Abstract:
A spatially modulated interferometer incorporates a beam shearing system having a plurality of reflective surfaces defining separate light paths of equal optical path length for two separate output beams. The reflective surfaces are arranged such that when the two beams emerge from the beam shearing system they contain more than 50 percent of the photon flux within the selected spectral pass band. In one embodiment, the reflective surfaces are located on a number of prism elements combined to form a beam shearing prism structure. The interferometer utilizing the beam sharing system of the invention includes fore-optics for collecting light and focusing it into a beam to be sheared, and a detector located at an exit pupil of the device. In a preferred embodiment, the interferometer has no moving parts.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on provisional patent application Serial No. 60/129,383 filed Mar. 13, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has certain rights in this invention pursuant to NAS7-1407 awarded by NASA. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates generally to beam shearing systems and their applications, particularly in spatially modulated or static interferometers, which have low volume, low mass, high spectra: resolution, a single instrument line shape function and are field widened. 
     Interferometers are a class of instruments that convert light from a source into an interference fringe pattern or interferogram. Interferometers make measurements on light within a certain portion of the spectrum. This portion is referred to as the predetermined spectral passband of the interferometer. Frequencies of light outside of the predetermined spectral passband are attenuated so that they do not cause inaccuracies in measurements made by the interferometer. 
     Spatially modulated or static interferometers use a beam shearing system to shear the input beam into two separate beams and a Fourier optical system can be used to recombine the two sheared beams at a detector array or photographic plate. This procedure generates an optical path difference across the wavefronts of the recombined beams, which results in the formation of a modulation pattern that is fixed in space. If a two dimensional detector array is used, then it can record the spatially modulated pattern as an interferogram in one dimension of the detector array and an image in the orthogonal direction. 
     Static interferometers are distinguished from other types of interferometers in that they do not require the movement of an optical component or the observational platform to generate an interferogram. Interferometers that require the movement of the observing platform or an optical element to generate their spectrum over a given time interval are prone to unrecoverable spectral errors. 
     The first reported static interferometer was a Michelson interferometer with tilted mirrors built by G. Stroke and A. Funkhouser (see G. W. Stroke et al., Physics Letters, 1965, Volume 16, Number 3, Pg. 272). Two decades later, T. Okamoto et al used a triangular Sagnac interferometer and a conventional camera lens to generate an interferogram (see T. Okamoto et al., Applied Optics, 1984, Volume 23, Number 2, Pg. 269). In all reported cases, traditional interferometer configurations and conventional Fourier Lenses have been used to generate the interferograms, and the detector arrays have operated in the ultra-violet to near infrared wavelength regimes. 
     The ability to operate a static interferometer at longer wavelengths than those used by existing static interferometers would offer certain advantages. For instance, operation in the thermal-infrared spectrum would enable fewer pixels to be used to sample the fringes of an interferogram because the frequency of fringes decreases with increased wavelength. Another advantage of operating in the thermal-infrared spectrum is that the surfaces of the optical components would no be required to meet the stringent surface quality and accuracy requirements of shorter wavelengths interferometers in order to prevent the generation of surface induced fringes that introduce errors by canceling the interferogram fringes. Thus, if a static interferometer were constructed that could operate in the thermal-infrared, then its optical components would be less costly and less time consuming to manufacture. 
     Conventional interferometer configurations, such as the Michelson and Sagnac interferometers, utilize beam shearing systems that waste at least 50% of the signal. These configurations typically require the beam of light input into the system to make two passes through a beam splitter during the shearing process. Light is therefore reflected back out the entrance through which it entered, resulting in the loss of at least one half of the light entering the static interferometer. 
     An alternative method that does not rely on the use of a beam splitter to generate a difference in optical path length was described by Padgett et al. U.S. Pat. No. 5,781,293. This method involves polarizing the input bean and then shearing it using birefringent crystals. Despite elimination of the beam splitter, at least 50% of the light entering this type of system is lost due to absorption by the input polariser. 
     A particular advantage of the Padgett et al static interferometer over other conventional interferometers is that it is field widened. Being field widened means that the slit can be increased to any reasonable width without influencing the spectral resolution. An interferometer will be field widened when it records the interferogram at a pupil plane. At a pupil plane, diffraction does not degrade spectral resolution. 
     A disadvantage of existing static interferometers is that their physical volume and mass increase significantly when high spectral resolution is required. This greatly increases cost in applications such as remote sensing devices mounted on satellites or space exploration vehicles. 
     Another disadvantage is that existing static interferometers do not have a single instrument line shape. The instrument line shape is the characteristic shape of the spectrum generated by the static interferometer when the instrument observes a particular frequency of radiation that is substantially narrower in bandwidth than the spectral resolution of the instrument. In existing static interferometers the line shape changes depending on the frequency of the radiation observed. These instruments must be calibrated for the line shape of each frequency in the instrument&#39;s bandwidth, which is a time consuming process. Data collected by these static interferometers are difficult to analyze and they are not suitable for generating high spectral resolution output in real time. It is desirable to use a static interferometer that possesses a single instrument line shape and which has near perfect spectral registration. This means that the detector array&#39;s output has a single line shape and the lines for the different frequencies are evenly spaced along the spectrum at equal wavenumber intervals. This simplifies the time required to calibrate the instrument and the time required to analyze the data recorded by the instrument. 
     Accordingly, it is desirable to develop a new static interferometer that is compact, makes use of the majority of incoming radiation, is field widened, can operate in the thermal-infrared region of the spectrum, has a single instrument line shape and has near perfect spectral registration. 
     SUMMARY OF INVENTION 
     In one aspect, the static interferometer of the present invention is capable of providing an instantaneous single-sided interferogram in a tangential exit pupil plane and an image in a sagiital image plane, both of which are located at the same point along the optical axis. The instrument can have an optical efficiency approaching 100 percent, has a high signal-to-noise ratio and is field widened. Because the interferogram is generated at a pupil plane by two perfectly collimated beams, the interferogram is not affected by diffraction. This characteristic enables the instrument to possess spectral radiometric purity, have a very broad spectral bandwidth and have the ability to operate within the thermal-infrared spectrum. In addition, this characteristic enables it to have a single instrument line shape and near-perfect spectral registration. Finally, the instrument is a compact and lightweight unit that is easy to align during construction and simple to calibrate. 
     In one particularly advantageous embodiment, the fore-optics collect light and focus it onto an entrance slit. The light passes through the entrance slit and into the beam shearing system, which splits it into two separate beams. The beam shearing system is constructed to ensure that the two beams of light emerging from it contain more than 50 percent of the collected light that is within the predetermined spectral passband of the instrument. The emerging beams are incident on a Fourier optical system, which collimates and recombines them onto the exit pupil plane. The recombined beams of light generate an interferogram on a detector line array located at a tangential exit pupil plane, enabling the intensity of the interferogram to be measured by the detector, read out by electronics and then digitized by an analogue to digital converter. The data processing system then manipulates the digital data to extract useful information concerning the spectral composition of the collected light. When fore-optics with a shifted pupil are used, measurements can be made using a single sided interferogram at the tangential exit plane. When a Fourier optical system is used that also focuses the light onto a sagiital image plane located at the same point on the optical axis as the tangential exit pupil plane, then a two-dimensional detector array can then be used to record the intensities of both the image and the interferogram. 
     The forgoing results are preferably achieved by static interferometers having: fore-optics for collecting light and focusing it into a beam; a spectral resolving system comprising of a beam shearing system to split :he beam of light having a photon flux within a predetermined spectral passband, an optical system for recombining the two split beams onto an exit pupil, and a detector located at the exit pupil. The beam shearing system preferably includes: an entrance slit structure having an entrance slit extending in a first direction for receiving the light collected by the fore-optics; a beam splitter aligned at an angle to the first direction so that the received beam of light is split into two separate beams; a reflective subsystem having a plurality of reflective surfaces defining separate light paths of equal optical path length for the two separate beams, the reflective surfaces arranged such that the two beams contain more than 50 percent of the photon flux that is within the predetermined spectral passband of the collected light. In this embodiment, the chief rays of the two separate beams are also substantially parallel to each other and the two light paths are of substantially equal optical path length. 
     In one form, the reflective surfaces are also arranged to ensure that the two beams remain substantially in phase relative to one another. In another form, a fore-optics may to have a shifted pupil design to generate single-sided interferograms at the exit pupil plane. In yet another form, the optical system has an optical axis and also recombines the beams that emerge from the beam shearing system to create a sagiital image plane located at the same point along that optical axis as the tangential exit pupil plane. In yet another form again, the interferometer contains a detector array, read out electronics and data processing system. The detector array records the intensity of the radiation incident on its pixels, the read out electronics digitizes these measurements and transfers them to the data processing system, and the data processing system manipulates the digitized measurements to obtain information about the spectrum of the incident radiation. In a still further form, the data processing system performs Fast Fourier Transforms (FFTs) on the digitized data to obtain the spectrum of the collected light. In a still further form again, the data processing system convolves the digitized data with digital filters to detect the presence or absence in the spectrum of the collected light of frequencies characteristically emitted or absorbed by particular chemicals. 
    
