Abstract:
Optical instruments having, inter alia, optics to process wavelengths of electromagnetic radiation to produce an interferogram. The instruments include an optical path and optical elements positioned along this path for splitting and recombining the wavelengths which interfere with each other to produce a plurality of different fringes of different wavelengths. The optics include matched gratings which are positioned along the optical path outside of the interferometer optics to produce first and second sets of spectrally dispersed beams. The interferometer optics includes a beam splitter and first and second mirrors. In two embodiments the beam splitter has an internal surface including three zones. The instruments can all include a detector for detecting the interferogram and means for processing the detected interferogram to produce spectral information that is spatially distributed.

Description:
PRIOR APPLICATIONS 
   This invention is a continuation-in-part of application Ser. No. 11/481,441, filed Jul. 5, 2006 and now abandoned, which is a continuation of application Ser. No. 10/723,901, filed on Nov. 25, 2003, now U.S. Pat. No. 7,167,249 B1, the disclosures of which are incorporated by reference. This application is also a continuation-in-part of application Ser. No. 11/078,019, filed Mar. 11, 2005, which is a continuation of application Ser. No. 10/651,491, now U.S. Pat. No. 6,992,775 B1. Insofar as these latter two relate to beam splitters, the disclosures are incorporated by reference. 

   GOVERNMENTAL RIGHTS CLAUSE 
   This invention is made with U.S. Government support under contracts F 29601-96-C-0096 and F29601-98-0204 awarded by the U.S. Air Force. The U.S. Government has certain rights in the invention. 

   FIELD OF THE INVENTION 
   Optical instruments which process wavelengths of electromagnetic radiation to produce an interferogram. More particularly, instruments (e.g., Fourier transform spectrometers) that produce interferograms of a spatially resolved scene, which instruments include an optical system which both splits the incoming light and physically separates it to produce two beams. U.S. Pat. No. 6,687,007 B1 to Meigs discloses embodiments wherein a matched pair of gratings is positioned inside the interferometer optics to further disperse the two beams. U.S. Pat. No. 7,167,249 B1 and application Ser. No. 11/481,441 (now abandoned) discloses embodiments wherein the matched pair of gratings are positioned outside the interferometer optics to further disperse the two beams. 
   The claimed invention relates to a new beam splitter which, like the beam splitter disclosed in the parent applications, eliminates the 50% light loss inherent in the Sagnac (or common path) interferometer. Two embodiments include the further improvement that both beams go through the same amount of glass (i.e., each have the same path length), so that the aberrations for both are identical. The net result is that such identical aberrations cancel each other out. 
   The instruments are useful in absorption, emission, or reflected spectroscopy where there is a need to image a time and spatially varying scene. This could be, but is not limited to imaging laser material interactions, an emission plume for a jet or rocket engine or a smoke-stack, environmental observations, non invasive blood monitoring, and other medical observations. 
   BACKGROUND OF THE INVENTION 
   Imaging spectrometers are, broadly speaking, optical instruments which process the electromagnetic radiation from a source into its fundamental components. For instance, an interferometric based spectrometer divides light from a source and interferes it to produce a fringe pattern of interfering light (i.e., an interferogram). The interference pattern can be captured on film or by, for instance, an electronic detector, for example, a semi-conductor array detector (e.g., a charged coupled device (CCD)). 
   There are numerous optical interferometer designs. The basic form of the Sagnac (or common path) interferometer is illustrated in  FIG. 1 . It is also illustrated in U.S. Pat. No. 4,976,542 to Smith. Other designs include the Mach-Zender interferometer, the Michelson interferometer and Twyman-Green interferometer (See W. L. Wolfe, Introduction to Imaging Spectrometers, SPIE Optical Engineering Press, pp. 60-64, 1997), the Fabry-Perot interferometer (see Wolfe, p. 70-73), the Lloyd&#39;s mirror interferometer (see the Smith patent) and, a variation of the common path interferometer (Sagnac) sometimes referred to as the Barnes interferometer (see T. S. Turner Jr., et al., A Ruggedized Portable Fourier Transform Spectrometer for Hyperspectral Imaging Applications, SPIE Vol. 2585 pp 222-232.) There are also dispersive spectrometers such as prism spectrometers and grating spectrometers. (See Wolfe, pp. 50-52 and 55-57). 
