observations, particularly from moving observation platforms. Furthermore, under conditions where the noise terms are dominated by the light source itself, the sensitivity of the instrument is limited by the so-called multiplex disadvantage.” 
         [0009]     U.S. Pat. No. 5,880,834 issued Mar. 9, 1999 to Michael Peter Chrisp for a convex diffraction grating imaging spectrometer, assigned to The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, 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. The spectrometer comprises an entrance slit, a catadioptric lens, a grating, and a detector array. The entrance slit directs light to the catadioptric lens; the mirrored surface in the lens receives the light and reflects the light back out of the lens to the grating. The grating receives the light from the lens and diffracts the light back to another portion of the lens. The lens then transmits and focuses the light onto the detector array.  
         [0012]     Small size for an 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. In one embodiment of the compact imaging spectrometer, the spectrometer has a front and a back. The entrance slit is located at or near the font and the detector is located at or near the back. The entrance slit, the mirror, the lens, the grating, and the detector array fit within an envelope located between the front and the back. In one embodiment the envelope is 71 mm long or smaller by 43 mm diameter or smaller.  
         [0013]     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. The invention is not intended to be limited to the particular uses disclosed and the invention covers all uses falling within the spirit and scope of the invention as defined by the claims.  
         [0014]     In one embodiment of the compact imaging spectrometer, the mirror surface is part of the lens. The entrance slit directs light to the lens and to the mirrored surface. The mirrored surface receives the light and reflects the light to the grating. The grating receives the light from the mirrored surface and diffracts the light onto the lens. The lens focuses the light onto the detector array.  
         [0015]     Another embodiment of the compact imaging spectrometer includes a second lens. The entrance slit directs light to the mirrored surface in the lens. The mirrored surface receives the light and reflects the light to the grating. The grating receives the light from the mirrored surface and diffracts the light through the lens to the second lens. The second lens focuses the light onto the detector array.  
         [0016]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     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.  
         [0018]      FIG. 1  is a raytrace illustrating an embodiment of a compact imaging spectrometer constructed in accordance with the present invention with a round aperture stop.  
         [0019]      FIG. 2  is a perspective view of the raytrace of the compact imaging spectrometer shown in  FIG. 1 .  
         [0020]      FIG. 3  is a raytrace illustrating an embodiment of a compact imaging spectrometer constructed in accordance with the present invention with a square aperture stop.  
         [0021]      FIG. 4  is a perspective view of the raytrace of the compact imaging spectrometer shown in  FIG. 3 .  
         [0022]      FIG. 5  is a raytrace illustrating another embodiment of a compact imaging spectrometer constructed in accordance with the present invention designed for a large format array in the mid-infrared.  
         [0023]      FIG. 6  is a perspective view of the raytrace of the compact imaging spectrometer shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     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.  
         [0025]     Referring to  FIG. 1  of the drawings, an embodiment of a 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  100 .  
         [0026]      FIG. 1  is a raytrace for the imaging spectrometer  100  with a round pupil. The structural elements of the compact imaging spectrometer  100  include an entrance slit  101 , an aspheric catadioptric lens  102  with a flat mirror surface  105 , a germanium immersion grating  103 , and detector  104 .  
         [0027]     The light goes from the entrance slit  101  to the lens  102  which transmits it to a mirrored flat surface  105  on the back of the lens  102 , then back through the lens  102  that refracts it to the ruled germanium immersion grating  103 . The diffracted order then propagates back to the lens  102  which focuses the light onto the 2D detector array  104 . On the detector array  104  the wavelength dispersion is in the Y-axis direction and the spatial direction is along the Z-axis.  
         [0028]     The germanium grating  103  is a wedged prism that is aspheric on the face and with the grating ruled on the flat reflective side. Baffles are inserted at select locations to meet stray light requirements. The zero order from the grating exits the front face is trapped by a v-shaped baffle.  
         [0029]     The catadioptric lens  102  in the spectrometer  100  consists of a rotationally symmetric front surface  106  and an asphere  107  on the back surface. A reflective plano surface is located in a small section of the lens  102  in order to redirect the light back to the grating  103 , thereby allowing the slit  101  and focal plane array  104  to be arranged at opposite ends of the optical system which in turn provides a practical packaging advantage for using standard packaged focal plane arrays. The flat surface can be diamond turned into a segment of the lens surface.  
         [0030]     The cold stop in the spectrometer  100  is at the germanium grating  103 . This ensures that the warm back radiation from outside the spectrometer entrance slit  101  does not reach the detector array  104 . This would cause an unacceptable degradation in the signal to noise ratio. The geometry of the spectrometer  100  allows a transmissive cold stop to be used ahead of the grating, for even better thermal background reduction, but this also increases the grating sizes. The stop geometry at the grating enables the input beam at the entrance slit to be telecentric which facilitates its use with the front end telescope system. The output beams on the detector are also telecentric which enables the loosening of the tolerances on the detector angular position and longitudinal position while still meeting the distortion requirements.  