    
     DESCRIPTION OF DRAWINGS 
     The above and other features of the present invention may be more fully understood from the following detailed description, taken together with the accompanying drawings, wherein similar reference characters refer to similar elements throughout and in which: 
     FIG. 1 is a simplified block diagram of a static interferometer constructed in accordance with one embodiment of the invention; 
     FIG. 2 is an optical ray trace diagram of a static interferometer of the type illustrated in FIG. 1, illustrating the Y-Z plane; 
     FIG. 3 is an optical ray trace diagram of a static interferometer of the type illustrated in FIG. 1, illustrating the X-Z plane; 
     FIG. 4 is a three dimensional perspective view of a beam shearing system of the type illustrated in FIG. 2; 
     FIG. 5 is an optical ray trace diagram of a beam shearing system of the type illustrated in FIG. 2; 
     FIG. 6 is an optical ray trace diagram of the rays reflected by the beam splitting surface of a first prism of the beam shearing system of FIG. 5; 
     FIG. 7 is an optical ray trace diagram of the path of the first split beam in the first and second prisms of the beam shearing system of FIG. 5; 
     FIG. 8 is an optical ray trace diagram of the path of the second split beam in the beam shearing system of FIG. 5; 
     FIG. 8A illustrates the dimensions of the first prism of the beam shearing system of FIG. 5; 
     FIG. 8B illustrates the dimensions of the second and third prisms of the beam shearing structure of FIG. 5; 
     FIG. 9 is an optical ray trace diagram illustrating the path lengths of the two beams sheared by the beam shearing structure o the type illustrated in FIG. 2; 
     FIG. 10 is an optical ray trace diagram illustrating the path lengths of the chief rays of the two beams sheared by the beam shearing structure of the type illustrated in FIG. 2; 
     FIG. 11 is an optical ray trace diagram illustrating the path lengths of the marginal rays of the two beams sheared by the beam shearing structure of the type illustrated in FIG. 2; 
     FIG. 11A is an optical ray trace diagram of an alternative embodiment of the beam shearing system illustrated in FIG. 1; 
     FIG. 11B is an optical ray trace diagram of a second alternative embodiment of the beam shearing system illustrated in FIG. 1; 
     FIG. 12 is an optical ray trace diagram of fore optics of the type illustrated in FIG. 2; 
     FIG. 13 is an optical ray trace diagram of a Fourier optical system of the type illustrated in FIG. 2 (view of the X-Z plane); 
     FIG. 14 is an optical ray trace diagram of a Fourier optical system of the type illustrated in FIG. 2 (view of the Y-Z plane); 
     FIG. 15 is an optical ray trace diagram of a first alternative embodiment to the Fourier optical system illustrated in FIG. 1 (view of the X-Z plane); 
     FIG. 16 is an optical ray trace diagram of a variation on the first alternative embodiment to the Fourier optical system illustrated in FIG. 1 (view of the Y-Z plane); 
     FIG. 17 is an optical ray trace diagram of a second alternative embodiment to the Fourier optical system illustrated in FIG. 1 (view of the X-Z plane); 
     FIG. 18 is an optical ray trace diagram of a second alternative embodiment to the Fourier optical system illustrated in FIG. 1 (view of the Y-Z plane); 
     FIG. 19 is a three dimensional perspective of a detector array of the type illustrated in FIG. 1, covered by a blocking filter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 illustrates a static interferometer  10  including fore-optics  12 , a beam shearing system  14 , a Fourier optical system  16 , a detector array  18  and the read out electronics and data processing unit  20 . As shown in FIGS. 2 and 3, input light  22  is focused by the fore-optics  12  into a beam  24 , which passes through a slit  26  and into the beam shearing system  14 . The beam shearing system  14  splits the beam into two separate beans,  30  and  32 , that are widely spaced when they emerge. These beams  30 , 32  enter the Fourier optical system  16 , which recombines them to create a pupil plane  34  and an image plane  36 . The detector array  18  is located at the pupil plane  34  and at the image plane  36 . The detector array  18  measures the intensity of the light incident on different areas within the pupil plane  34  and the image plane  36 . These intensity measurements are then recorded by the read out electronics and data processing unit  20 , which performs additional manipulations to extract useful information from the raw measurement data. 
     The static interferometer  10  of FIG. 1 is preferably fabricated in the configuration shown in FIGS. 2 and 3, with telecentric fore-optics and a telecentric Fourier optical system. This configuration provides spectral radiometric purity, ensures a single instrument line shape, provides, perfect spectral registration and ensures that the interferometer is field widened. 
     The performance of the static interferometer  10  is enhanced when the beam shearing system  14  takes the form of a prism structure  50  as illustrated in FIGS. 4 and 5. This configuration results in an extremely compact form of the static interferometer  10 . The higher the refractive index of the material used to fabricate the prism structure  50 , the more compact the prism&#39;s design. The prism structure  50  also possesses characteristics that eliminate dispersion and astigmatism, maximize the efficiency of light utilization and ensure the wavefronts of the two beams emerging from the beam shearing system  30 , 32  are not out of phase with respect to each other. 
     The prism structure  50  is typically made of KBr to avoid the need for anti-reflection coatings, but it can also be made from CsI for very broad spectral bandwidth, from Ge to avoid chromatic defocus and to increase durability, from ZnSe for high durability. Ge and ZnSe require the use of anti-reflective coatings and the performance of CsI is considerably enhanced if antireflective coatings are used. In the illustrated embodiment, all prisms in the prism structure  59  are made from the same material to ensure that both the beams emerging from the beam shearing system  30 , 32  travel through optical paths of identical length. 
     Considering the prism structure  50  of FIG. 4 in further detail, the first prism  52  consists of an entrance surface  54  and a beam splitting surface  56 . A portion of the entrance surface  54  is covered in a reflective coating  58  and a portion of the beam splitter surface  54  is covered in a beam splitter coating  60 . The second prism  62  and third prisms  66  each have a surface that is completely coated in a reflective coating  64  and  66 . All other surfaces in the prism are optically transparent. The reflective coatings of the prism structure  50  are typically made from Al, Au or Ag and the beam splitter coating  60  is usually made from Ge. 
     The material chosen to construct the prism structure illustrated in FIG. 4 effects the optical efficiency of the beam splitter. The variation in optical efficiency with the choice of material is demonstrated by TABLE 1A. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1A 
               