   In a non-imaging Fourier transform spectrometer the point source of radiation is split into two virtual points a fixed distance apart to yield a fringe pattern at the detector. If one wants to attain a fine spectral resolution, the distance between the two virtual points should be large; for a course spectral resolution, it should be very small. This distance may be controlled by shifting one of the mirrors (typically referred to as lateral shear) of, for instance, the common path interferometer. With this arrangement, a wide spectral range measurement loses resolution, while a high resolution measurement reduces the effective spectral range. In an imaging spectrometer, the point source is imaged with a set of imaging optics and a slit is inserted giving the instrument the capability of one-dimensional imaging in the direction perpendicular to the shear. 
   Shear, both lateral and angular, is discussed in Turner, Jr. et al. (supra). For the Sagnac, translation of either mirror in the plane of  FIG. 1  produces lateral shear. Mirror tilt about an axis perpendicular to the drawing plane also produces lateral shear. Conversely, in the Barnes interferometer only angular shear is possible and is produced only by mirror tilt. See FIGS. 2 and 3 of Turner, Jr., et al. 
   U.S. Pat. No. 4,976,542 to W. H. Smith discloses a Fourier transform spectrometer which incorporates the common path (or Sagnac) interferometer and in which a charge-coupled device (CCD) is placed in the image plane instead of film. The CCD has pixels aligned along two dimensions to provide both spectral resolution and spatial resolution. The CCD is characterized by greater dynamic range, lower pixel response variation, and is photon nose limited, all of which enhances its use as a detector for a spectrometer. See also Digital Array Scanned Interferometers for Astronomy, W. H. Smith, et al., Experimental Astronomy 1: 389-405, 1991. In these devices, the interferometer introduces lateral shear in one direction and a two dimensional camera is aligned so a row of pixels is parallel to this geometric plane. In the perpendicular direction, a set of cylindrical lenses is used to provide an imaging capability along the columns of pixels. A row plot from the detector is an interferogram similar to the interferogram collected in a temporally modulated Michelson interferometer. 
   OBJECTS OF THE INVENTION 
   It is an object of the present invention to provide an improved beam splitter. 
   It is also an object of the present invention to provide an improved beam splitter in which both portions of the split beam pass through an amount of glass so that they both have the same optical path length. 
   It is an additional object of the present invention to provide a Fourier transform spectrometer with the improved beam splitters which has all the advantages of the spectrometers disclosed in U.S. Pat. No. 6,687,007 B1 to Meigs, but which has an increased optical throughput. 
   It is another object of the present invention to provide a Fourier transform spectrometer with the improved beam splitters of the present invention which both splits and spectrally disperses incoming wavelengths to produce two spectrally dispersed beams in which the long wavelengths within the range of wavelengths of interest do not overlap. 
   It is yet another object of the present invention to provide a Fourier transform spectrometer with the improved beam splitters of the present invention which utilizes a matched pair of gratings to spectrally disperse the incoming beam of wavelengths, which pair of gratings are located outside that portion of the spectrometer&#39;s optical system, the interferometer optics portion, which splits the incoming beam into two paths. 
   It is still another object of the present invention to provide an improved beam splitter which can be used in a common path interferometer without the use of dispersive gratings. 
   The foregoing and other objects will be apparent from the drawings and the description set forth herein. 
   SUMMARY OF THE INVENTION 
   The present invention relates to novel beam splitters including an optically transmissive member. The incumbent beam to be split is directed onto the first surface of the optically transmissive member, which surface may be an internal surface. The first surface is divided into first, second and third zones. In each of these zones, the percentage of light that is either reflected or transmitted is described as “substantially” for the reason that no optics can be perfect. While there will be a minimal amount of loss inherent to the optics, high quality parts can reduce this loss to, in some cases less than 1% of the total irradiance of the wavelengths incident. The first zone has a first coating which, for the wavelengths being split, is substantially 100% reflective. The second zone has a second coating which allows for substantially 50% of the wavelengths to be reflected and 50% to be transmitted. The third zone is substantially 100% transmissive. The second zone is between the first and the third zone. Optionally, there are uncoated regions between the first and second zones and between the second and third zones. The novel beam splitter designs are incorporated into interferometers and can be used with or without the dispersive gratings of, for instance, the type disclosed herein. 
