Patent Publication Number: US-2009225314-A1

Title: Spectrometer designs

Description:
PRIORITY APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/022,054 filed on Jan. 29, 2008 entitled “Spectrometer Designs”, which is a continuation of U.S. patent application Ser. No. 11/141,355 filed on May 31, 2005 entitled “Spectrometer Designs”, which claims priority to U.S. Provisional Patent Application No. 60/685,217 filed May 27, 2005, entitled “Spectrometer Designs” (Attorney Docket No. NOSOL.002PR). Each of the foregoing applications, is hereby incorporated by reference in its entirety and made part of this specification. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention described herein relates to spectrometers including, for example, spectrometers having gratings monolithically integrated with other optical elements. 
     2. Description of the Related Art 
     Spectrometers are optical instruments that determine the spectral content of an optical signal. The output of a spectrometer is a spectral distribution of intensity versus wavelength, referred to herein as a spectrum. 
     Spectrometers are very useful in a myriad of scientific and technological applications and are the basis of spectroscopy. Spectroscopy may, for example, help identify the composition of materials and may provide information regarding different physical and chemical processes. 
     Imaging spectrometers are a special type of spectrometer that produces wavelength spectrums for different spatial locations in a two-dimensional field. Imaging spectroscopy can be accomplished by producing spectrums for a plurality of sites along one swath of the two-dimensional field. A recording device, such as an array of photodetectors, is located at an image plane to record the spectral information for locations across the swath. The instrument is then swept over the next swath and the spectral response of each portion of the new swath is measured in a like manner. Spectroscopic information can thereby be obtained for a two-dimensional array of locations. 
     Spectrometers are used both in the laboratory and in the field. For various applications imaging spectrometers may be included as payloads in satellites, airplanes, or unmanned aerial vehicles (UAVs). Such spectrometers may be used, for example, for remote sensing and reconnaissance. In the case of imaging spectrometers on satellite and airplane platforms, the instrument can be scanned over the two-dimensional field by the motion of the platform itself. In this way, a map over the spectral response of the entire two-dimensional field can be created. 
     Two characteristics of spectrometers that are therefore desirable are rigidity and small size. Rigidity can be important to ensure that the instrument maintain precise alignment of optical components to achieve desired performance. Over its lifetime the instrument can be subjected to vibration and other physical stresses that can degrade instrument performance if proper alignment of the optical components is lost. These types of physical stresses can occur during rocket launch of a satellite payload or during turbulence or during maneuvering and landing of an airplane or UAV, for example. Small size is also important because space is generally limited for airplane and UAV based instruments and especially in the case of satellite missions where extra size and weight can add significantly to the cost of placing the satellite in orbit. There is a need, therefore, for a spectrometer with increased ruggedness and decreased size. 
     SUMMARY 
     One embodiment of the invention comprises a spectrometer comprising: a first body portion comprising substantially optically transmissive material; first and second reflective regions disposed on a first side of said first body portion; a reflective grating disposed on a second side of said first body portion; and a second body portion comprising substantially optically transmissive material joined to said first body portion with said reflective grating disposed therebetween, wherein said first and second reflective regions and said reflective grating are arranged with respect to each other such that light incident on said first reflective region is reflected to said grating, diffracted from said grating to said second reflective portion, and reflected from said second reflective portion into said second body portion. 
     Another embodiment of the invention comprises a spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first reflector; a curved reflective grating, said first reflector and said curved reflective grating defining a first optical path therebetween, said first reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material; and a detector defining a second optical path extending from said curved reflective grating to said detector, said second optical path substantially comprising said substantially transmissive material. 
     Another embodiment of the invention comprises a spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first curved reflector; and a reflective grating, said first curved reflector and said reflective grating defining a first optical path therebetween, said first curved reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material, wherein said reflective grating is configured to reflect broadband light having a bandwidth of at least about 400 nanometers to a detector via a second optical path comprising said substantially transmissive material. 
     Another embodiment of the invention comprises a spectrometer comprising: a first body portion comprising substantially optically transmissive material; first and second reflective regions disposed on a first side of said first body portion; and a reflective grating disposed on a second side of said first body portion; wherein said first and second reflective regions and said reflective grating are arranged with respect to each other such that broadband light at least about 300 nanometers in bandwidth propagating through said substantially optically transmissive material incident on said first reflective region is reflected to said grating, diffracted from said grating through said substantially optically transmissive material to said second reflective region, and reflected from said second reflective region through said optically transmissive material. 
     Another embodiment of the invention comprises a spectrometer comprising: a medium comprising substantially optically transmissive material; a slit; and a reflective grating, said slit and said reflective grating defining a first optical path therebetween, said slit and said reflective grating disposed with respect to each other and said medium such that said first optical path substantially comprises said substantially transparent material. 