         [0031]     The diffraction grating  103  has the rulings immersed into a prism. The grating can be diamond flycut with a blazed profile that will have maximum diffraction efficiency at a desired wavelength. In the spectrometer  100  conventional gratings are used with equally spaced straight rulings on a flat surface. For the diffraction grating  103 , light enters from the front germanium surface, which has power, and then passes through the germanium to diffract off the grating rulings at the back surface. The diffracted light then propagates through the prism and out. The grating is cut on the back of a wedged prism. The refractive face of the prism may be spherical or aspherical. For the spectrometer  100  shown the diffraction grating is on a flat surface. For stray light control the angular orientation of the front face is arranged so that its Fresnel reflection misses the detector surface.  
         [0032]     The spectrometer  100  is diffraction limited over the infrared wavelength range with excellent spatial and spectral resolutions. 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 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 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 design meets the requirements in Table 1 below.  
                                               TABLE 1                                       Spectral Range   7.5-13.5 microns           F-number (round or square)   4           Detector array   256 spatial × 256 spectral           Pixel Size   40 microns           Entrance Slit Length   10.24 mm           Spatial Distortion:   &lt;0.1 pixel (&lt;±2 microns)            Change in 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                      
 
         [0033]     The imaging spectrometer  100  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  101 , flat mirror  105 , aspheric lens  102 , germanium grating  103 , and detector  104  fit within the envelope. The X axis and the Y axis are shown in the plane of the paper. The Z axis extends perpendicular to both the X axis and the Y axis. The envelope is 60 mm by 40 mm by 40 mm or smaller. As shown in  FIG. 1 , the X axis is 60 mm, the Y axis is 40 mm, and the Z axis is 40 mm. The compact imaging spectrometer  100  has a front and a back. The entrance slit  101  is located at or near the font and the detector  104  is located at or near the back.  
         [0034]     The optical prescription for the imaging spectrometer in  FIG. 1  is given in TABLE 2. The origin of the global coordinate system is at the center of the lens front face  106 , and positive X rotation angles are anti-clockwise about the X axis. The lens  102  and the grating  103  are made from germanium, and the grating period is 0.01863 mm. All the powered surfaces are convex, and the sagittal equation of the rotationally symmetric aspheric lens surface  107  is given by: 
 
 z =( y   2   /R   2 )/(1+ sqrt (1−(1+ K )( y   2   /R   2 )) 
 
         [0035]     where R is the radius of curvature and K is the conic constant. This is an example of a typical design prescription.  
                                                                     TABLE 2                                       X angle   Radius   Conic       reference   Surface notes   Y (mm)   Z (mm)   (degrees)   (mm)   constant                                101   slit   14.06   −27                   106   lens front surface   0   0   0   189.493       105   lens mirror surface   14   2.6   0   flat       107   lens back surface   0   3.5   0   −184.029   −9.538       103   grating front surface   −0.57   −25.38   0.369   −1088.34       103   grating ruled surface   −0.58   −27.78   −1.751   flat       104   detector surface   −5.09   33.91   −0.466   flat                  
 
         [0036]     Referring to  FIG. 2 , a raytrace of a compact imaging spectrometer with a round pupil is shown. This is a perspective view of  FIG. 1 . This embodiment of the present invention is designated generally by the reference numeral  200 . The structural elements of the compact imaging spectrometer  200  include an entrance slit  201 , an aspheric catadioptric lens  202  with a flat mirror surface  205 , a germanium immersion grating  203 , and detector  204 .  
         [0037]     Light goes from the entrance slit  201  to the lens  202  which transmits it to a mirrored flat surface  205  on the back of the lens  202 , then back through the lens  202  that refracts it to the ruled germanium immersion grating  203 . The diffracted order then propagates back to the lens  202  which focuses the light onto the 2D detector array  204 . The germanium grating  203  is a wedged prism that is aspheric on the face and with the grating ruled on the flat reflective side. Baffles are inserted at select locations to meet stray light requirements.  
         [0038]     The catadioptric lens  202  in the spectrometer  200  consists of a rotationally symmetric front surface and an asphere on the back surface. A reflective plano surface is located in a small section of the lens  202  in order to redirect the light the grating  103 , thereby allowing the slit  101  and focal plane array  104  to be arranged at opposite ends of the optical system which in turn provides a practical packaging advantage for using standard packaged focal plane arrays. The flat surface can be diamond turned into a segment of the lens surface.  
         [0039]     The cold stop in the spectrometer  200  is at the germanium grating  203 . This ensures that the warm back radiation from outside the spectrometer entrance slit  201  does not reach the detector array  204 . This would cause an unacceptable degradation in the signal to noise ratio. The geometry of the spectrometer  200  allows a transmissive cold stop to be used ahead of the grating, for even better thermal background reduction, but this also increases the grating sizes.  