             
             
               
                   
               
               
                 PRISM EFFICIENCIES AT 10 μm WAVELENGTH. 
               
             
          
           
               
                 Substrate/Coating 
                 Ge + Al 1   
                 Ge + Al + Protective 2   
                 Ge + Au + AR 3   
               
               
                   
               
               
                 KBr 
                 80% 
                 85% 
                 95% 
               
               
                 Csl 
                 72% 
                 80% 
                 95% 
               
               
                 ZnSe 
                 — 
                 — 
                 91% 
               
               
                 Ge 
                 — 
                 — 
                 90% 
               
               
                   
               
               
                 NOTE 1: “Ge + Al” implies a germanium beam splitter plus aluminum coatings on all reflection surfaces. No coatings are placed on transmission surfaces.  
               
               
                 NOTE 2: “Ge + Al + Protective” implies a germanium beam splitter coating, aluminum reflection coatings, and protective coatings on all transmission surfaces. The protective coatings protect the water-soluble salt crystals from moisture.  
               
             
          
         
       
     
     NOTE 3: “Ge+Au+Ar” implies a germanium beam splitter coating, gold reflection coatings, and anti-reflection coatings on transmission surfaces. The gold coatings require an adhesive layer. No germanium beam splitter coating is required for the germanium substrate. The AR coatings on the salt crystals are assumed to be narrow-band, and the AR coatings on the metallic substrates are assumed to be broad-band. 
     A closer examination of FIG. 5 demonstrates how the prism structure  50  shears the input beam  70  of FIG. 5 into two separate beams  72  and  74 . The input beam  70  enters the prism through the entrance surface  54  and is incident upon the beam splitter coating  60 . This splits the beam and approximately 50% of the light is reflected in a first split beam  72  and the remainder of the light is transmitted in a second split beam  74 . It is advantageous for the prism structure to be manufactured with the beam splitting surface  56  having a minimal tilt relative to the entrance surface  54 , shown as π on FIG.  5 . This improves the efficiency of the beam splitting coating  60 , reduces polarization and prevents the first split beam  72  from being totally internally reflected at the air gap between the beam splitter surface  56  and the second prism  62  by the beam reflected off the reflective coating on the entrance surface  58 . 
     The first split beam  72  is reflected by the reflective coating on the entrance surface  58  and is then reflected again by the second prism&#39;s reflective surface  64 . The first split beam  72  then exits the prism structure  50  through the exit surface  76 . The second split beam  74  continues until it strikes the third prism&#39;s reflective surface  68 , where it is reflected and exits the prism structure  50  through the exit surface  76 . The surfaces of the prism structure are arranged so that the first split beam of light  72  and the second split beam of light  74  have traveled the same optical distance and are parallel when they exit the exit surface  76 . Overall, the prism structure  50  is extremely efficient in its utilization of light. The only way that light entering the system through the entrance surface  54  can exit the prism structure  50  is through the exit surface  76 . 
     The prism structure  50  reflects the first split beam  72  three times and the second split beam  74  only once ensure that the wavefronts of the two beams do not undergo a 180° phase change with respect to each other. If the two wavefronts are in-phase, then the image is preserved across the entrance slit  26 , single sided interferograms can be generated without loss of signal, and the optical aberrations generated by the Fourier optical system  14  tend to cancel out when the two beams  30 , 32  recombine at the detector array  18 . 
     The shape of the prism structure  50 , as shown in FIG. 4, affords easy mounting and alignment. Proper alignment of the prisms is important because it ensures that the modulation efficiency of the interferometer is maximized. The modulation efficiency is a measure of the fringe visibility or contrast and directly affects the signal amplitude. When the chief rays emerging from the prism are not parallel to each other or the optical system is not telecentric, then the modulation efficiency, η(β), decreases as a function of the angle, β, between the two chief rays. 
     Therefore, a special mounting system (not shown) manufactured from aluminum is used that hard mounts the prisms  52 , 62 , 66  without the need for epoxies. The prisms are held in aluminum caps that are held in place by springs. The springs hold the prisms in alignment when the spectrometer temperature is reduced. The air gaps between the prisms are not sealed with adhesives because this introduces strong absorption features in the thermal infrared region of the spectrum. 
     A closer examination of FIGS. 5 through 11 reveals how the prism structure  50  is manufactured to ensure that it possesses the properties necessary for it to function effectively as the beam shearing system  14  in the static interferometer  10 . Three requirements apply to the construction of all of the three prisms  52 , 62 , 66 . The first is that the prism structure is constructed so that the entrance surface  54  is substantially perpendicular to the chief ray of the input beam  78  and the exit surface  76  is substantially perpendicular to both chief rays of the exit beams  30  and  32 . The purpose of constructing the prism structure  50  in this way is to eliminate dispersion and astigmatism. Dispersion alters the instrument line shape as a function of wavenumber and astigmatism creates different wavefront errors in each of the two exit beams  30  and  32 , broadening the instrument line shape. The following equation and explanation illustrates the effect of astigmatism on the instrument line shape. Let Δε represent the difference in wavefront errors between the two beams at the exit pupil. Wavefront errors are caused by optical aberrations. If the optical aberrations or wavefront errors of the two beams are identical and vary slowly across the wavefront, then there is no influence on the spectral resolution or instrument line shape. Astigmatism would cause a difference in wavefront errors between the two beams. For a given Δε, the width of the instrument line shape at half maximum, ΔV ½ , will increase according to the following equation:          Δ                   v     1   /   2         =       0.605     δ   max       +       1     y   max                  v                 Δ                 ɛ       2        cos        (       FOV   F     2     )                {         [     F     Δ                 S                   cos        (       FOV   F     2     )           ]     2     -     1   4       }                                    
     where 
     ΔS is shown on FIG. 5 
     F is the focal length of the Fourier Lens, 
     δ max  is the maximum optical path difference, 
     Y max  is the distance between the center of the exit pupil and the edge of the exit pupil, 
     and 
     FOV F =2 arctan(ΔS/2F) is the field of view of the Fourier Lens. 
     The second is that the breadth of each of the prisms  52 , 52 , 66 , shown as b p  on FIG.  4 ,must be large enough so that all the light entering through the length of the slit is contained within the prism. In addition to these two requirements, there are manufacturing requirements that apply to each of the prisms  52 , 62 , 66  individually. The third is that the entrance surface must be perpendicular to the exit surface to minimize optical aberrations. 