     Another embodiment of the invention comprises a spectrometer configured to be mounted in an unmanned airborne vehicle, the spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first reflector; a reflective grating, said first reflector and said reflective grating defining a first optical path therebetween, said first reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material, said reflective grating being configured to reflect light to a detector via a second optical path comprising said substantially transmissive material; and a housing in which said body, said first reflector, and said reflective grating are positioned, wherein said housing is no greater than 1000 cubic centimeters, and said housing is configured to be mounted in an unmanned airborne vehicle receiving area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of a spectrometer comprising a convex diffraction grating and a pair of concave mirrors integrated together in a monolithic structure; 
         FIG. 2  is a schematic view of an example spectrometer entrance slit; 
         FIG. 3  is a schematic view of light incident upon the entrance slit of an example spectrometer where the propagation medium on either side of the slit has the same index of refraction; 
         FIG. 4  is a schematic view of light incident upon the entrance slit of an example spectrometer comprising material having a high index of refraction that causes the light entering the spectrometer through the slit to have a reduced cone angle within the instrument; and 
         FIG. 5  is a schematic view illustrating one configuration where optical components comprising concentric surfaces are used to produce well-corrected images relatively free of optical aberrations. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates one embodiment of an imaging spectrometer  100 . The spectrometer instrument  100  comprises a generally monolithic assemblage of optical components that are integrated together using a rigid support structure or main body  110  comprising substantially optically transmissive material. As shown in  FIG. 1  and discussed more fully below, light collected by the example spectrometer  100  propagates through the optically transmissive material comprising the support structure  110  interacting with optical components integrated thereon and/or therein. 
     In this example, these optical components include an entrance slit  112 , a reflective surface  114 , first and second curved mirrors  116 ,  118 , a diffraction grating  120 , and a sensor  122 . The entrance slit  112  is disposed on a turn mirror block  124  that includes the reflective surface  114 . The first and second curved mirrors  116 ,  118  and diffraction grating  120  are disposed on different sides of a meniscus block  126 . This meniscus block  126  has first and second curved surfaces  128 ,  130 . The first and second curved mirrors  116 ,  118  are disposed on the first curved surface  128  and the grating  120  is disposed on the second curved surface  130 . 
     The spectrometer instrument  100  further comprises a plano-convex block  132  having a first curved surface  134  and a second flat surface  136 . The first curved surface  134  on the plano-convex block  132  mates with the second curved surface  130  on the meniscus block  126 . The turn block  124  also has a planar surface  138  that is butt up against the second flat surface  136  of the plano-convex block  132 . The spectrometer  100  further comprises an output block  140  having first and second planar surfaces  142 ,  143 . The first planar surface  142  on the output block  140  is butt up against the second flat surface  136  of the plano-convex block  132 . A substantially optically transmissive adhesive may be used to attach the turn block  124  and the output block  140  to the plano-convex block  132  and the plano-convex block to the meniscus block  126 . In various preferred embodiments, the substantially optically transmissive adhesive has an index of refraction substantially matching that of the turn block  124 , the output block  140 , the plano-convex block  132 , and the meniscus block  126  to reduce Fresnel reflection. 
     In certain preferred embodiments, the spectrometer is substantially compact. For example, the monolithic structure  110  comprising the turn block  124 , the output block  140 , the plano-convex block  132 , and the meniscus block  126  bonded together with adhesive may have a length, L, as shown in  FIG. 1 , between about 25 millimeters (mm) and 75 mm and a width, W, as shown, between about 20 mm and 60 mm. The thickness, not shown in  FIG. 1 , may be between about 10 mm and 30 mm. Discussions outside these ranges are also possible. In one embodiment, the monolithic structure  110  has dimensions of approximately 50 mm long, 40 mm wide, and 22 mm high, although the structure may be larger or smaller. In certain embodiments, the spectrometer  100  is also light. The monolithic structure comprising the turn block  124 , the output block  140 , the plano-convex block  132 , and the meniscus block  126  together may, for example, weigh between about 50 grams and 1000 grams, but may be heavier or lighter in other embodiments. 
     The spectrometer  100  further comprises imaging optics  144  shown in block diagram form in  FIG. 1 . An optical path extends from the imaging optics  144  to the slit  112 . The spectrometer  100  also may comprise a processor or computer  146  configured to collect images from the sensor  122 . This processor  146  may comprise electronics electrically connected to the sensor  122 , which may comprise a two-dimensional detector array such as a CMOS or CCD solid state detector array or a HgCdTe or InSb solid state detector array. The image sensor  122  may be included in a package and may optionally be cooled by a cooler. 
     In certain preferred embodiments, light propagates along an optical path through the instrument  100  substantially as follows. Light emanates or reflects from a remote object (not shown) and is collected by the imaging optics  144 . The remote object can be, for example, the ground below in a UAV application. The imaging optics  144 , which represents optics suitable for conditioning light to serve as the input to the instrument  100 , forms an image of the remote object onto the entrance slit  112  of the spectrometer instrument  100 . The light passes through the entrance slit  112 , diverging, and enters the optically transmissive medium which comprises the majority of the optical path through the instrument  100 . The emerging light beam is directed toward the reflective surface  114  in the turn block  124 . This reflective surface  114  is oriented to redirect the light about 90° in the embodiment shown in  FIG. 1  through the plano-convex block  132  and the meniscus block  126  to the first mirror  116 . The diverging light beam reflects from the first mirror  116 , a concave mirror that produces a converging beam. 