   In one embodiment, in addition to the first surface, there are second and third surfaces which are parallel to each other and to the first surface. In the third embodiment, the beam splitter is in the form of a cube formed of two prisms, with the coatings on one of the two diagonal mating surfaces. The outer surfaces may have anti-reflective coatings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an optical schematic of a prior art spectrometer incorporating a common path or Sagnac interferometer. 
       FIG. 2  is an optical schematic of the first embodiment of the present invention incorporated in a Sagnac interferometer in which the grating pair is placed in front of the interferometer. 
       FIG. 3  is an optical schematic of the second embodiment of the present invention, in which the grating pair is positioned along the optical path of the interferometer after the interferometer optics. 
       FIG. 4  is an additional partial optical schematic of the first embodiment of the present invention, particularly illustrating the single element beam splitter and the reflected portion of the beam. 
       FIG. 5  is an additional partial optical schematic of the first embodiment of the present invention, particularly illustrating the single element beam splitter and the transmitted portion of the beam. 
       FIG. 6  is a partial optical schematic of the first embodiment of the present invention illustrating, inter alia, the convergence of the transmitted and reflected portions of the beam of  FIGS. 4 and 5 . 
       FIG. 7  is an optical schematic of the second embodiment of the present invention ( FIG. 2 ) in which the prior art beam splitter is replaced by the two element beam splitter. 
       FIG. 8  is an optical schematic of the novel beam splitter composed of two elements and three surfaces. 
       FIG. 9  is an optical schematic of the novel beam splitter in which the two elements form a cube, whereby all the exterior surfaces of the beam splitter are perpendicular to, as the case may be, the incoming or exiting beam. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , Fourier transform spectrometer  11  processes an incident light source  13  through an aperture  15 , to a beam splitter  17 , where source  13  is divided into a reflected beam (represented by central ray path  19 ) and a transmitted beam (represented by central ray path  21 ). The portion of source  13  represented by path  19  is reflected from the front surface a first mirror  23  to the front surface a second mirror  26 , and then back to beam splitter  17 . The second, transmitted portion of source  13  is reflected off second mirror  26 , back to first mirror  23  and through beam splitter  17 . Thus, beam splitter  17 , together with mirrors  23  and  25 , serve to split incident source  13  into two portions. Spectrometer  11  also includes a detector  27  which is placed at the back focal plane of spherical (Fourier) lens  29 . (Aperture  15  is in the front focal plane.) A second, cylindrical lens  31  is interposed between detector  27  and spherical lens  29 , which images spatial locations from  15  onto detector  27 . As is well known in the art, spherical lens  29  and cylindrical lens  31  may be assembled from one of several optical elements in various sequences to minimize optical aberrations. As is well understood in the art, this basic arrangement produces an interfering light pattern or fringe pattern at the focus of spherical lens  29 , commonly referred to as an interferogram. Cylindrical lens  31  images the relative physical distribution of source  13  as selected by aperture  15  such that as it impinges on detector  27  it represents the relative spacing for the various sources and their locations in source  13 . The detector  27  may be a charge-coupled device, or CCD. Alternatively, any photon counting array noise limited device, or other means of recording the optical signature, may be used. For an electric device, the output of detector  27  is processed by a computer  33  using Fourier transform techniques as is well known in the art to resolve the spectrum detected by detector  27 . In this configuration fully half of the light that enters beam splitter  17  and leaves the device is lost when beams  21  and  19  pass through beam splitter  17  the second time. 