     A closer examination of FIG. 6 demonstrates that the first prism  52  is manufactured so that its shape ensures the following. Firstly, the portion of the entrance surface  54  that is not coated is large enough to ensure that none of the input beam  70  is incident upon the portion of the entrance surface that is coated in reflective coating  58 . Secondly, the portion of he beam splitting surface that is coated in a reflective coating  6 C is wide enough to ensure that none of the input beam  70  is incident on the portion of the beam splittIng surface  56  that is not coated in beam splitting coating. Thirdly, the width of the first prism w 1 , the height of the first prism, shown as h 1 , on FIG.  6  and the tilt angle of the beam splitting surface relative to the entrance surface, shown as π on FIG. 6, ensure that the entire first split beam  72  is incident on the portion of the entrance surface  54  that is coated in reflective coating. Fourthly, the width of the first prism w 1 , the height of the first prism h 1 , the tilt angle π and the length of the beam splitting surface  56 , shown as 1 1  on FIG. 6, ensure that the entire beam  80  reflected from the portion of the entrance surface coated in reflective coating is incident on the portion of the beam splitting surface  56  that is not coated in beam splitting coating. Finally, the width of the first prism w 1 , the height of the first prism h 1 , the tilt angle π and the length of the beam splitting surface 1 1  ensure that the length of the optical paths taken by the first splat beam  72  and the second split beam  74  throughout the entire prism structure  50  are equivalent. 
     A closer examination of FIGS. 7 and 8 reveals that the second prism  62  is manufactured so that its shape ensures the following. Firstly, that the surface of the second prism adjacent to the beam splitting surface of the first prism  56  is matched to that surface. Secondly, that the width of the second prism, shown as w 2  on FIG.  7  and the angle of the second prism&#39;s reflective surface  64  relative to the entrance surface  54 , shown as θ on FIG. 7 ensure that the entire beam  80  reflected from the portion of the entrance surface coated in reflective coating is incident upon the second prism&#39;s reflective surface  64  and that the chief ray of the beam of light reflected from this surface  82  is perpendicular to the second prism&#39;s exit surface  84 . Thirdly, the width of the second prism w 2 , the angle of the second prism&#39;s reflective surface relative to the entrance surface θ and the height of the surface of the second prism that is adjacent to the third prism  86 , shown as h 2  on FIG. 7, ensure that the entire beam reflected from the second prism&#39;s reflective surface  88  emerges from the exit surface of the second prism  84  and that none of second split beam  74  is incident on the second prism&#39;s reflective surface  64 . Fourthly, the angle between the surface of the second prism adjacent to the third prism  86  and the second prism&#39;s exit surface  84 , shown as φ in FIG. 7, is as close to 90° as possible to prevent any total internal reflection that may result due to the tiny air gap between he two prisms. Finally, the width of the second prism w 2 , the angle of the second prism&#39;s reflective surface relative to the entrance surface θ and the height of the surface of the second prism adjacent to the third prism h 2  ensure that the length of the optical paths taken by the first split beam  72  and the second split beam  74  throughout the entire prism structure  50  are equivalent. 
     A closer examination of FIG. 8 reveals that the third prism  66  is manufactured so that its shape ensures the following. Firstly, the surfaces of prisms two  62  and three  66  that are adjacent to each other  86  and  90  are matched to eliminate dispersion and aberration. Secondly, that its height, shown as h 3  on FIG. 8, its width, shown as W 3  on FIG.  8  and the angle of its reflective surface  68  relative to the entrance surface  54 , shown as a on FIG. 8, ensure that the chief ray of the beam of light reflected from the third prism&#39;s reflective surface  92  is perpendicular to the third prism&#39;s exit surface  94  and hat the entire beam of light reflected from the third prism&#39;s reflective surface  96  exits the third prism through the third prism&#39;s exit surface  94 . Finally, that the third prism&#39;s height h 3 , width W 3  and the angle of its reflective surface relative to the entrance surface a ensure that the length of the optical paths taken by the first split beam  72  and the second split beam  74  throughout the entire prism structure  50  are equivalent. FIGS. 8A and 8B show the dimensions of a prism structure  50  manufactured to receive an F/4 input beam from a slit with a length of 12.8 mm. FIG. 8A shows the dimensions of prism one  52  and FIG. 8B shows the dimensions of prisms two and three  62 , 68 . An optical prescription for these prisms is contained in TABLES 1B-1D for an optical system presented In TABLE 2. The description of the tables is presented in the form utilized by the optical design program marketed under the tradename ZEMAX. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1B 
               
             
             
               
                   
               
               
                 Non-Sequential Components Parameters 
               
             
          
           
               
                 Obj Type 
                 Obj Name 
                 Y Position 
                 Z Position 
                 Tilt X 
                 Material 
                 X ½ Width 
                 Y ½ Width 
               
               
                   
               
             
          
           
               
                 Poly Obj 
                 Prism A 
                   
                   
                   
                 KBr 
                 Scale 1 
                 Is Vol 1 
               
               
                 Poly Obj 
                 Prism B 
                   
                 5.851 
                   
                 KBr 
                 Scale 1 
                 Is Vol 1 
               
               
                 Poly Obj 
                 Prism C 
                   
                 21.851 
                   
                 KBr 
                 Scale 1 
                 Is Vol 1 
               
               
                 Rectangle 
                 A1 Mirror 
                 4.15 
                 —0.01 
                 10 
                 Mirror 
                 16 
                 5.85 
               
               
                 Rectangle 
                 A2 BmSp 
                 −5.40 
                 6.65 
                 −45 
                 mirror/KBr 
                 16 
                 4.64 
               
               
                 Rectangle 
                 C2 Mirror 
                   
                 37.0 
                   
                 Mirror 
                 16 
                 14 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1C 
               
               
                   
               
               
                 Prism A Vertex Parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Front Face Vertices 
                   
                 Back Face Vertices 
                   
               
             
          
           
               
                   
                 V1 
                 −16 
                 −10 
                 0 
                 V5 
                 −16 
                 −10 
                 5.85 
               
               
                   
                 V2 
                 −16 
                 10 
                 0 
                 V6 
                 −16 
                 10 
                 9.38 
               