     The first mirror  116  directs the light through the substantially transmissive material in the meniscus block  126  to the grating  120 . The grating  120  is a convex surface that converts the converging beam from the first mirror  116  into a diverging beam. The grating  120  also diffracts the beam. One order of the diffracted beam is directed to the second mirror  118 . The grating  120  also introduces dispersion such that different wavelengths are diffracted at distinct angles thereby spatially separating the different wavelength components. The diffracted beam propagates through the substantially optically transmissive material in the meniscus block  126  to the second mirror  118 , which comprises a concave reflecting surface. Accordingly, the divergent diffracted beam is converted into a convergent beam directed through the substantially optically transmissive material in the meniscus block  126 , the plano-convex block  132 , and the output block  140 . After exiting the output block  140 , the light strikes the sensor  122 , which converts optical energy across the spatial extent of the sensor into electrical signals which may be recorded and/or processed. The sensor  122 , a two-dimensional detector array, produces signals indicative of the spatial distribution of the intensity on the sensor. These signals are conveyed to the computer  146  for image processing. 
     In certain preferred embodiments, the spectrometer is configured for light having a broad bandwidth, e.g., about 200, 300, or 400 nanometers (nm) or more. For example, certain embodiments are configured for visible light having a wavelength between about 400-800 nm or 450-900 nm. The design (e.g., shape, size, materials, etc.) and location of the optical components (e.g., grating, reflectors, detector, etc.) may be selected such that broadband light can be processes by the spectrometer  100 . Larger or smaller bands are also possible. 
     The imaging optics  144  may comprise one or more lens elements in certain embodiments. For example, the imaging optics  144  may comprise a lens or lens system similar to that used in a camera. Zoom or wide field optics may be employed. The imaging optics  144  may have a focal length between about 20-100 millimeters (mm) and an f-number between about 2 and 4 in some embodiments, although values outside these ranges are possible. The imaging optics  144  are disposed with respect to the slit  112  to form an image on the slit. This image may, for example, be about 5 mm to 25 mm wide and high in certain embodiments, and the imaging optics  144  may be a distance, e.g., between about 5 mm and 100 mm from the slit. The type and configuration of the imaging optics, however, is not limited as other types of imaging optics and other designs can be employed. The image formed on the slit  112  will also vary in size and shape. 
     An exemplary slit  112  is depicted in  FIG. 2 . The slit  112  may have a height, a, between about 5 mm and 25 mm, e.g., about 10 mm, and a width, b, between about 5 microns and 50 microns, e.g., about 10 microns. More generally, the slit  112  is an aperture through which light enters the spectrometer  100 . This aperture  112  may have different shapes and different dimensions as well. In certain preferred embodiments where the spectrometer  100  comprises an imaging spectrometer, the slit  112  is elongated so as to pass light corresponding to an elongated portion of the image field, referred to above as a swath. In some embodiments, however, the aperture  112  may not be elongated and may, e.g., be a point aperture comprising a small round hole. Such apertures may be used for non-imaging spectrometers, for example. 
     The aperture  112  may comprise an opening in a mask  148  comprising, for example, metal or other opaque material. This metal may be deposited on a front surface of the turn block  124 ; see  FIG. 1 . The mask  148  and aperture  112  may be formed using photolithographic techniques. In other embodiments, the mask  148  may be attached to the front surface of the turn block  124 , for example, by adhesive. Other types of apertures  112  may be used and the aperture may be secured to the spectrometer  100  in other ways. The aperture  112  may also be located elsewhere. For example, in embodiments that do not employ the turn block  124 , the aperture  112  may be disposed on the surface  136  of the plano-convex block  132 . Alternatively, the aperture  112  may be separate from the main body  110  of the spectrometer  100 . Other configurations are also possible. 
     The turn block  124 , shown in  FIG. 1 , may comprise substantially optically transmissive material such as, for example, glass. Fused silica may be employed in certain preferred embodiments. Other materials may be employed as well. The front face of the turn block  124  may be polished. As described above, the aperture  112  can be disposed on this front face. The turn block  124  may further be polished to form the reflective surface  114 . Light propagating from the slit  112  through the turn block  124  may be reflected from this reflective surface  114  by total internal reflection. Alternatively, the reflective surface  114  may comprise a reflective material such as metal or a dielectric reflector. Other designs are possible. 
     As discussed above, the reflective surface  114  may be oriented to direct the light propagated through the slit  112  along a path toward the first mirror  116 . This reflective surface  114  may therefore be oriented at about 90° in some embodiments; however, the orientation may vary depending on the configuration and design. 
     The turn block  124  may assist in the packaging and arrangement of the components. For example, the turn block  124  may allow the imaging optics  144  to be farther away from the sensor  122 . In certain embodiments, however, the turn block  124  and/or the reflective surface  114  may be excluded. For example, the light beam from the slit  112  may directly propagate to the first mirror  116  without being redirected by a reflective element. 
     As demonstrated by  FIGS. 3 and 4 , the substantially optically transmissive material of the turn block  124  and other components comprising the main body  110  through which the light propagates in the spectrometer may advantageously enable a more compact design. These components include, for example, the plano-convex block  136  and the meniscus block  126  shown in  FIG. 1 . 