   To correct the problems inherent in the interferometer of Okamoto et al., discussed in parent application Ser. Nos. 10/723,901 and 11/481,441, and achieve the objectives set forth above, a pair of matched gratings is incorporated into the interferometer of the present invention. Thus, with reference to  FIGS. 2 and 3 , matched gratings have been integrated into the conventional interferometer design.  FIG. 2 , spectrometer  41 , includes an aperture  43 , first and second matched gratings  45  and  47 , beam splitter  49 , first mirror  51 , second mirror  53 , lens system  55  and detector  57 . For grating pair  45 ,  47 , the choice of grating pitch, grating order, etc., is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1. Beam splitter  49 , is, in this embodiment, of conventional design. Mirrors  51  and  53  are typically first surface mirrors. Beam splitter  49 , together with mirrors  51  and  53  is sometimes referred to as interferometer optics  59 . Lens system  55  is of the conventional Fourier lens/cylindrical lens combination used to image spatial locations from aperture  43  onto detector  57 , and interfere the separated beams as discussed above with regard to  FIG. 1 . Further, as with the prior embodiment, the output of detector  57  is processed by a computer (not shown) using well known Fourier transform technique to resolve the spectrum detected. 
   In operation, incident light, represented by central ray path  61 , passes through aperture  43  and onto first and second gratings  45  and  47  where it is dispersed to produce the desired amount of lateral wavelength dependant spectral spreading of the light. This is illustrated in FIG. 2 of U.S. Pat. No. 6,687,007 B1. As with, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1, in addition to producing the desired lateral wavelength dependant spectral spreading, the use of grating pair  45  and  47  allows for an easy adjustment of the amount of lateral spectral spread in the system, without introducing any optical aberrations, because the gratings diffract light only in a plane perpendicular to the grooves. 
   After being dispersed by gratings  45  and  47 , the light is divided into a reflected beam, represented by central ray path  63  and a transmitted beam, represented by central ray path  65 . As those skilled in the art appreciate, ray paths  63  and  65  separated by mirrors  53  and  51  and are recombined by lens system  55  and focused onto detector  57 . This invention has the advantage over the prior art of allowing spectral mapping to be conducted at selectable wavelengths, defined by the choice of grating specifications, and thus increasing the spectral resolution over the selected waveband. 
     FIG. 3 , spectrometer  71  illustrates an alternate embodiment of the present invention in which the grating pair is positioned after the interferometer optics. Spectrometer  71  includes aperture  43 , beam splitter  49 , first mirror  51  and second mirror  53  of interferometer optics  59 , and detector  57 , which are identical to those depicted in  FIG. 2  and function to split incident radiation into two separate beams.  FIG. 3  also depicts lens system  85  which is the functional equivalent of lens system  55  of  FIG. 2 . As is well known in the art, lens system  85  includes a spherical (Fourier) lens  87  and a cylindrical lens  89  and functions to recombine the split beam paths onto detector  57  and image spatial locations defined by aperture  43  onto detector  57 . As is well known in the art, spherical lens  87  and cylindrical lens  89  may be assembled from one of several optical elements in various sequences to minimize optical aberrations.  FIG. 3  further depicts the incorporation of first pair of gratings  73  and  75 , and second pair of gratings  77  and  79 . For grating pairs  73 ,  75  and  77 ,  79 , the choice of grating pitch, grating order etc. is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1. The embodiment of  FIG. 3  of the current application differs from the invention of the prior application in that the reflected beam and transmitted beam are dispersed by separate and distinct pair of gratings. This arrangement is necessary as the beam of light is dispersed subsequent to being split. This arrangement, as in the previous embodiment, has the benefit of allowing spectral mapping to be conducted over selectable wavelengths based on the grating parameters thus increasing the spectral resolution over the selected waveband. 
   In operation, with reference to  FIG. 3 , incident light, represented by central ray path  61  is divided into a reflected beam, represented by central ray path  81 , and a transmitted beam (represented by central ray path  83 ) by interferometer optics  59 . Path  81  is directed onto a first pair of gratings  73  and  75 . The gratings, as is well known in the art, function to spectrally disperse the incoming beam path to produce the required amount of lateral spread in the wavelengths. The dispersed beam is depicted in  FIG. 3  as three beam paths collectively referred to as dispersed beam path  91 . Beam path  91  is then focused by lens system  85  onto detector  57 . Path  83  is spectrally dispersed in the same manner by a second pair of gratings  77  and  79  to produce dispersed beam path  93 , which is depicted in  FIG. 3  as three beam paths. Gratings  73  and  75  are symmetrically opposed and identical in all other respects to gratings  77  and  79 . Dispersed paths  91  and  93 , as those skilled in the art appreciate, are recombined by lens system  85  and both interfered and focused onto detector  57 . 
   The beam splitter  49  (and ultimately interferometer  59  which is commonly referred to as a Sagnac or common path interferometer) of  FIGS. 1 ,  2  and  3  is of conventional design and is well known in the art. One inherent disadvantage of this conventional beam splitter design is that it ultimately loses at least 50% of the incident light during the beam splitting process as those skilled in the art will appreciate. This deficiency is evident from  FIGS. 1 ,  2  and  3 . In  FIG. 3 , for example, incoming beam path  61  is split into central ray paths  81  and  83 , each representing 50% of the original light. Path  81  is reflected by mirrors  51  and  53  and directed back toward beam splitter  49 . Beam splitter  49  functions in exactly the same manner as described above in that it reflects 50% of the incident radiation light and transmits 50%. The 50% that is transmitted is not available to be directed to the detector, representing a loss in optical throughput. Thus the remaining light represents at a maximum only 25% of the total incident light. The total light of path  83  is similarly reduced with the difference that after being reflected by mirrors  51  and  53  onto beam splitter  49 , the reflected portion of the incident light is lost from the system. Thus, the total throughput of the spectrometer is reduced by 50% relative to the light which is inputted into the system. 
   To solve this inherent problem of the Sagnac (or common path) interferometer, beam splitter  103  (as referenced on  FIGS. 4 ,  5 , and  6 ) is incorporated into the interferometer optics  105  of spectrometer  101 , replacing beam splitter  49 .  FIG. 4  is a partial optical schematic that illustrates the use of beam splitter  103  to reflect a portion of the incident light.  FIG. 5  is a partial optical schematic that illustrates the use of beam splitter  103  to transmit a portion of the incident light.  FIG. 6  represents the combination of reflected light path of  FIG. 4  and the transmitted light path of  FIG. 5 . Spectrometer  101  further includes lens system  121  which is functionally equivalent to lens system  55  of  FIG. 2 , comprised of a cylindrical lens and a Fourier lens combined to recombine and focus the radiation on detector  57 . 
   Beam splitter  103  is constructed with an optically transmissive and reflective material. Beam splitter  103  includes first surface  107  and parallel second surface  109 . First surface  107  is divided into first zone  111 , second zone  113 , and third zone  115 . In one embodiment all three zones are of equal lengths along first surface  107 . In other embodiments, first zone  111  and third zone  115  are of equal lengths along first surface  107  with second zone  113  occupying the balance of length on first surface  107 . In another embodiment zones  111 ,  113  and  115  may be of different lengths. Optionally, there are uncoated regions between the first and second zones and between the second and third zones. Third zone  115  (on surface  107 ) is coated to reflect substantially 100% of the incident light in a spectral bandwidth compatible with the desired operating optical bandwidth of the system. Second zone  113  is coated to partially reflect and partially transmit the incident light, normally 50% each. First zone  111  is coated with an anti-reflective (or transmissive) coating to transmit substantially 100% of the incident light. 
   In operation, with reference to  FIGS. 4 ,  5  and  6 , incident light, represented by ray path  61  is spatially filtered with aperture  43  and partially reflected and partially transmitted by the coating of zone  113  on surface  107  of beam splitter  103 .  FIG. 4  depicts the path of the reflected light as two paths as it is naturally dispersed, collectively referred to as path  117 .  FIG. 5  depicts the path of the transmitted light as two paths as it is naturally dispersed, collectively referred to as path  119 .  FIG. 4  further illustrates that reflected beam path  117  is reflected by mirrors  51  and  53  and is directed toward third zone  115  of beam splitter  103 . Path  117  is incident upon second surface  109  of beam splitter  103  and is refracted toward the third zone  115  of first surface  107  where path  117  is reflected back through beam splitter  103  toward second surface  109 . Beam  117  is refracted upon exiting beam splitter  103  toward lens system  121  where one axis is focused upon detector  57  to image aperture  43  and the orthogonal axis is interfered on detector  57 . The angles of refraction, as is well known in the art, can be calculated by standard methodology. 