         [0040]     The diffraction grating  203  has the rulings immersed into a prism. The grating can be diamond flycut with a blazed profile that will have maximum diffraction efficiency at a desired wavelength. In the spectrometer  200  conventional gratings are used with equally spaced straight rulings on a flat surface. For the diffraction grating  203 , light enters from the front germanium surface, which has power, and then passes through the germanium to diffract off the grating rulings at the back surface. The diffracted light then propagates through the prism and out. The grating is cut on the back of a wedged prism. The refractive face of the prism may be spherical or aspherical. For the spectrometer  200  shown the diffraction grating is on a flat surface.  
         [0041]     Referring to  FIG. 3 , another embodiment of a compact imaging spectrometer with a square pupil 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  with a square pupil. The differences between the round pupil embodiment illustrated in  FIG. 1  and the square pupil imaging spectrometer  300  is the curvature of the grating front surface—spherical for the round pupil, aspherical for the square pupil. In comparison, the square pupil provides larger etendue while the round pupil requires a simpler spherical surface versus the aspheric surface on the grating. The square pupil also reduces the amount of aliasing of the spectrum sampled by the detector array.  
         [0042]     The imaging spectrometer  300  has applicable to a wide range of focal plane array formats, scaled accordingly to accommodate different focal plane array physical dimensions. The imaging spectrometer  300  is also adaptable to a modest range of F-numbers, as suitable optical solutions have been obtained for an F/3 point design.  
         [0043]     The imaging spectrometer  300  has use for Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. The imaging spectrometer  300  also has use for commercial remote sensing where portability is important. The imaging spectrometer  300  can be used for pollution detection, and remote sensing of agricultural crops, and geological identification among the various potential applications. The imaging spectrometer  300  can be used for the remote monitoring of industrial processes.  
         [0044]     The structural elements of the compact imaging spectrometer  300  include an entrance slit  301 , an aspheric catadioptric lens  302  with a flat mirror surface  105 , a germanium immersion grating  303 , and detector  304 .  
         [0045]     The light goes from the entrance slit  301  to the lens  302  which transmits it to a mirrored flat surface  305  on the back of the lens  302 , then back through the lens  302  that refracts it to the ruled germanium immersion grating  303 . The diffracted order then propagates back to the lens  302  which focuses the light onto the 2D detector array  304 . On the detector array  104  the wavelength dispersion is in the Y-axis direction and the spatial direction is along the Z-axis.  
         [0046]     The germanium grating  103  is a wedged prism that is aspheric on the face and with the grating ruled on the flat reflective side. Baffles are inserted at select locations to meet stray light requirements.  
         [0047]     The catadioptric lens  302  in the spectrometer  300  consists of a rotationally symmetric front surface  306  and an asphere  307  on the back surface. A reflective piano surface is located in a small section of the lens  302  in order to redirect the light back to the grating  103 , thereby allowing the slit  101  and focal plane array to be arranged at opposite ends of the optical system which in turn provides a practical packaging advantage for using standard packaged focal plane arrays. The flat surface can be diamond turned into a segment of the lens surface.  
         [0048]     The cold stop in the spectrometer  300  is at the germanium grating  303 . This ensures that the warm back radiation from outside the spectrometer entrance slit  301  does not reach the detector array  304 . This would cause an unacceptable degradation in the signal to noise ratio. The geometry of the spectrometer  300  allows a transmissive cold stop to be used ahead of the grating, for even better thermal background reduction, but this also increases the grating sizes.  
         [0049]     The diffraction grating  303  has the rulings immersed into a prism. The grating can be diamond flycut with a blazed profile that will have maximum diffraction efficiency at a desired wavelength. In the spectrometer  300  conventional gratings are used with equally spaced straight rulings on a flat surface. For the diffraction grating  303 , light enters from the front germanium surface, which has power, and then passes through the germanium to diffract off the grating rulings at the back surface. The diffracted light then propagates through the prism and out. The grating is cut on the back of a wedged prism. The refractive face of the prism may be spherical or aspherical. For the spectrometer  300  shown the diffraction grating is on a flat surface.  
         [0050]     The spectrometer  300  is diffraction limited over the wavelength range with excellent spatial and spectral resolutions. 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 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 design meets the requirements in Table 1 above.  
         [0051]     The imaging spectrometer  300  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  301 , flat mirror  305 , aspheric lens  302 , germanium grating  303 , and detector  304  fit within the envelope. The X axis and the Y axis are shown in the plane of the paper. The Z axis extends perpendicular to both the X axis and the Y axis. The envelope is 62 mm by 41 mm by 41 mm or smaller. As shown in  FIG. 3 , the X axis is 62 mm, the Y axis is 41 mm, and the Z axis is 41 mm.  