     In particular, the substantially optically transmissive material may have a higher index of refraction than that of the environment surrounding the spectrometer  100  such that light is refracted upon entering the spectrometer  100 . A comparison is presented in  FIGS. 3 and 4  of the cases where light propagates through air within the spectrometer  100  and light propagates through a medium having a higher refractive index in the spectrometer  100 . In particular,  FIG. 3  shows light propagating through a region  150  comprising air and passing through an aperture  152  into a region  154  also comprising air. No refraction is present as the medium is the same on both sides of the aperture  152 . In contrast,  FIG. 4  shows light propagating through a region  156  comprising air and passing through an aperture  158  into a region  160  comprising material such as glass, which has a higher refractive index than air. The light is refracted.  FIG. 4  shows how a cone of light rays  162  passing through the aperture  158  has a smaller size within the material than in the air. This reduction in the size of the cone of rays  162  enables optics having a smaller size to be used in the spectrometer design. 
     As shown in  FIG. 1 , a cone of rays entering the spectrometer  100  through the slit  112  is refracted in the turn block  124  comprising a substantially optically transmissive material having a higher refractive index. The resultant cone of rays in the turn block  124  and similarly in the spectrometer  100  comprising the substantially optically transmissive material is likewise reduced. Smaller optics can therefore be used. Accordingly, the optics within the spectrometer  100  may have a higher f-number (or reduced numerical aperture). Nevertheless, the spectrometer  100  collects a larger cone of light equivalent to a smaller f-number (or increased numerical aperture). A compact spectrometer design that collects more light can thus be achieved by propagating the light within the spectrometer  100  through a medium having a higher refractive index than the medium outside the spectrometer, which will likely comprise air or vacuum. Accordingly, the light is propagated through the main body  110  in the spectrometer  100 , which comprises a substantially optically transmissive material such as glass, which has an index of refraction of about 1.5 in some cases. 
     As discussed above in connection with  FIG. 1 , the main body  110  of the spectrometer  100  further comprises the plano-convex block  132 . This plano-convex block  132  comprises substantially optically transmissive material, such as glass, having an index of refraction greater than air or vacuum. The plano-convex block  132  may also comprise fused silica in certain preferred embodiments. The plano-convex block  132  may be polished to provide the first curved surface  134  and a second flat surface  136 . The curved surface  134  may be spherically shaped. The distance separating the first curved surface  134  and a second flat surface  136  at the center may be between about 5 mm and 20 mm. This plano-convex block  132  may have a width between about 20 mm and 60 mm. The radius of curvature of the first curved surface  134  may be between about 15 mm and 30 mm and may match the curvature of the grating  120  in certain preferred embodiments. Other dimensions can also be used. 
     As discussed above, the planar surface  138  of the turn block  124  is butt up against the second flat surface  136  of the plano-convex block  132 . Light from the slit  112  thus is reflected by the reflective surface  114  on the turn block  124  into the plano-convex block  132 . This light propagates through the higher index material comprising the plano-convex block  132 . 
     Adhesive may be used to bond the planar surface  138  on the turn block  124  to the second flat surface  136  on the plano-convex block  132 . This adhesive may have an index of refraction substantially similar to the substantially optically transmissive material comprising the turn block  124  and the plano-convex block  132 . Such index matching may reduce Fresnel reflections. 
     As discussed above in connection with  FIG. 1 , the main body  110  of the spectrometer  100  further comprises the meniscus block  126 . This meniscus block  126  also comprises substantially optically transmissive material, such as glass, having an index of refraction greater than air or vacuum. The meniscus block  126  may comprise fused silica in certain preferred embodiments. The meniscus block  126  may be polished to provide the first and second curved surfaces  128 ,  130 . These curved surfaces  128 ,  130  may be rotationally symmetrical, and more particularly, spherically shaped. The distance separating the first and second curved surfaces  128 ,  130  at the center (e.g., through the optical axis) may be between about 10 mm and 30 mm. This concave-convex block  126  may have a width between about 20 mm and 60 mm. The radius of curvature of the first curved surface  128  may be between about 30 mm and 60 mm and may have the same center of curvature of the second curved surface  130  as discussed more fully below. The curvature of the second curved surface  130  may be between about 15 mm and 30 mm and may be about one-half the radius of curvature of the first curved surface  128  in some embodiments also discussed below. Other dimension can be used as well. In certain embodiments, the radius of curvature of the second curved surface  130  matches the curvature of the grating  120  in certain embodiments. 
     The first and second curved mirrors  116 ,  118  are disposed on the first curved surface  128  and the grating  120  is disposed on the second curved surface  130 . The first and second curved mirrors  116 ,  118  may be formed, for example, by metallizing portions of the first curved surface  128  on the meniscus block  126 . A metallized region may extend through and include both the first curved mirror  116  and the second curve mirror  118 . In other embodiments, separated regions of metallization may be used to form the first and second curved mirrors  116 ,  118 . Dielectric coatings may be employed in some embodiments. 
     Other methods of forming mirrors on the first curved surface  128  may also be used. In some embodiments, the first and/or second mirrors  116 ,  118  are mounted proximal to, possibly spaced apart from, the first curved surface  128 . Index matching material may be used in such embodiments to reduce reflection. In some embodiments, adhesive having suitable refractive index to reduce Fresnel reflection may be employed to adhere the mirrors  116 ,  118  to the first curved surface  128 . Still other methods may be used to provide reflective surfaces near the first curved surface. 
