Abstract:
A compact imaging spectrometer with an immersive diffraction grating that compensates optical distortions. The imaging spectrometer comprises an entrance slit for transmitting light, means for receiving the light and directing the light, an immersion grating, and a detector array. The entrance slit, the means for receiving the light, the immersion grating, and the detector array are positioned wherein the entrance slit transmits light to the means for receiving the light and the means for receiving the light directs the light to the immersion grating and the immersion grating receives the light and directs the light to the means for receiving the light, and the means for receiving the light directs the light to the detector array.

Description:
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 
   Related subject matter is disclosed and claimed in the following commonly owned, co-pending, U.S. Patent Applications with at least one common inventor: U.S. patent application Ser. No. 10/646,666 filed Aug. 21, 2003 by Charles L. Bennett, Jay V. Bixler, Paul J. Kuzmenko, Scott A. Lerner, and Isabella T. Lewis, titled “Compact Refractive Imaging Spectrometer Utilizing Immersed Gratings,” and U.S. patent application Ser. No. 10/658,150 filed Sep. 9, 2003 by Michael P. Chrisp, titled “Compact Reflective Imaging Spectrometer Utilizing Immersed Gratings,” and U.S. patent application Ser. No. 10/658,141 filed Sep. 9, 2003 by Scott A. Lerner, titled “Compact Catadioptric Imaging Spectrometer Utilizing Immersed Grating” and U.S. patent application Ser. No. 10/680,788 filed Oct. 6, 2003 by Scott A. Lerner, titled “Compact Catadioptric Imaging Spectrometer Utilizing Reflective Grating,” and U.S. patent application Ser. No. 10/680,847 filed Oct. 6, 2003 by Scott A. Lerner, titled “Compact Imaging Spectrometer Utilizing Immersed Gratings,” and U.S. patent application Ser. No. 10/844,086 filed May 11, 2004 by Scott A. Lerner, titled “Compact Imaging Spectrometer Design Utilizing An Immersed Grating and Anamorphic Mirror,” and U.S. patent application Ser. No. 10/877,622 filed Jun. 24, 2004, 2004 by Michael P. Chrisp and Scott A. Lerner, titled “Imaging Spectrometer Utilizing Immersed Gratings with Accessible Entrance Slit.” 
   BACKGROUND 
   1. Field of Endeavor 
   The present invention relates to a spectrometer and more particularly to a compact imaging spectrometer. 
   2. State of Technology 
   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.” 
   U.S. Patent Application No. 20020135770 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.” 
   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.” 
   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 
   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. 
   The present invention provides a compact imaging spectrometer with an immersive diffraction grating that compensates optical distortions. The imaging spectrometer comprises an entrance slit for transmitting light, means for receiving the light and directing the light, an immersion grating, and a detector array. The entrance slit, the means for receiving the light, the immersion grating, and the detector array are positioned wherein the entrance slit transmits light to the means for receiving the light and the means for receiving the light directs the light to the immersion grating and the immersion grating receives the light and directs the light through an optical element to the detector array. 
   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 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 
     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. 
       FIG. 1  illustrates an embodiment of a compact imaging spectrometer utilizing a single lens constructed in accordance with the present invention. 
       FIG. 2  illustrates an embodiment of a compact imaging spectrometer utilizing a plano convex lens constructed in accordance with the present invention. 
       FIG. 3  illustrates an embodiment of a compact imaging spectrometer utilizing reflective mirrors constructed in accordance with the present invention. 
       FIG. 4  illustrates a perspective view of a compact imaging spectrometer utilizing reflective mirrors shown in  FIG. 3 . 
       FIG. 5  illustrates an embodiment of a compact imaging spectrometer utilizing a refractive lens constructed in accordance with the present invention. 
       FIG. 6  illustrates an embodiment of a compact imaging spectrometer utilizing a reflective grating constructed in accordance with the present invention. 