               
                   
                 V3 
                 16 
                 10 
                 0 
                 V7 
                 16 
                 10 
                 9.38 
               
               
                   
                 V4 
                 16 
                 −10 
                 0 
                 V8 
                 16 
                 −10 
                 5.85 
               
             
          
           
               
                   
                 Repeated Vertices 
                   
               
             
          
           
               
                   
                 Front 
                 R 12340 
                 Back 
                 R 56780 
               
               
                   
                 Top 
                 R 26730 
                 Bottom 
                 R 15840 
               
               
                   
                 Left side 
                 R 12650 
                 Right side 
                 R 43780 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 1D 
               
               
                   
               
               
                 Prism B Vertex Parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Front Face Vertices 
                 Back Face Vertices 
               
             
          
           
               
                 V1 
                 −16 
                 −10 
                 0 
                 V5 
                 −16 
                 −10 
                 16 
               
               
                 V2 
                 −16 
                 10.00357 
                 3.53 
                 V6 
                 −16 
                 1.27 
                 16 
               
               
                 V3 
                 16 
                 10.00357 
                 3.53 
                 V7 
                 16 
                 1.27 
                 16 
               
               
                 V4 
                 16 
                 −10 
                 0 
                 V8 
                 16 
                 −10 
                 16 
               
             
          
           
               
                 Repeated Vertices 
               
             
          
           
               
                   
                 Front 
                 R 12340 
                 Back 
                 R 56780 
               
               
                   
                 Top 
                 R 26730 
                 Bottom 
                 R 15840 
               
               
                   
                 Left side 
                 R 12650 
                 Right side 
                 R 43780 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1E 
               
               
                   
               
               
                 Prism C Vertex Parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Front Face Vertices 
                   
                 Back Face Vertices 
                   
               
             
          
           
               
                   
                 V1 
                 −16 
                 −10 
                 0 
                 V5 
                 −16 
                 −10 
                 25.15 
               
               
                   
                 V2 
                 −16 
                 10 
                 0 
                 V6 
                 −16 
                 10 
                 5.15 
               
               
                   
                 V3 
                 16 
                 10 
                 0 
                 V7 
                 16 
                 10 
                 5.15 
               
               
                   
                 V4 
                 16 
                 −10 
                 0 
                 V8 
                 16 
                 −10 
                 25.15 
               
             
          
           
               
                   
                 Repeated Vertices 
               
             
          
           
               
                   
                 Front 
                 R 12340 
                 Back 
                 R 56780 
               
               
                   
                 Top 
                 R 26730 
                 Bottom 
                 R 15840 
               
               
                   
                 Left side 
                 R 12650 
                 Right side 
                 R 43780 
               
               
                   
                   
               
             
          
         
       
     
     A requirement in the manufacture of all the prisms, is that they must ensure the length of the optical paths taken by the first split beam  72  and the second split beam  74  through the prism structure  50  are equivalent. This requirement means that each of the rays in the split beams of light  72 , 74  must travel the same distance in each of the materials present in the prism structure  50  as the corresponding ray in the other split beam of light. A closer examination of FIGS. 9,  10  and  11  demonstrates that the prism structure  50  ensures that this occurs. In each of these figures the optical distance of the ray of the first split beam of light is the sum of the three distances d 1 , d 2  and d 3  and the optical distance traveled by the corresponding ray of the second split beam of light is the sum of the two distances d 4  and d 5 . In each of FIGS. 9,  10  and  11 , d 1 +d 2  +d 3 =d 4 +d 5 . Minor variations in the width of the air gaps between prisms one and two and prisms two and three are inconsequential. The optical path length for the prism structure shown in FIGS. 8A and 8B is 47 mm and the width of the air gaps is approximately 10 microns. This difference is unimportant because it does not have a significant effect on the marginal ray angles of the two split beams  72 , 74 , when they are incident on the Fourier optical system  16 . 
     A variation on the embodiment of the beam shearing system  14 ′ described above is shown in FIG. 11A, this arrangement replaces the prism structure  50  with an structure involving parallel plates acting as a beam splitter  150  and three mirrors  152 ,  154  and  156 . The basic arrangement of  3  reflections for the first split beam  72 ′ and one reflection for the second split beam  74 ′ is maintained. Use of this beam shearing system  14 ′ places a limitation on the F/# of the input beam and the possible size of the separation between the two emerging beams  30 ′ and  32 ′, shown as Δs on FIG. 11A because the beams diverge more rapidly in air than in a higher index material. 
     A closer examination of FIG. 11A reveals that the static interferometer must be manufactured in the following way. The entrance slit  26 ′ must be mounted inside the first mirror  152  and the parallel plate beam splitter  150  oriented perpendicular to the chief ray of the entrance beam  158 . The parallel plate beam splitter is coated on the second surface of the first parallel plate, and the second parallel plate is of equal thickness to the first plate to ensure that the total optical path length traveled by both beams is equal. 
     The first mirror is tilted at an angle that ensures that the first split beam  72 ′ is reflected from the beam splitter  150  to the second mirror  154 . The slit must be small compared to the returning beam  72 ′ to minimize the amount of light lost. The second and third mirrors are aligned so that the chief rays of the two emerging beams  30 ′ and  32 ′ are parallel. Overall, the mirrors must be arranged to ensure that both split beams of light  72 ′, 74 ′ travel the same optical distance. 
     Another variation of the beam shearing system is shown as  14 ″ in FIG. 11B This variation is designed for use with a collimated input beam. It uses a similar parallel plate and mirror configuration as in FIG. 11A except that the beam splitter  150 ′ is tilted with respect to the input beam  70 ′. The beam splitter  150 ′ splits the input beam into two beams  72 ″ and  74 ″. These beams are then reflected in a similar fashion to the beam shearing system shown as  14 ′ in FIG.  11 A. However, because the beams are collimated, the beam shearing system  14 ″ is able to recombine the two split beams at an exit pupil plane without the need for the Fourier optical system. The beam shearing system places a limitation on the F/# of the fore-optics to ensure that the exit pupil is sufficiently far from the fore-optics to enable the configuration shown. 
     The best performance of the static interferometer  10  is achieved, when the fore-optics are as shown in FIG.  12 . Manufacturing the fore-optics in this way ensures that the pupil of the fore-optics is shifted to one side of the optical axis. This results in the chief ray of the input beam  22  striking the exit pupil  34  at the edge of the detector array  18 , enabling a single sided interferogram to be recorded. Using a single sided interferogram instead of a complete interferogram reduces the size of the other components of the static interferometer  10  by a factor of two, without losing any information or signal. 
     Examining the illustration in FIG. 12 in greater detail, reveals that the fore-optics  12  comprise a scan mirror  160  and three coaxial aspheric mirrors  162 ,  164  and  166 . The first coaxial aspheric mirror  162  is an even asphere and the other two mirrors are simple conics  164 ,  166 . The input beam  22  is reflected by the scan mirror onto the first coaxial asphere  162 , which reflects the light onto the second coaxial asphere  164 . The light is finally reflected by the third coaxial asphere  166 , which focuses the light onto the entrance slit  26 . The focused beam  168  is telecentric and it is observed that its chief ray strikes one side of the aperture stop  170  of the second coaxial aspheric mirror because the pupil of the fore-optics is shifted. The four mirrors  160 ,  162 ,  164 ,  166  are co-axial, which means that the fore-optics are simple to construct. 
     A specific example of the mirror configuration of the fore-optics  12 , illustrated in FIG. 12, is provided in part of TABLE 2. The description of TABLE 2 is presented in the form utilized by the optical design program marketed under the tradename ZEMAX. The prescription in TABLE 2 assumes that the output light  24  is F/4 in the tangential y-plane and F/5 in the sagiital X-plane. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 ZEMAX PRESCRIPTION: Telescope and Prism in 
               
               
                 1.2 cm −1  Imaging Spectrometer. 
               