     The curved grating  120  may be formed on the second curved surface  130  of the meniscus block  126 , for example, using photolithographic techniques. For example, metal may be deposited on the second curved surface  130  and patterned using, e.g., photoresist. Other approaches may be employed. The grating  120  comprises a holographic optical element formed by holographic techniques. In some embodiments, low diffractive orders (e.g., n=1, etc.) are used. Low diffractive orders such as n=2, 3, 4, or 5 could also be used. The low order may, for example, be directed to the second mirror  118  and conveyed to the sensor  122 . Other orders may also be used. 
     In some embodiments, the grating  120  is mounted proximal to, possibly spaced apart from, the second curved surface  130 . Index matching material may be used in such embodiments to reduce reflection. Accordingly, in some embodiments, the first and second curved mirrors  116 ,  118  can be spaced apart from (but disposed on a first side of) the meniscus block  126  and the grating  120  can be spaced apart from (but disposed on a second side of) the meniscus block. Still other configurations are possible. 
     As discussed above, the plano-convex block  132  is mated with the meniscus block  126 . The first curved surface  134  on the plano-convex block  132  may be butt up against the second curved surface  130  on the meniscus block  126 . Adhesive may be used to bond the two surfaces  134 ,  130  together. This adhesive may have an index of refraction substantially similar to the substantially optically transmissive material comprising the meniscus block  132  and the plano-convex block  132 . Such index matching may reduce Fresnel reflections. 
     Accordingly, light from the slit  112  is reflected by the reflective surface  114  on the turn block  124  into the plano-convex block  132  and through the substantially optically transmissive material in the meniscus block  126 . The light reaches the first mirror  116  where the light is reflected back through the substantially optically transmissive material in the meniscus block  126  to the diffraction grating  120 . This light is diffracted by the reflective grating  120  and is directed once again through the substantially optically transmissive material comprising the meniscus block  126  to the second mirror  118 . The light is reflected from the second mirror  118  again through the optically transmissive material in the meniscus block  126  and proceeds into the plano-convex block  132 . Accordingly, the light propagates substantially through high index material between the slit  112  and the first mirror  114 , the first mirror and the grating  120 , the grating and the second mirror  118  and to the plano-convex block  132 . The light also propagates through high index material within the plano-convex block  132 . 
     As discussed above in connection with  FIG. 1 , the main body  110  of the spectrometer  100  also comprises the output block  140 . This output block  140  similarly comprises substantially optically transmissive material, such as glass, having an index of refraction greater than air or vacuum. The output block  140  may comprise fused silica in certain preferred embodiments. The output block  140  may be polished to provide first and second planar surfaces  142 ,  143 . The distance separating the first and second planar surfaces  142 ,  143  at the center may be between about 5 mm and 20 mm. This output block  140  may have a width between about 10 mm and 30 mm. In certain preferred embodiments, the length of the output block  140  is sufficiently large such that a substantial portion of the path from the second mirror  118  to the sensor  122  comprises high index material. The width of the optical block  140  may also be at least as large to accommodate the width of the beam from the second mirror  118  to the sensor  122  in certain embodiments. Other sizes and shapes for the output block  140  are also possible. 
     As discussed above, the output block  140  is mated with the plano-convex block  132 . The first planar surface  142  of the output block  140  may be butt up against the second planar surface  136  of the plano-convex block  132 . Adhesive may be used to bond the two surfaces  142 ,  136  together. This adhesive may have an index of refraction substantially similar to that of the substantially optically transmissive material comprising the plano-convex block  132  and the output block  140 . Such index matching may reduce Fresnel reflections. 
     The spectrometer  100  further comprises the sensor  122  as discussed above. The sensor  122  may comprise a detector array comprising a two-dimensional array of detectors or pixels. Such a sensor  122  may comprise a CMOS detector or a CCD detector. Other types of detectors may also be used. For example, the sensor  112  may comprise mercury cadmium telluride (HgCdTe) or indium antimonide (InSb). Still other types of sensors are possible. The sensor  122  may be sensitive to UV, visible or IR radiation. 
     Also shown in  FIG. 1  is a camera window  145  used to package and protect the image sensor  122 . The image sensor  122  may be in a package (not shown). This package may include the window  145  through which light passes to reach the detector array. 
     The sensor  122  also may be located at the focal plane of the second mirror  118 . The sensor  122  may also be located at the conjugate image plane to the slit  112  established by the optics (e.g., the first relay mirror  116 , the grating  120 , and the second relay mirror  118 ). Accordingly, the distance from the second mirror  118  to the sensor  122  may be between about 30 mm to 60 mm in some embodiments. The sensor  122  may be located elsewhere as well. Although not shown in  FIG. 1 , the sensor  122  may be mounted to the output block  140  in some embodiments. As discussed above, the sensor  122  may be located at an image plane conjugate to the slit  112 . 
     In certain preferred embodiments, the sensor  122  is in communication with the imaging processing computer  146  such that intensity spectrums may be recorded. This image processing computer  146  may be in communication with other devices including but not limited to display devices, storage media, or other computing or processing apparatus. 