     FIG. 5  further illustrates the transmitted beam path  119  of spectrometer  101 . Transmitted beam path  119  is reflected by mirrors  51  and  53  and is directed toward first zone  111  of beam splitter  103  where it is further transmitted and refracted through first zone  111  of beam splitter  103  toward lens system  121  where one axis is focused upon detector  57  to image aperture  43  and the orthogonal axis is interfered on detector  57 . The angles of refraction, as is well known in the art, can be calculated by standard methodology. 
     FIG. 6  shows the combination of beam paths  117  and  119  of spectrometer  101 .  FIG. 6  further illustrates beam paths being recombined by lens system  121  where one axis is focused upon detector  57  to image aperture  43  and the orthogonal axis is interfered on detector  57 . 
   In the spectrometer  131  of  FIG. 7 , beam splitter  103  is incorporated into the invention as depicted in  FIG. 3 . Beam splitter  103  takes the place of the conventionally designed beam splitter  49 , and the disclosure of  FIGS. 4 ,  5  and  6  is incorporated into the description of  FIG. 7 . The embodiment of the invention as depicted in  FIG. 7  has multiple advantages over the prior art. First, it incorporates the benefits of gratings being placed after the beam splitter apparatus as described with reference to  FIG. 3  above. Second, the new beam splitter increases the throughput of the interferometer by approximately 100% as compared to the conventional interferometer design. Grating pairs  73 ,  75  and  77  and  79  function in the same manner as is disclosed with reference to  FIG. 3  above. The choice of grating pitch, grating order, etc., is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1, and will not be affected by the index of refraction of the substrate material of beam splitter  103 .  FIG. 7  also depicts lens system  133  which is the functional equivalent of lens system  85  of  FIG. 3 . As is well known in the art, lens system  133  includes a spherical (Fourier) lens  135  and a cylindrical lens  137  and functions to recombine the split beam paths onto detector  57  and to image in one axis aperture  43 . 
   In  FIG. 8  beam splitter  141  is comprised of two optical elements each of which has the same optical path length (e.g., the same refractive index and thickness) which, collectively, include first surface  143 , second surface  145  and third surface  147 . Similar to the first surface of beam splitter  103  (shown with reference to  FIGS. 4-7  above), second surface  145  is divided into first zone  149 , second zone  151  and third zone  153 . In one embodiment all three zones are of equal length along second surface  145 . In other embodiments, first zone  149  and third zone  153  are of equal length along second surface  145  with second zone  151  occupying the balance of length on second surface  145 . In a further embodiment, first zone  149 , second zone  151  and third zone  153  are of different lengths. As with beam splitter  103 , optionally there are uncoated regions between the first and second zones and between the second and third zones. First zone  149  (on surface  145 ) is coated to reflect substantially 100% of the incident light in a spectral bandwidth compatible with the desired operating optical bandwidth of the system in which beam splitter  141  is incorporated. Second zone  151  is coated to partially reflect and partially transmit the incident light, normally 50% each. Third zone  153  is coated with an anti-reflective (or transmissive) coating to transmit substantially 100% of the incident light. 
   In operation, incident light, represented by ray path  155 , is partially reflected and partially transmitted by second zone  151  on second surface  145  after being refracted by first surface  143 . Beam path  157  represents the path of the reflected light and beam path  159  represents the path of the transmitted light. Reflected beam path  157  is reflected by mirrors  163  and  161  and is refracted by third surface  147  such that it is incident upon first zone  149  of second surface  145 . Path  157  is then reflected back through beam splitter  141  toward third surface  147 . Beam  157  is refracted upon exiting beam splitter  141  toward lens system (not shown) where one axis focuses an image of an aperture (not shown) upon a detector (not shown) and the orthogonal axis is interfered on a detector (not shown). The angles of refraction, as is well known in the art, are calculated by standard methodology. 
     FIG. 8  further illustrates the transmitted beam path  159 . Transmitted beam path  159  is reflected by mirrors  161  and  163 , refracted by first surface  143  such that it is directed toward third zone  153  of second surface  145 , where it is further transmitted until being refracted by third surface  147  and directed toward a lens system (not shown) where one axis is focuses an image of the aperture (not shown) upon a detector (not shown) and the orthogonal axis is interfered on a detector (not shown). Spacing  165  is set to Nyquist sampling and can be adjusted by adjusting mirror  161 . 
   Beam splitter  141  can replace beam splitter  103  as described in previous embodiments (shown with reference to  FIGS. 4-7 ). Beam splitter  141  has been shown to make the aberrations identical for the two optical paths, as compared to beam splitter  103 , and thus reduce aberrations in the interferogram 
   In  FIG. 9  beam splitter  171  is composed of two prisms  173  and  175  which, when assembled together from a cube  177 . The optical path lengths (e.g., refractive indices and thicknesses) of both prisms are identical. With this design there are four external optically functional surfaces  181 ,  183 ,  185  and  187 , as opposed to just two (surfaces  143  and  147 ) in case of beam splitter  141 . Further, there is an internal diagonal interface, referred to as surface  189  for convenience, formed by the mating surfaces of prisms  173  and  175 , divided into first zone  191 , second zone  193  and third zone  195 . In one variation all three zones are of equal length along surface  189 . In other variations, first zone  191  and third zone  195  are of equal length along surface  189  with second zone  193  occupying the balance of the length on surface  189 . In a further variation, first zone  191 , second zone  193  and third zone  195  are of different lengths. Further, as with beam splitter  103  and  141 , there may be uncoated regions between each of these zones. First zone  191  is coated to reflect substantially 100% of the incident light in a spectral bandwidth compatible with the desired operating optical bandwidth of the system in which beam splitter  171  is incorporated. Second zone  193  is coated to partially reflect and partially transmit the incident light, normally 50% each. Third zone  195  is coated with an anti-reflective (or transmissive) coating to transmit substantially 100% of the incident light. Finally, each of surfaces  181 ,  183 ,  185  and  187  can be coated with an anti-reflective coating. 
   In operation, incident light, represented by ray path  201 , is partially reflected and partially transmitted by second zone  193  on surface  189  after passing through surface  181  without being refracted. Beam path  203  represents the path of the reflected light and beam path  205  represents the path of the transmitted light. Reflected beam path  203  is reflected by mirrors  207  and  209  and passes through surface  185  without being refracted such that it is incident upon first zone  191  of surface  187 . Path  203  is then reflected back through prism  173  through surface  187 , without being refracted, exiting toward a lens system (not shown) where one axis focuses an image of an aperture (not shown) upon a detector (not shown) and the orthogonal axis is interfered on a detector (not shown). 
     FIG. 9  further illustrates the transmitted beam path  205 . Transmitted beam path  205  is reflected by mirrors  209  and  207 , passes through surface  183  without being refracted and directed toward third zone  195  of surface  189 , where it is further transmitted through surface  187  (again without being refracted) and directed toward a lens system (not shown) where one axis is focuses an image of the aperture (not shown) upon a detector (not shown) and the orthogonal axis is interfered on a detector (not shown). Spacing  211  is set to Nyquist sampling and can be adjusted by adjusting mirror  209 . 
   Beam splitter  171  can replace beam splitter  103  or beam splitter  143  in the above described interferometers (e.g.,  FIGS. 4-7 ). As with beam splitters  103  and  143 , beam splitter effectively eliminates the loss of light, and resulting inefficiency, inherent in the prior art (e.g., Smith). It, like beam splitter  143 , has the further improvement that both beams have the same optical path length so that the aberrations for both beams are identical. Finally, the cube design of beam splitter  171  is very rugged and is advantageous in the construction of Sagnac interferometers. 
   Whereas the drawings and accompanying description have shown and described the preferred embodiment, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.