         [0052]      FIG. 4 . is a perspective view of the imaging spectrometer in  FIG. 3 . This embodiment of the present invention is designated generally by the reference numeral  400 . The structural elements of the compact imaging spectrometer  400  include an entrance slit  401 , an aspheric catadioptric lens  402  with a flat mirror surface  405 , a germanium immersion grating  403 , and detector  404 .  
         [0053]     Referring to  FIG. 5 , another embodiment of a 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  for midwave infrared covering approximately the 3 to 5 micron band. The imaging spectrometer  500  performance meets all the requirements in Table 3.  
                                               TABLE 3                                       Spectral Range   3.2-5.3 microns           F-number (round or square)   6.5           Detector array   480 spatial × 640 spectral           Pixel Size   27 microns           Entrance Slit Length   12.96 mm           Spatial Distortion:   &lt;0.1 pixel (&lt;±1.3 microns)            Change in Spatial Mapping with Wavelength                Spectral Distortion:Spectral Smile   &lt;0.1 pixel (&lt;±1.3 microns)           Optical Performance   Diffraction Limited           Ghosting   &lt;0.1% of the primary image                      
 
         [0054]     The imaging spectrometer  500  has use for Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. The imaging spectrometer  500  also has use for commercial remote sensing where portability is important. The imaging spectrometer  500  can be used for pollution detection, and remote sensing of agricultural crops, and geological identification among the various potential applications. The imaging spectrometer  500  can be used for the remote monitoring of industrial processes.  
         [0055]     The structural elements of the compact imaging spectrometer  500  include an entrance slit  501 , an aspheric catadioptric lens  502  with a flat mirror surface  505 , a germanium grating  503 , and additional lens  508  and detector  504 . The light goes from the entrance slit  501  to the lens front surface  506  which transmits it to a mirrored flat surface  505  on the back of the lens  502 , then back through the lens surface  506  that refracts it to the ruled germanium immersion grating  503 . The diffracted order then propagates back to the lens  502  which focuses the light onto the 2D detector array  504 . The light is dispersed spectrally on the detector array in the Y axis direction and the spatially resolved direction is in the Z axis direction.  
         [0056]     The germanium grating  503  is a wedged prism that is spherical on the input face and with the grating ruled on the flat reflective side. The cold stop is at the grating, which provides the advantage of a telecentric input beam at the exit slit and a telecentric exit beam at the detector. The angular position of the front face of the grating is set so that the Fresnel reflection falls outside the detector area. Baffles are inserted at select locations to meet stray light requirements. The zero order from the grating exits from the front face of the prism and is trapped by a baffle.  
         [0057]     In the imaging spectrometer  500 , the grating  503  and the catadioptric lens  502  are germanium, and the final lens  508  is zinc selenide. The mirror annulus  505  can be diamond turned at the same time as the powered back lens surface is turned. For greater compactness a flat segment on the lens can be flycut instead of the annulus going completely around the lens. The additional zinc selenide lens  508  provided additional performance to meet the tighter requirements of the large format array with the smaller pixel sizes. The imaging spectrometer  500  is diffraction limited at all wavelengths and meets the spatial and spectral distortion requirements over the full detector area.  
         [0058]     The imaging spectrometer  600  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  601 , flat mirror  605 , aspheric lens  602 , germanium grating  603 , the lens  60 - 8 , and detector  604  fit within the envelope. The X axis and the Y axis are shown in the plane of the paper. The Z axis extends perpendicular to both the X axis and the Y axis. The envelope is 71 mm by 43 mm by 43 mm or smaller. As shown in  FIG. 5 , the X axis is 71 mm, the Y axis is 43 mm, and the Z axis is 43 mm.  
         [0059]     Referring to  FIG. 6 , is the perspective view of the imaging spectrometer in  FIG. 5 . This embodiment of the present invention is designated generally by the reference numeral  600 . The imaging spectrometer  600  is for midwave infrared covering approximately the 3 to 5 micron band. The imaging spectrometer  600  has use in checking for the presence of biological or chemical weapons, in commercial remote sensing, in pollution detection, in remote sensing of agricultural crops, in geological identification, in the remote monitoring of industrial processes, and other sensing.  
         [0060]     The structural elements of the compact imaging spectrometer  600  include an entrance slit  601 , a flat mirror  605 , an aspheric lens  602 , a germanium grating  603 , a zinc selenide lens  608 , and detector  604 . The light goes from the entrance slit  601  to the lens  602  which transmits it to a mirrored flat surface  605  on the back of the lens  602 , then back through the lens  602  that refracts it to the ruled germanium immersion grating  603 . The diffracted order then propagates back through the lens  602  to the zinc selenide lens  608 . The zinc selenide lens  608  focuses the light onto the 2D detector array  604 .  
         [0061]     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.