     The spectrometer components, e.g., the main body  110 , possible the imaging optics  144 , sensor  122 , and/or image processing computer  146 , can be included in or mounted on a housing (not shown). The spectrometer housing may include sockets, threaded holes, bolts, brackets, clamps, and/or other fastening arrangements for mounting, for example, in a compartment or bay or other appropriate location. The housing may further include a connection including power and electrical signals which can be coupled to the sensor  122 , scanning actuators, computer or processor  146 , cooler, etc., if present. 
     This housing may protect the spectrometer components. The spectrometer  100  may be included in satellites, airplanes/helicopters, unmanned aerial vehicles or on other platforms as well, such as in boats, ships, trucks, cars, balloons, rockets, or other vehicles. The spectrometers  100  may be located elsewhere such as in stations (e.g., weather or research stations, buoys, etc.) in the field, in laboratories, in manufacturing plants, and in medical facilities. The location and use of these spectrometers is not limited. 
     As described above, the spectrometer  100  may comprise an imaging spectrometer that produces wavelength spectrums for different spatial locations in a two-dimensional field. The two-dimensional image field can be imaged by the imaging optics  144  onto the slit  122 . The slit  144  can selectively pass one swath across the two-dimensional image at a time. Spectrums for a plurality of sites along the swath can be produced as a result of the wavelength dispersion of the grating  120 . These spectral distributions are mapped onto the sensor  122  by the second mirror  118 . One spectral distribution may, for example, be mapped across a row of photodetectors in the detector array  122 . Multiple rows of photodetectors in the sensor  122  may record the spectra for locations across the swath in this example. The two-dimensional image field is shifted with respect to the slit  112  to produce the next swath and the spectra of each portion of the new swath is measured in a like manner. Measurements for multiple swaths can be obtained and assembled to produce spectra for a two-dimensional array of locations. Shifting of the spectrometer  100  which may be mounted on a movable platform such as a satellite, airplane, or UAV may permit multiple swaths to be obtained. In other embodiments, the object may be moved with respect to the spectrometer  100  in other ways. In certain embodiments, for example, movable optics coupled to a processor controlled actuator may be included to sweep through multiple swaths. 
     In addition to being configured to provide imaging, the optical components  112 ,  116 ,  120 ,  118  may yield well-corrected imaging. As described above, the first and second mirrors  116 ,  118  and the grating  120  may have substantially the same center of curvature as discussed more fully below. Additionally, the radius of curvature of the first and second mirrors  116 ,  118  may be substantially the same and may be about one-half the radius of curvature of the grating  120 . In certain embodiments, however, the first and second mirrors  116 ,  118  and the grating  120  may be nearly concentric. The centers may be slightly offset. Such a design provides for improved imaging. 
       FIG. 5  schematically illustrates such an embodiment where optical components  500  comprising concentric surfaces are used to produce a well-corrected image of an object with substantially reduced optical aberrations. The optical path through the configuration shown in  FIG. 5  begins at the object plane  570  from which light propagates to a first curved reflector  550 . The first curved reflector  550  reflects the light to the curved reflection grating  540  where the light is diffracted and redirected to the second curved reflector  560 . The second curved reflector  560  reflects the light to the image plane  580 . 
     The reflection grating  540  has a spherical, convex surface defined by a sphere  520  with radius R 1 . Similarly, both of the curved reflectors  550  and  560  are spherical concave mirrors whose surfaces are defined by a second sphere  530  with radius R 2 . Thus, reflectors  550  and  560  have positive power and cause the light passing through the system  500  and incident thereon to converge. Conversely, the reflection grating  540  has negative power and causes the light incident thereon to diverge, as shown. The distance from the object plane  570  to the first curved reflector  550  and from the second curved reflector  560  to the image plane  580  can be approximately equal to the focal length of the curved reflectors  550  and  560 . In certain preferred embodiments the radius R 2  of the second sphere  530  is approximately twice the radius R 1  of the first circle  520 . Additionally, the sphere  520  and  530  are substantially concentric about a shared center  510 . 
     The configuration illustrated in  FIG. 5  has many advantages from the perspective of an optical imaging system. For example, the illustrated configuration of optical components  500  is characterized by unity magnification which may help to eliminate distortion of the object image formed at the image plane  580 . Furthermore, the exact curvatures of the reflection grating  540 , the first curved reflector  550 , and the second curved reflector  560  can be chosen to display excellent spatial imaging characteristics over the entire image plane  580  by correcting the image for astigmatism and field curvature. Such correction can be accomplished by choosing the curvatures such that the reflection grating  540  and the curved reflectors  550  and  560  each substantially compensate for optical aberrations introduced by one another. Field curvature can thereby be decreased. Chromatic aberration is also reduced as reflecting optical elements are employed. Finally, the configuration of optical elements  500  is relatively simple to manufacture due to the spherical surfaces of the optical components. 
     The spectrometer  100  in  FIG. 1  may be configured according to the design in  FIG. 5 . In particular, the first and second mirrors  116 ,  118  and the grating  120  may be substantially concentric. The radii of curvature of the first and second mirrors  116 ,  118  may be substantially the same. Additionally, the radius of curvature of the grating  120  may be about one-half the radii of curvature of the first and second mirrors  116 ,  118 . Substantial aberration reduction may thereby be provided. 