             
          
           
               
                   
                   
                   
                 Radius 
                 Thickness 
                   
                   
                   
                   
               
               
                 Surface 
                 Type 
                 Name 
                 Mm 
                 mm 
                 Conic 
                 Coeff on r4 
                 Coeff on r6 
                 Coeff on r8 
               
               
                   
               
             
          
           
               
                 Object 
                   
                   
                 Infinity 
                 infinity 
                   
                   
                   
                   
               
               
                 1 
                 apert 
                 ent port 
                   
                 130 
               
               
                 2 
                 coord 
                 tilt Y 
                   
                   
                   
                 6° 
               
               
                 3 
                 mirror 
                 Scan 
                 Infinity 
                 −140 
               
               
                 4 
                 coord 
                 tilt Y 
                   
                   
                   
                 6° 
               
               
                 5 
                 mirror 
                 TM1 asph 
                 631.6257 
                 122.6101 
                 −3.2 
                 −8.326e-10 
                 3.06657e-14 
                 −2.45337e-18 
               
               
                 STOP 
                 mirror 
                 TM2 conic 
                 176.0148 
                 −122.6101 
                 0.155589 
               
               
                 7 
                 mirror 
                 TM3 conic 
                 244.9272 
                 215.1279 
                 0.182058 
               
               
                 8 
                 coord 
                 Decenter Y 
                   
                   
                   
                 31.2483 mm 
               
               
                 9 
                 apert 
                 Slit 
                   
                 3 
               
               
                 10 
                 coord 
                 Decenter Y 
                   
                   
                   
                 3.2 mm 
               
               
                 11 
                 NSC 
                 Prism 
                   
                   
                 Reverse 
                 exit location Y: 
                 exit location Z: 
                 exit tilt X: 
               
               
                   
                   
                   
                   
                   
                 rays: 1 
                 −10.001 mm 
                 16.1112 mm 
                 −90°  
               
               
                 12 
                 standrd 
                 Gap 
                   
                 −0.042 
               
               
                 13 
                 coord 
                 Decenter Y 
                   
                   
                   
                  −12 mm 
               
               
                 14 
                 coord 
                 tilt Y 
                   
                 −134.468 
                   
                 −13.383° 
               
               
                 15 
                 coord 
                 Decenter X 
                   
                   
                   
                 13.927 mm 
               
               
                 16 
                 mirror 
                 X toroid 
                 136.1947 
                 113.6148 
                   
                 −9.0040886e-9 
                 −2.775225e-13 
               
               
                   
                   
                 FM1 
                 148.5778 
               
               
                 17 
                 mirror 
                 X toroid 
                 −74.663 
                 −77.3079 
                   
                 5.3235734e-6 
                 −2.8462433e-7 
                 −8.10146e-10 
               
               
                   
                   
                 FM2 
                 99.7735 
               
               
                 18 
                 mirror 
                 asphere 
                 171.811 
                 77.3079 
                   
                 −9.2187783e-8 
                 1.719915e-11 
                 −2.72977e-15 
               
               
                   
                   
                 FM3 
               
               
                 19 
                 mirror 
                 Y toroid 
                 −298.0 
                 −132.6079 
                   
                 7.7831648e-8 
                 −1.816862e-11 
               
               
                   
                   
                 FM4 
                 −209.474 
               
               
                 20 
                 coord 
                 tilt Y 
                   
                   
                   
                 14.582° 
               
               
                 Image 
                   
                 focal plane 
                 infinity 
               
               
                   
               
               
                 NOTE 1:  
               
               
                 Spectral resolution is 1.21 cm −1 . Pupil width is 50 mm, X-Field is +/− 1.888°, Y-Field is 9°, and wavelength is 13 μm. Design is valid between 5 and 20 μm with KBr or 5 and 50 μm with CsI. Tangential width of stop is 30.6 mm; sagittal width is 24.6 mm. Stop is shifted along tangential plane away from slit so that chief ray is 6.2 mm from short side and 23.8 mm from long side of stop. Detector is 12.8 × 16 mm.  
               
               
                 NOTE 2:  
               
               
                 In Zemax the X toroids are generated by placing coordinate breaks of 90° and −90° before and after a standard “Y” toroid. However, many optical design packages explicitly call out Y and X toroids to omit the two coordinate breaks. Therefore the coordinate breaks for the X toroids have been omitted in the above prescription. The top number is the radius, and the bottom number is the radius of rotation.  
               
               
                 NOTE 3:  
               
               
                 Decenters, tilts, and NSC parameters are listed in the columns with the headings “Conic”, “Coeff. on r4”, “Coeff. on r6”, and “Coeff. on r8”.  
               
             
          
         
       