       FIG. 7  illustrates an embodiment of a compact imaging spectrometer utilizing a catadioptric lens constructed in accordance with the present invention. 
       FIG. 8  illustrates an embodiment of a compact imaging spectrometer utilizing an anamorphic mirror constructed in accordance with the present invention. 
       FIG. 9  illustrates an embodiment of a compact refractive imaging spectrometer constructed in accordance with the present invention. 
       FIG. 10  illustrates an embodiment of an imaging spectrometer utilizing a catadioptric lens constructed in accordance with the present invention. 
       FIG. 11  illustrates another embodiment of a compact imaging spectrometer constructed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
   Referring now 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 .  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 lens  102 , a germanium immersion grating  103 , and detector  104 . 
   The compact imaging spectrometer  100  utilizes immersion grating  103  for correction of optical distortion. Both the front and back surfaces of the immersion grating  103  are plano. The immersion diffraction grating  103  can be ruled on a germanium, zinc selenide, or other refractive prism. The immersion grating  103  has 115 grooves/mm. 
   One embodiment of the immersion grating  103  consists of a reflective grating that is index matched to a refractive prism such that the combination of the refractive grating and the refractive prism are nearly optically identical to the immersion grating ruled on the prism itself. Furthermore, the immersion grating assembly may consist of one or more prisms of the same or different glass types for additional degrees of freedom in the correction of optical distortion and wavefront aberrations. Ruling a reflective grating may be significantly less costly than the ruling of an immersion prism. The utility in index matching a reflective grating to a refractive prism is that it may be significantly less costly and it will provide nearly identical optical performance. The immersion grating  103  provides the mating of a square pupil to the immersion grating. The design of the immersion grating may use any shape pupil. However, a square pupil that matches the bilateral symmetry of the detector allows additional etendue through the system with little or no additional optical correction necessary to meet the same system requirements as for a round pupil. 
   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. 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  104  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. 
   For the imaging spectrometer  100 , light from slit  101  is collimated by the lens  102 . The immersion grating  103 , located at the stop of the system, disperses the collimated light and then the lens  102  focuses light onto the detector  104 . For the nominal design, the immersion grating  103  has three degrees of freedom: wedge, tilt, and grating spacing. The prism wedge of the immersion grating  103  defines design symmetry such that the smile distortion from the immersion grating balances the smile from the focusing lens. The tilt directs the front surface ghost reflection such that it does not fall on the detector  104 . The groove spacing controls the spectral dispersion. 
   The cold stop is at the germanium grating  103 . This ensures that the warm background radiation from outside the spectrometer entrance slit  101  does not reach the detector array  104 . This would cause unacceptable degradation in the signal to noise ratio. The geometry of imaging 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 size. 
   The imaging spectrometer  100  has use for Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. The imaging spectrometer  100  also has use for commercial remote sensing where portability is important. The imaging spectrometer  100  can be used for pollution detection, and remote sensing of agricultural crops, and geological identification among the various potential applications. The imaging spectrometer  100  can be used for the remote monitoring of industrial processes. 
   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. The imaging spectrometer  100  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  101 , 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 3.2 cm by 2.0 cm by 1.5 cm or smaller. As shown in  FIG. 1 , the X axis is 3.2 cm, the Y axis is 2.0 cm, and the Z axis is 1.5 cm. The compact imaging spectrometer  100  has a front and a back. The entrance slit  101  is located at or near the font and the grating  103  is located at or near the back. 
   