       FIGS. 1 and 5  illustrate one possible configuration of optical elements. Many other configurations are possible. Additionally, the design may deviate from perfectly concentric. For example, the center points of spheres  520  and  530  can be offset by as much as 3 mm. This offset may correspond, for example, to as much as about 15% of the radius of curvature, R 1 . In certain embodiments, the offset is in a direction parallel to the z-axis shown in  FIG. 5 . Similarly, the length of the radius R 2  of sphere  530  can deviate from twice the length of the radius R 1  of sphere  520  by as much as 10% of R 2 . Such offsets may improve performance in certain embodiments. The center of curvatures of the first and second mirrors  116 ,  118  can also be offset by as much as 5 mm. Similarly, the radii of curvatures of the first and second mirrors  116 ,  118  can be different by as much as 10%. Values outside these ranges are also possible. 
     Spectrometer designs other than those specifically recited herein are also possible. For example, additional optical and mechanical components can be incorporated within the instrument  100 . In certain embodiments it is also possible to exclude or substitute one or more of the components illustrated in  FIG. 1 . Similarly, different arrangements and configurations may be used. Different shapes, sizes, and materials may be employed. 
     For example, an embodiment of the invention may incorporate a single mirror rather than the two mirrors  116 ,  118  illustrated in  FIG. 1 . In some embodiments, the mirrors may be excluded. Conversely, additional mirrors may be added. The mirror or mirrors  116 ,  118  as well as the grating  120  may be shaped differently. The mirrors  116 ,  118  may be convex or planar. Aspheric, cylindrical, or other shapes are also possible. Similarly the grating  120  may be concave or planar in other embodiments. The grating  120  may be aspheric or cylindrical. As described above, the aperture  112  may comprise a slit, may be a circular or point aperture, or have other shapes. In some embodiments, the aperture  112  may be excluded. 
     Also as discussed above, the aperture  112  need not be formed on the turn block  124 . In some embodiments, for example, the aperture  112  may be formed on the plano-convex block  132 . The grating  120  may also be formed on the plano-convex block  132  in certain embodiments; however, forming the grating on a concave surface offers manufacturing advantages. The aperture  112 , mirrors  116 ,  118 , and the grating  120  need not be formed directly on the surface of the main body  110 , e.g., turn block  124  and the meniscus block  126 . Separate structures may be used for the aperture  112 , one or more of the mirrors  116 ,  118 , and the grating  120  in other embodiments and these components may be spaced apart from the main body  110 . Index matching may be provided in some embodiments. In some embodiments, the first and second curved mirrors  116 ,  118  can be spaced apart from (but on a first side of) the meniscus block  126  and the grating  120  can be spaced apart from (but on a second side of) the meniscus block. Still other configurations are possible. 
     The main body  110 , for instance, can be configured differently. For example, the main body  110  may be shaped differently and may comprise different components. For example, any one of the turn block  124 , plano-convex block  132 , meniscus block  126 , and output block  140  can be excluded, split into more than one portion, or shaped differently. For example, the meniscus block  126  can be replaced with a block having planar surfaces rather than curved surfaces  128 ,  130 . Similarly, the plano-convex block  132  can have a planar surface instead of a curved surface  134 . Alternatively, the shape of the curved surfaces can be changed, for example, from concave to convex or convex to concave or may have other shapes as well. Aspheric, cylindrical, or other shapes may be used in some embodiments. Additionally, curvature may be added in some case. For example, the flat surface  136  on the plano-convex block  132  may be non-flat. Similarly, the surfaces (e.g., front surface, surface  138 , and reflective surface  114 ) on the turn block  124 , the second surface  136  on the plane-convex block  132 , as well as the surfaces  142 ,  143  on the output block may be other than flat. These surfaces may be curved to include optical power in some embodiments or may be matched with complementary surfaces. 
     Additionally, the main body  110  may comprise more or fewer portions. For example, the meniscus-block  126  can be split up into more than one part. Similarly, any of the plano-convex block  132 , the turn block  124 , and the output block  140  can be split into two or more sections. These sections may be bonded together and index matched in certain embodiments. One or more of the portions  126 ,  132 ,  124 ,  140  of the main body  110  described with reference to  FIG. 1  can also be excluded. The output block  140  may be excluded and light may propagate through air to the sensor  122 . Alternatively, the plano-convex block  132  may be shaped differently to provide additional material through which the light propagates to the sensor  122 . The turn block  124  may be excluded (or arranged so as not to provide a turn.) In some embodiments, the monolithic structure  110  may comprise simply a single portion such as the meniscus block  126  or a differently shaped portion that replaces the meniscus block. A grating and one or more mirrors or a slit may be integrated together with this portion. The sensor  122  may also be included. 
     While glass and, in particular, fused silica have been disclosed as a suitable substrate material, this does not preclude the use of other materials. Other materials substantially optically transmissive to the wavelength for which the spectrometer is to operate may be used. Other materials may be chosen based on their mechanical and optical properties, such as rigidity, coefficient of thermal expansion, transparency in the selected band of wavelengths, and index of refraction. 