     
     The best performance of the static interferometer  10  is achieved, when the two beams emerging from the beam shearing system  30 , 32  are recombined using the Fourier optical system  16  shown in FIGS. 13 and 14. The Fourier optical system  16  is manufactured to maximize the image focal length and spatial resolution of the Fourier optical system by ensuring that the imaging and collimating functions are performed by the same optical components. It is also telecentric in the tangential plane to ensure spectral radiometric purity. 
     Examining the illustration in FIGS. 13 and 14 in greater detail, it shows a form of the Fourier optical system  6  having four mirrors and which is completely anamorphic. The first and second mirrors  180 , 182  are aspheric toroids that each have an aspherical surface in the X plane and a spherical surface in the Y plane. The third mirror  184  is an even asphere and the fourth mirror  186  is an aspheric toriod with an aspheric surface in the Y plane and a spherical surface in the X plane. All four mirrors are co-axial, with a vertices along the common axis  190 . This property ensures that no alignment of the mirrors of the Fourier optical system is required. Their mounting (not shown) is a simple ‘bolt and go’ configuration. 
     The mirrors are manufactured to ensure that the two beams emerging from the beam shearing system  30 , 32  strike the first mirror  180  and are reflected onto he second mirror  182 , which is located inside a cut out of the fourth mirror  186 . The mirrors also ensure that the two beams are then reflected onto the third mirror  184  and from there onto the fourth mirror  186 . The combination of the four mirrors ensures that the light reflected from the fourth mirror  186  forms two collimated exit beams  192  and  194  that exit the Fourier, optical system through a cut-out of the third mirror  184 . The manufacture of the mirrors ensures that the two exit beams  192 , 194  combine and form a pupil plane  34  in the Y plane and an image plane  36  in the X plane, with both planes located at the same point along the optical axis  190 . 
     A specific example of the mirror configuration of he Fourier optical system  16 , illustrated in FIGS. 13 and 14, is also provided in TABLE 4. The description of TABLE 3 is presented in the form utilized by the optical design program marketed under the tradename ZEMAX. Manufacturing the Fourier optical system  16  according to the prescription in TABLE 4 results in it having focal length in the X and Y plane of 64 mm. 
     Manufacturing the fore-optics  12 , the beam shearing system  14  and the Fourier optical system  16  according to the prescriptions contained in TABLES 1B-1E and 2, results in the static interferometer achieving a spectral resolution of 1.2 cm −1  over the spectral bandpass of the material used to construct the prism structure  50 . If the length of the entrance slit  26  is chosen to be 12.8 mm, then the static interferometer  10  will have a 3.8° field of view (FOV). 
     An alternative method of manufacturing the Fourier optical system  16 ′ to achieve a broadband, high spectral resolution, off-axis point spectrometer is shown in FIG.  15 . It consists of four off-axis aspheres,  180 ′,  182 ′,  184 ′ and  186 ′, which generate two very high quality collimated beams. These beams generate a 0.5 cm −1 , single-sided interferogram across a 25 mm detector array. 
     FIG. 16 shows a variation on this alternative method of manufacturing the Fourier optical system  16 ″. This variation uses the same four off-axis aspheres,  180 ″,  182 ″,  184 ″ and  186 ″, but uses a fold mirror  200  and a cylindrical mirror  202  to compress the collimated beam to 1 mm in the X direction to improve the signal efficiency and reduce the width of the detector array  18 . A cylindrical mirror  202  is preferable to a refractive cylindrical lens because the rays of the collimated beams passing through the thicker part of a lens would be shifted in the Y direction with respect to the rays passing through the thinner part of that lens. An optical prescription for this Fourier optical system  16 ″ is contained in TABLE 2. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 ZEMAX PRESCRIPTION: 0.5 cm −1  Point Spectrometer 
               
             
          
           
               
                   
                   
                   
                 Radius 
                 Thickness 
                   
                   
               
               
                 Surface 
                 Type 
                 Name 
                 mm 
                 mm 
                 Coeff on r4 
                 Coeff on r6 
               
               
                   
               
             
          
           
               
                 Object 
                   
                   
                 infinity 
                 infinity 
                   
                   
               
               
                 STOP 
                 stanrd 
                 ent pupil 
                   
                 100 
               
               
                  2 
                 paraxial 
                 telescope 
                 100 
                 100 
               
               
                  3 
                 apert 
                 slit 
                   
                 3 
               
               
                  4 
                 KBr 
                 prism 
                   
                 93 
               
               
                  5 
                 coord 
                 tilt X 
                   
                   
                   −45° 
               
               
                  6 
                 mirror 
                 C2 
               
               
                  7 
                 coord 
                 tilt X 
                   
                 −15 
                   −45° 
               
               
                  8 
                 standrd 
                 prism exit 
               
               
                  9 
                 coord 
                 decenter Y 
                   
                   
                 −27.75 mm 
               
               
                 10 
                 coord 
                 tilt Y 
                   
                 −138.7318 
                 −4.6515° 
               
               
                 11 
                 coord 
                 decenter X 
                   
                   
                 ±13.5551 mm 
               
               
                 12 
                 mirror 
                 M1 asphere 
                 146.0153 
                 83.9595 
                 1.5855e-9 
                 −4.1844e-14 
               
               
                 13 
                 mirror 
                 M2 asphere 
                 109.167 
                 −95.2057 
                 4.1951e-7 
                 −8.3999e-10 
               
               
                 14 
                 mirror 
                 M3 asphere 
                 235.8198 
                 95.2057 
                 4.1801e-8 
                 −1.0609e-13 
               
               
                 15 
                 mirror 
                 M4 asphere 
                 −295 
                 −120.4057 
                 4.3190e-8 
                 2.7327e-12 
               
               
                 16 
                 coord 
                 dec X tilt Y 
                   
                   
                 12.1 mm 
                 4.6515° 
               
               
                 17 
                 coord 
                 tilt Y 
                   
                   
                   20° 
               
               
                 18 
                 mirror 
                 fold mirror 
               
               
                 19 
                 coord 
                 tilt Y 
                   
                 26.12 
                   20° 
               
               
                 20 
                 coord 
                   
                   
                   
                 −20° 
               
               
                 21 
                 mirror 
                 X toroid 
                 −56 
               
               
                 22 
                 coord 
                   
                   
                 −26 
                 −20° 
               
               
                 Image 
                   
                 focal plane 
                 infinity 
               
               
                   
               
               
                 NOTE 1:  
               
               
                 Spectrometer can achieve 0.5 cm −1 . Pupil width is 50 mm, X,Y-Fieids are +/− 0.55° and wavelength is 13 μm. Design is valid between 5 and 20 μm with KBr or 5 and 50 μm with CsI. Tangential width of stop is 25 mm; sagittal width is 12.5 mm. Stop is shifted along tangential plane by −9.3 mm. A paraxial lens is used to simulate the telescope. Shifts in the ± X direction simulate the prism. Cylindrical mirror compresses beam without adversely  
               
               
                 # affecting interferogram. Detector array is 1 × 25 mm  
               
               
                 NOTE 2:  
               
               
                 Decenters and tilts are listed in the column with the heading “Coeff. on r4”.  
               
               
                 An alternative method of manufacturing the Fourier optical system 16′′′ to achieve a large FOV, on-axis refractive imaging spectrometer is shown in FIGS. 17 and 18. This configuration is appropriate where the spectral bandwidth requirements are not too broad. The imaging focal length can be made significantly shorter than the interferogram focal length of the Fourier optical system 16′′′ to increase the FOV. The design, shown in FIGS. 17  
               
               
                 # and 18, uses three aspheric Ge lenses 210, 212 and 214 to collimate the beams emerging from the beam shearing system 30,32 to form a pupil plane 34 in one axis and to form an image plane 36 in the other axis. An optical prescription for this Fourier optical system 16′′′ is contained in TABLE 4.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 ZEMAX PRESCRIPTION: 1.2 cm −1  On-Axis Imaging 
               
               
                 Spectrometer 
               
             
          
           
               
                   
                   
                   
                 Radius 
                 Thickness 
                   
                   
                   
               
               
                 Surface 
                 Type 
                 Name 
                 mm 
                 mm 
                 Coeff on r4 
                 Coeff on r6 
                 Coeff on r8 
               
               
                   
               
             
          
           
               
                 Object 
                   
                   
                 infinity 
                 infinity 
                   
                   
                   
               
               
                 STOP 
                 standrd 
                 ent pupil 
                   
                 64 
               
               
                 2 
                 paraxial 
                 Telescope 
                 64 
                 64 
               
               
                 3 
                 apert 
                 Slit 
                   
                 3 
               
               
                 4 
                 KBr 
                 Prism 
                   
                 40.158 
               
               
                 5 
                 coord 
                 tilt X 
                   
                   
                 −45° 
               
               
                 6 
                 mirror 
                 C2 
                   
                   
               
               
                 7 
                 coord 
                 tilt X 
                   
                 −6.842 
                 −45° 
               
               
                 8 
                 standrd 
                 prism exit 
                   
                 21.2106 
               
               
                 9 
                 Ge 
                 L1 asphere 
                 292.1871 
                 3 
                 −3.1404e-7 
                 −4.477e-11 
                 4.1884e-13 
               
               
                 10 
                 — 
                 Y toroid 
                 −285.5098 
                 14.5723 
               
               
                 11 
                 Ge 
                 L2 standard 
                 67.2090 
                 2.35 
               
               
                 12 
                 — 
                 asphere 
                 −60.0 
                 38 
                 −1.0316e-7 
                 1.76843e-11 
                 −3.6181e-11 
               
               
                 18 
                 Ge 
                 L3 window 
                 infinity 
                 1.5 
               
               
                 21 
                   
                 Y toroid 
                 −63.6822 
                 15 
                 −1.1044e-5 
                 1.78802e-7 
               
               
                 Image 
                   
                 focal plane 
                 infinity 
               
               
                 Object 
                   
                   
                 infinity 
                 infinity 
               
               
                 STOP 
                 standrd 
                 ent pupil 
                   
                 64 
               
               
                 2 
                 paraxial 
                 telescope 
                 64 
                 64 
               
               
                 3 
                 apert 
                 slit 
                   
                 3 
               
               
                 4 
                 KBr 
                 prism 
                   
                 40.158 
               
               
                 5 
                 coord 
                 tilt X 
                   
                   
                   45° 
               
               
                 6 
                 mirror 
                 C2 
                   
                   
               
               
                 7 
                 coord 
                 tilt X 
                   
                 −6.842 
                   45° 
               
               
                 8 
                 standrd 
                 prism exit 
                   
                 21.2106 
               
               
                 9 
                 Ge 
                 L1 asphere 
                 292.1871 
                 3 
                 −3.1404e-7 
                 −4.477e-11 
                 −4.1884e-13 
               
               
                 10 
                 — 
                 Y toroid 
                 −285.5098 
                 14.5723 
               
               
                 11 
                 Ge 
                 L2 standard 
                 67.2090 
                 2.35 
               
               
                 12 
                 — 
                 asphere 
                 −60.0 
                 38 
                 −1.0316e-7 
                 1.76843e-11 
                 −3.6181e-11 
               
               
                 18 
                 Ge 
                 L3 window 
                 infinity 
                 1.5 
               
               
                 21 
                   
                 Y toroid 
                 −63.6822 
                 15 
                 −1.1044e-5 
                 1.78802e-7 
               
               
                 Image 
                   
                 focal plane 
                 infinity 
               
               
                   
               
               
                 NOTE 1:  
               
               
                 Pupil width is 16 mm, Y-Fields are +/−12° and wavelength is 13 μm. Design is valid between 5 and 16 μm. Width of stop in X direction is 16 mm. Width in Y direction is 12.8 mm. Stop can be shifted along tangential plane to generate single-sided interferogram and achieve 1.2 cm −1  spectral resolution. A paraxial lens is used to simulate the telescope. Coordinate break shifts in ± X  
               
               
                 # simulate the prism. Detector array is 12.8 × 16 mm.  
               
               
                 NOTE 2:  
               
               
                 In this design the pupil plane is in the X direction and the image plane is in the Y direction. This enabled “Y” toroids to be used to generate the image, and “Y” toroids do not require coordinate brakes to rotate them about the Z axis.  
               
               
                 NOTE 3:  
               
               
                 Tilts are listed in the column with the heading “Coeff. on r4”.  
               
             
          
         
       
     
     Closer inspection of FIG. 2 illustrates that a 2 dimensional detector array is located at the pupil  34  and image  36  planes. This array makes measurements of light intensity in both the pupil and image planes. Large format AlGaAs Quantum Well Infrared Photoconductor (QWIP) arrays are the most appropriate detectors because their pixel to pixel responsivity is uniform. AlGaAs QWIP arrays can be fabricated with an operability greater than 99.99%, an uncalibrated uniformity better than 2% and a calibrated uniformity better than 0.3%. They can also be thermally cycled indefinitely, are radiation hard and remain stable after calibration. 
     AlGaAs QWIP detector arrays are not the only type of detector that can be used. Other examples of detector arrays that could be utilized by the static interferometer  10  include InSb, HgCdTe, microbolometers, thermopiles, CCDs and active pixel sensor (APS) arrays. 
     The processor unit  20  analyzes the data recorded by the detector array  18 . This unit digitizes the data, stores it, can analyze it in a number of ways, including the two following methods and can output any results. The data recorded by the detector array  18  corresponds to a series of interferograms. The first possible method of analyzing this data is to perform a digital fast fourier transform (FFT) to convert each interferogram into a spectrum, which can then be further analyzed. An example would be where the spectrum s analyzed to determine the overall chemical composition of the scene being viewed. 
     An alternative method of analyzing the data is to use the interferograms as matched filters to detect distinct features within the interferograms. An example of this would be to detect the presence of a single chemical in the scene being viewed. This process involves using a filter shape that indicates the presence or absence of hat feature, when the filter is convolved with the interferogram. An advantage of using this second technique is that it is less computationally intensive than performing numerous FFTs. Another advantage is that the interferogram automatically separates broadband features, such as emissions from a planet&#39;s surface from narrowband features, such as are created by constituent gases in the planet&#39;s atmosphere. 
     At shorter wavelengths it may be necessary to use heterodyning at the detector array. Heterodyning is appropriate if a full cycle of the interferogram  220  of FIG. 19 is located inside one pixel  222  of the detector array  18 . When this occurs, the constructive  224  and destructive  226  half cycles of the interferogram will cancel each other out and the detector  18  will detect zero light intensity for that pixel  222 . This phenomenon can be avoided by using a blocking filter  228 . This filter discards the destructive portion of the interferogram  226 , ensuring that only the constructive portion of the interferogram  224  is incident on the pixel of the detector array  222 . In a broadband system, the blocking filter must be manufactured so that its pattern has a frequency that is equal to or lower than the lowest frequency in the predetermined spectral passband. The detector will then detect that light to be frequency zero and light with higher frequency will be detected as having a frequency equal to the actual frequency of the detected radiation minus the frequency of the radiation of lowest frequency. interferogram  220 , using a detector  18  that would not have enough pixels to measure the intensity of the interferogram without the use of a blocking filter. 
     While the preferred embodiment has been described and illustrated, various substitutions and modifications may be made thereto without departing from the scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.