     
       
             
             
             
           
         
             
                 
               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: Change 
               &lt;0.1 pixel (&lt;±2 microns) 
             
             
                 
               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 
             
             
                 
                 
             
           
        
       
     
   
   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. 
   Referring now to  FIG. 2  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  200 .  FIG. 2  is a raytrace of a compact imaging spectrometer  200  that utilizes an immersion grating for correction of optical distortion. The structural elements of the compact imaging spectrometer  200  include an entrance slit  201 , a lens  202 , a germanium immersion grating  203 , and detector  204 . The immersion grating  203  has a curved front surface that mitigates stray light in the design and provides additional wavefront correction for the spectrometer design. The immersion grating  203  has 84 grooves/mm. 
   The compact imaging spectrometer  200  utilizes immersion grating  203  for correction of optical distortion. The imaging spectrometer  200  meets the requirements in Table 1 and is diffraction limited over the wavelength range with excellent spatial and spectral resolution. 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  204  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. 
   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. The imaging spectrometer  200  has a size envelope that is smaller than spectrometers currently in use. 
   Referring now to  FIG. 3  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  300 .  FIG. 3  is a raytrace of a compact imaging spectrometer  300  with an immersion grating used to correct the spectral slit curvature. The structural elements of the compact imaging spectrometer  300  include an entrance slit  301 , a mirror  305 , a germanium immersion grating  303 , a mirror  306 , and detector  304 . 
   The compact imaging spectrometer  300  utilizes immersion grating  303  for correction of optical distortion. The immersion grating  303  can be ruled on a germanium, zinc selenide, or other refractive prism. A reflective grating that is index matched to a refractive prism such that the combination of the refractive grating and the refractive prism are nearly optically identical to the immersion grating ruled on the prism itself could be used for the immersion grating  303 . Furthermore, the immersion grating assembly may consist of one or more prisms of the same or different glass types for additional degrees of freedom in the correction of optical distortion and wavefront aberrations. Ruling a reflective grating may be significantly less costly than the ruling of an immersion prism. 
   The imaging spectrometer  300  meets the requirements in Table 1 and 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  304  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 compact imaging spectrometer  300  is smaller than spectrometers currently in use. The reduced cryogenic cooling requirements of the compact imaging spectrometer  300  allow its use in small unmanned aerial vehicles. The reduced cryogenic cooling requirements of the compact imaging spectrometer  300  also allows it to be used for man portable instruments. The compact imaging spectrometer  300  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  301 , the first mirror  305 , the immersive diffraction grating  303 , the second mirror  306 , and the detector array  304  fit within the envelope. The envelope is 2.5 cm by 3.2 cm by 2.2 cm or smaller. As shown in  FIG. 3  the X axis is 3.2 cm and the Y axis is 2.5 cm and the z-axis 2.2 cm. 
     FIG. 4 , is a perspective view of the compact reflective imaging spectrometer in  FIG. 3 . This embodiment is designated generally by the reference numeral  400 .  FIG. 4  is a raytrace for the compact imaging spectrometer  400 . The embodiment  400  of a compact reflective imaging spectrometer includes a two-dimensional detector array to enable spectral analysis of the spatial structure of objects within the scene. 
   The structural elements of the compact imaging spectrometer  400  include an entrance slit  401 , a germanium or zinc selenide grating  402 , an array detector  403 , a concave mirror  405 , and a concave mirror  405 . The compact imaging spectrometer  400  provides an infrared reflective imaging spectrometer apparatus that comprises an entrance slit for directing light, a concave reflective primary mirror for reducing the divergence of said light from said entrance slit, a wedged germanium or zinc selenide grating dispersing said light and a concave reflective secondary mirror focusing said light onto a two-dimensional detector array. 
   The compact imaging spectrometer  400  is smaller than spectrometers currently in use. The reduced cryogenic cooling requirements of the compact imaging spectrometer  400  allow its use in small unmanned aerial vehicles. The reduced cryogenic cooling requirements of the compact imaging spectrometer  400  also allow it to be used for man portable instruments. The compact imaging spectrometer  400  meets the requirements in Table 1 and is diffraction limited at all wavelengths and spatial points on the detector. 
   The concave mirrors  404  and  804  in the compact imaging spectrometer  400  can be diamond turned and are sections of spheres or rotational aspheres. The germanium or zinc selenide diffraction grating  402  has the rulings immersed into a flat germanium or zinc selenide surface. The grating can be diamond flycut with a blazed profile that will have maximum diffraction efficiency. In the compact imaging spectrometer  400  conventional gratings are used with equally spaced straight rulings. Performance improvement is obtained with curved gratings with varying the ruling spacings. For the diffraction grating, light enters from the front germanium or zinc selenide surface (which usually 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 germanium and out through the front surface. The grating is cut on the back of a wedged plano-convex or plano-concave lens. In some of these designs the power can be eliminated from the lens resulting in the grating being cut on the back of a wedged germanium prism. 
   Referring to  FIG. 5  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  500 .  FIG. 5  is a raytrace for the imaging spectrometer  500 . The structural elements in the compact imaging spectrometer  500  include slit  501 , lens  502 , immersed diffraction grating  503 ,  2 D detector  504 , and baffle  505 . The imaging spectrometer  500  has a size envelope that is smaller than spectrometers currently in use. The slit  501 , lens  502 , immersed diffraction grating  503 , and 2D detector  504  fit within the envelope. The envelope is 3.5 cm by 1.9 cm by 1.2 cm or smaller. 
   As shown in  FIG. 5 , light goes from the entrance slit  501  to the lens  502 , which refracts it to the ruled germanium grating  503 . The diffracted order then propagates back to the lens  502 , which focuses onto the 2D detector array  503 . The germanium grating  503  is a wedged prism that is plano on both faces and with the grating ruled on the flat reflective side. The baffle  504  mitigates stray light at the detector  504 . The compact imaging spectrometer  500  solves the requirements for compact imaging spectrometers meeting the performance requirements given in Table 1. 
   Referring to  FIG. 6  of the drawings, an embodiment of a compact imaging spectrometer utilizing a catadioptric lens constructed in accordance with the present invention is illustrated. Where a catadioptric lens is a compound lens with both refractive and reflective surfaces. This embodiment of the present invention is designated generally by the reference numeral  600 .  FIG. 6  is a raytrace for the imaging spectrometer  600 . The structural elements of the compact imaging spectrometer  600  include an entrance slit  601 , a rotationally symmetric zinc selenide aspheric lens  602 , a germanium grating  603 , and a 2D detector array  604 . 
   The imaging spectrometer  600  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  601 , rotationally symmetric zinc selenide aspheric lens  602 , germanium grating  603 , and 2D detector array  604  fit within the envelope. The envelope is 4.5 cm by 5.6 cm by 2.2 cm or smaller. As shown in  FIG. 6 , the X axis is 4.5 cm and the Y axis is 5.6 cm. 
   As shown in  FIG. 6 , light goes from the entrance slit  601  to the rotationally symmetric zinc selenide aspheric lens  602 . The rotationally symmetric zinc selenide aspheric lens  602  reflects and refracts the light back to the reflective grating  603 . The grating  603  is a flat reflective grating with 45 lines/mm. The rotationally symmetric aspheric lens  602  is zinc selenide allowing for transmission of visible light. The diffracted order then propagates back to the lens  602 , which reflects and focuses the light onto the 2D detector array  604 . 
   Referring to  FIG. 7  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  700 .  FIG. 7  is a raytrace for the imaging spectrometer  700 . The structural elements of the compact imaging spectrometer  700  include an entrance slit  701 , a germanium grating  702 , an array detector  703 , and a zinc selenide catadioptric lens  704 . The imaging spectrometer  700  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  701 , the grating  702 , the detector array  703 , and the zinc selenide catadioptric lens  704  fit within the envelope. The envelope is 3.0 cm by 2.7 cm by 1.6 cm or smaller. As shown in  FIG. 7  the X axis is 3.0 cm, the Y axis is 2.7 cm and the Z axis is 1.6 cm. 
   The imaging spectrometer  700  provides a compact imaging spectrometer based on catadioptric lenses and an immersive diffraction grating. The zinc selenide catadioptric lens  704  in the compact imaging spectrometer  700  consist of rotationally symmetric surfaces. In another embodiment anamorphic aspheric surfaces are used. The cold stop in the compact imaging spectrometer  700  is at the germanium grating. This ensures that the warm background radiation from outside the spectrometer entrance slit does not reach the detector array. This would cause and unacceptable degradation in the signal to noise ratio. The geometry of the compact imaging spectrometer  700  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. 
   Referring to  FIG. 8  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  800 .  FIG. 8  is a raytrace for the imaging spectrometer  800 . The structural elements of the compact imaging spectrometer  800  include an entrance slit  801 , a germanium grating  802 , an array detector  803 , and an anamorphic mirror  804 . The imaging spectrometer  800  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  801 , the grating  802 , the detector array  803 , and the anamorphic mirror  804  fit within the envelope. The envelope is 3.5 cm or smaller by 5.0 cm or smaller by 2.0 cm or smaller. As shown in  FIG. 8  the X axis is 5.0 cm and the Y axis is 3.5 cm. 
   The imaging spectrometer  800  provides a compact imaging spectrometer based on an anamorphic mirror and an immersive diffraction grating. The compact imaging spectrometer  800  is smaller than those currently in use and has a reduced cryogenic cooling requirement enabling its use in small unmanned aerial vehicles and for man portable instruments. The compact imaging spectrometer  800  can be utilized for remote sensing imaging spectrometers where size and weight are of primary importance. The compact imaging spectrometer  800  has very good spectral and spatial registration providing accurate spectral data for spectral algorithm retrievals. This avoids having to resample the images to correct for these defects, which has the disadvantage of creating spectral mixing between pixels reducing the sensitivity and accuracy of the retrieval algorithms. 
   Referring to  FIG. 9  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  900 . 
     FIG. 9  is a raytrace for the imaging spectrometer  900 . The structural elements in the compact imaging spectrometer  900  include slit  901 , collimating lens  902 , immersed diffractive grating  903 , objective triplet lens L 1   904 , objective triplet lens L 2   905 , objective triplet lens L 3   906 , cold filter  907 , and image plane  908 . 
   The imaging spectrometer  900  has a size envelope that is smaller than spectrometers currently in use. The slit  901 , collimating lens  902 , immersed diffractive grating  903 , objective triplet lens L 1   904 , objective triplet lens L 2   905 , objective triplet lens L 3   906 , cold filter  907 , and image plane  908  fit within the envelope. The envelope is 8.2 cm by 7.9 cm by 1.4 cm or smaller. As shown in  FIG. 9  the X axis is 8.2 cm and the Y axis is 7.9 cm. 
   The immersed diffractive grating  903  material is germanium. The immersed diffractive grating is on side B. The grating consists of equally spaced straight grooves. The grating has 65 grooves per mm. Angle AB is 51.7°, angle BC is 36.2°, and angle CA is 92.1°. 
   In operation of the compact imaging spectrometer  900 , rays R 1  diverge from slit  901 . Collimating lens  902  collimates rays R 1  to the immersed diffractive grating  903 . Rays R 2  are collimated. The immersed diffractive grating  903  angularly separates rays R 2  according to wavelength. Rays R 3  are collimated and angularly separated in wavelength. Longer wavelengths are angled to the left and shorter wavelengths to the right. Lenses  904 ,  905 , and  906  in combination focus rays R 3  with minimal distortions. Rays R 6  converge and pass through cold filter  907 . The cold filter  907  serves to filter out background thermal radiation that is not of interest. Ray R 7  focuses onto the image plane  908 . 
   All the lenses in the compact imaging spectrometer  900  have spherical surfaces. The diffraction grating has the rulings immersed into a flat germanium surface. The grating can be diamond flycut with a blazed profile that will have maximum diffraction efficiency. In the compact imaging spectrometer  900 , conventional gratings are used with equally spaced straight rulings. For the diffraction grating, light enters from the front germanium surface (which may have power) and then passes through the germanium to diffract off the grating rulings at the back surface. The diffracted light then propagates through the germanium and out. The grating is cut on the back of a wedged prism. The refractive faces of the prism may be spherical or plano. In the compact imaging spectrometer  900  the power has been eliminated from the prism resulting in the grating being cut on a side of a wedged germanium prism. Although the grating is cut into germanium in this design, other refractive materials such as zinc selenide are also suitable. 
   Referring to  FIG. 10  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  1000 .  FIG. 10  is a raytrace for the imaging spectrometer  1000  with a round pupil. The structural elements of the compact imaging spectrometer  1000  include an entrance slit  1001 , an aspheric catadioptric lens  1002  with a flat mirror surface  1005 , a germanium immersion grating  1003 , and detector  1004 . The light goes from the entrance slit  1001  to the lens  1002  which transmits it to a mirrored flat surface  1005  on the back of the lens  1002 , then back through the lens  1002  that refracts it to the ruled germanium immersion grating  1003 . The diffracted order then propagates back to the lens  1002  which focuses the light onto the 2D detector array  1004 . On the detector array  1004  the wavelength dispersion is in the Y-axis direction and the spatial direction is along the Z-axis. 
   The germanium grating  1003  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. 
   The catadioptric lens  1002  in the spectrometer  1000  consists of a rotationally symmetric front surface  1006  and an asphere  1007  on the back surface. A reflective plano surface is located in a small section of the lens  1002  in order to redirect the light back to the grating  1003 , thereby allowing the slit  1001  and focal plane array  1004  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. 
   The diffraction grating  1003  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  1000  conventional gratings are used with equally spaced straight rulings on a flat surface. Additional aberration correction can be obtained with curved grooves with a varied groove spacing or with a holographically formed grating. For the diffraction grating  1003 , 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  1000  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. 
   The spectrometer  1000  meets the requirements is Table 1 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 imaging spectrometer  1000  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  1001 , flat mirror  1005 , aspheric lens  1002 , germanium grating  1003 , and detector  1004  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. 10 , the X axis is 60 mm, the Y axis is 40 mm, and the Z axis is 40 mm. The compact imaging spectrometer  1000  has a front and a back. The entrance slit  1001  is located at or near the front and the detector  1004  is located at or near the back. 
   Referring to  FIG. 11 , 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  1100 .  FIG. 11  is a raytrace of the imaging spectrometer  1100  for midwave infrared covering approximately the 3 to 5 micron band. The imaging spectrometer  1100  performance meets all the requirements in Table 2. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
           
           
             
                 
               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: Change 
               &lt;0.1 pixel (&lt;±1.3 microns) 
             
             
                 
               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 
             
             
                 
                 
             
           
        
       
     
   
   The imaging spectrometer  1100  has use for Homeland Defense to check for the presence of biological or chemical weapons without entering the contaminated areas. The imaging spectrometer  1100  also has use for commercial remote sensing where portability is important. The imaging spectrometer  1100  can be used for pollution detection, and remote sensing of agricultural crops, and geological identification among the various potential applications. The imaging spectrometer  1100  can be used for the remote monitoring of industrial processes. 
   The structural elements of the compact imaging spectrometer  1100  include an entrance slit  1101 , an aspheric catadioptric lens  1102  with a flat mirror surface  1105 , a germanium grating  1103 , and additional lens  1108  and detector  1104 . The light goes from the entrance slit  1101  to the lens front surface  1106  which transmits it to a mirrored flat surface  1105  on the back of the lens  1102 , then back through the lens surface  1106  that refracts it to the ruled germanium immersion grating  1103 . The diffracted order then propagates back to the lens  1102  which focuses the light onto the 2D detector array  1104 . 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. 
   The germanium grating  1103  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 provide 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. 
   In the imaging spectrometer  1100 , the grating  1103  and the catadioptric lens  1102  are germanium, and the final lens  1108  is zinc selenide. The mirror annulus  1105  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  1108  provided additional performance to meet the tighter requirements of the large format array with the smaller pixel sizes. The imaging spectrometer  1100  is diffraction limited at all wavelengths and meets the spatial and spectral distortion requirements over the full detector area. 
   The imaging spectrometer  1100  has a size envelope that is smaller than spectrometers currently in use. The entrance slit  1101 , flat mirror  1105 , aspheric lens  1102 , germanium grating  1103 , the lens  60 - 8 , and detector  1104  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. 
   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.