     Additionally, in certain embodiments, different portions of the main body  110  may comprise different materials. For example, the meniscus block  126  may comprise a first material and the plano-convex block  132  may comprise a second different material. These materials may be closely index matched in certain embodiments and may have coefficients of thermal expansion that yield reduced movement of the optical components with temperature variation. The concentric design and other designs may provide compensation for thermal stresses and thermal expansion in some embodiments. 
     Different techniques may be used to connect the different portions of the main body  110 . As described above, an adhesive or cement may be used to bond the different blocks  124 ,  132 ,  124 ,  140  together. In certain preferred embodiments, these adhesives or cements are substantially optically transmissive to the wavelength of operation and may be have an index similar to that of the blocks to provide index matching. In some embodiments, the blocks contact one another directly and are held in place using methods other than an adhesive. These blocks  124 ,  132 ,  124 ,  140  may be mounted on a structure that holds the blocks in place. Index matching fluid or material may be disposed between the blocks  124 ,  132 ,  124 ,  140 . In other embodiments the blocks  124 ,  132 ,  124 ,  140  may be otherwise fused together. 
     Some benefits and advantages of the embodiments of the present invention described above include rigidness and compact size. As discussed, certain preferred embodiments of the invention have a substantially monolithic design where the optical components of the spectrometer  100  can be disposed on the surface of or embedded in an optically transmissive material, for example fused silica. Because fused silica is a rigid, durable material, the spectrometer  100  exhibits good durability and rigidity. This characteristic presents a significant advantage in terms of alignment of the optical components comprising the instrument. In some embodiments, for example, the optical components of the spectrometer  100  can be fabricated on a surface of one or more blocks of fused silica substrate. The blocks can then be carefully aligned and may be bonded in place. Once bonded together, the instrument acts as a single body of material and can therefore be substantially resistant to the effects of vibration and mechanical shock which might otherwise disrupt the precise optical alignment of the spectrometer instrument  100 . Fused silica also has a low thermal expansion coefficient, making for a temperature stable design. 
     In contrast, other spectrometers may instead employ mechanical alignment mechanisms in the optical mounts for each component, which can increases the size, weight, and cost of the instrument. Furthermore, the non-monolithic design of traditional spectrometers may be prone to misalignment during normal use and its accompanying vibrations and mechanical shocks. 
     Embodiments of the present invention also advantageously can be designed to have a relatively compact size without necessarily sacrificing important optical performance. Though the size of traditional spectrometers can be decreased by incorporating smaller optical components, decreasing the usable aperture of the instrument can adversely affect its light-gathering power which may have a detrimental affect on the speed of the instrument and the resolution of the image produced. As has been previously discussed, embodiments of the invention alleviate the problem associated with reducing the aperture size of the optical components in the spectrometers by incorporating a substantially optically transmissive material such as fused silica in a majority of the optical path inside the instrument. A spectrometer laid out in fused silica increases the acceptance angle of light entering the instrument. The optical throughput of a spectrometer in fused silica (or other optically transmissive material having an index of refraction greater than that of air or vacuum) is higher than the optical throughput of a like-sized spectrometer where the optical path comprises air or a vacuum. Use of a higher refractive index medium allows for the design of a smaller instrument without sacrificing optical throughput. 
     Accordingly, since the F-number of light increases as it enters the instrument  100 , the instrument can be designed to have a relatively high effective F-number with smaller diameter optical components while still accepting an amount of light comparable to a larger instrument with a lower effective F-number. Thus, utilizing an optically transmissive material (e.g., glass) that has a high index of refraction relative to the environment surrounding the spectrometer  100  enables a small instrument with optical speed comparable to that of a larger instrument to be realizable. Small size can be an important advantage for imaging spectrometers. Since the instrument  100  has a relatively high optical speed for its size, exposure times of the detector array can also be short, which can in turn allow for higher resolution scanning since the time between scans can be shorter. 
     In one embodiment of the invention substantially incorporating fused silica throughout the optical path of the light within the spectrometer, a compact imaging spectrometer  100  is obtained that is approximately 50 mm long, 40 mm wide, and 22 mm high. In some embodiments of the invention, materials with indexes of refraction greater than that of fused silica may be used to further increase light throughput and achieve even smaller designs. 
     This strikingly small spectrometer instrument  100  can be used in a wide range of applications. As discussed above, the spectrometer instrument  100  may be used for military, research, manufacturing, medical, and other applications. The spectrometers  100  may be located in stations (e.g., weather or research stations, buoys, etc.) in the field, in laboratories, in manufacturing plants, in medical facilities, etc. The spectrometer  100  may be included in satellites, airplanes and helicopters, unmanned aerial vehicles or on other platforms as well, such as in boats, ships, trucks, cars, balloons, rockets, or other vehicles. The location and use of these spectrometers is not limited. 
     The spectrometer  100  may be used in the Visible-Near IR band (400-1000 nm), the Short Wave Infrared (SWIR) band (900-2500 nm), the Midwave Infrared (MWIR) band (3-5 microns), and the Longwave Infrared (LWIR) band (8-12 microns). Furthermore, spectrometers designed for use in other spectral bands are possible as well. For example, embodiments of the invention could be designed for use in the UV band. 
     Various embodiments of the invention have been described above. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims.