Abstract:
A spectrometer having slit and detector elements located on the optical axis of the spectrometer, resulting in substantially increased spectral and spatial fields of the spectrometer. The spectrometer being more compact than current designs, while providing superior spatial and spectral image quality and resolution.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/782,546, filed on Mar. 14, 2013, which is incorporated by reference herein in its entirety and for all purposes. 
    
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with U.S. Government support from the U.S. Army under Contracts W15P7T-06-C-F001 and W15P7T-08-C-P212. The U.S. Government has certain rights in the invention. 
    
    
     SUMMARY OF THE INVENTION 
     The various embodiments of the present invention locate slit and detector elements on the optical axis, resulting in substantially increased spectral and spatial fields of the spectrometer. These embodiments are more compact than current designs, while providing superior spatial and spectral image quality and resolution. 
     The various embodiments of this invention provide at least, but not limited to the following: 
     a spectrometer design that is compact in physical size; a spectrometer design that is low in mass 
     a spectrometer design that has a high spectral resolution; 
     a spectrometer design that has a large spatial field; 
     a spectrometer design that has a large spectral field; 
     a spectrometer design that has a high degree of spatial and spectral image quality; 
     a spectrometer design that has a small spectral smile distortion; 
     a spectrometer design that has a fast optical speed; and 
     a spectrometer design that has a combination of the characteristics described above with superior trade-offs than have been previously attainable. 
     For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and the scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a compact refractive relay spectrometer, taken along its optical axis in the plane parallel to the direction of dispersion; 
         FIG. 2A  is a schematic view of the spectrometer illustrated in  FIG. 1 , taken along a plane perpendicular to its optical axis just prior to the image plane; 
         FIG. 2B  is a graphical representation of the maximum optical field height as a function of spectral resolution that results from the off-axis spatial and spectral field components of the spectrometer illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic view of a compact spectrometer with high spectral resolution in accordance with an embodiment of the present invention, taken along its optical axis in the plane parallel to the direction of dispersion; 
         FIG. 4  is a schematic view of a compact spectrometer with high spectral resolution in accordance with another embodiment of the present invention, taken along its optical axis in the plane parallel to the direction of dispersion; 
         FIG. 5A  is a schematic view of the embodiments of the present invention illustrated in  FIG. 3  and  FIG. 4 , taken along a plane perpendicular to their respective optical axes just prior to the image plane; 
         FIG. 5B  is a graphical representation of the maximum optical field height as a function of spectral resolution that results from the on-axis spatial and spectral field components of the embodiments of the present invention illustrated in  FIG. 3  and  FIG. 4 ; 
         FIG. 6A  is a graphical representation of the spectral smile distortion as a function of spatial field for the embodiment of the present invention illustrated in  FIG. 3 ; 
         FIG. 6B  is a graphical representation of the spectral smile distortion as a function of spatial field for the embodiment of the present invention illustrated in  FIG. 4 ; 
         FIG. 7  is a schematic view of a compact spectrometer with high spectral resolution in accordance with a further embodiment of the present invention, taken along its optical axis in the plane parallel to the direction of dispersion; 
         FIG. 8  is a graphical representation of the spectral smile distortion as a function of spatial field for the embodiment of the present invention illustrated in  FIG. 7 ; 
         FIG. 9  is a schematic view of a compact spectrometer with high spectral resolution in accordance with a further embodiment of the present invention, taken along its optical axis in the plane parallel to the direction of dispersion; and 
         FIG. 10  is a schematic view of a compact spectrometer with high spectral resolution in accordance with a still further embodiment of the present invention, taken along its optical axis in the plane parallel to the direction of dispersion. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention relates generally to spectrometers, and, more particularly, but not limited to, novel spectrometer designs which are more compact in physical size and having higher spectral resolution than previous designs. 
     Due to their dispersive nature, the slit or slit aperture, hereinafter referred to generally as the slit, and detector elements of many spectrometer design forms are located off-axis, thereby increasing the effective optical field of the design. This often results in larger diameter spectrometers that have significantly increased size and weight, increased aberration content that degrades their spatial and spectral image quality, and increased spectral distortions that complicate spectral algorithms and data processing of the captured imagery. Spectral resolution is often compromised to balance these negative aspects of spectrometers of this type. 
     Reference is made to  FIG. 1 , which illustrates a refractive relay spectrometer  100 , which includes therein a first optical subassembly and a second optical subassembly, of the type described in U.S. Pat. No. 7,061,611, which is incorporated herein by reference in its entirety and for all purposes. While the optical elements  12 ,  14 ,  16 ,  18 ,  22 ,  24 ,  26 , and  28  are co-located along a single optical axis  30 , the slit or slit aperture  40  and detector element  50  are displaced, in this embodiment, but not limited to, substantially an equal distance from the optical axis  30 . This displacement is often necessary to compensate for the angle of diffraction at the dispersing element  60  and is typical for many spectrometer designs. 
     This displacement of the slit  40  and detector or detector element  50  from the optical axis limits the overall compactness of the spectrometer  100  due to the size of the optical field at the image plane that results. To better understand this effect, the spatial field (Δx) and spectral field (Δλ) of the dispersed image are shown from the view of a plane perpendicular to the optical axis  30  just prior to the detector  50  in  FIG. 2A , where the detector  50  is shown at its off-axis position. The optical field radius (Hρ) of the spectrometer  100  must be sufficiently large enough to accommodate the furthest extents of both the spatial field (Δx) of the slit  40  and detector  50 , as well as the spectral field (Δλ) that results from the dispersion of the grating  60 . The more dispersive the grating  60  becomes, the larger the size of the dispersed image at the detector  50  and the further off-axis the spectral field (Δλ) becomes, thereby increasing the optical field radius (Hρ) of the spectrometer. The relationship between the size of the spatial field (Δx), the focal length (f) of the optical subassembly of the system or spectrometer  100  optically disposed between the diffraction grating  60  and the detector  50 , the period (λ) of the diffraction grating  60 , and the longest wavelength (λ max ) in the operational spectral band of the sensor on the optical field (Hρ) is also provided in  FIG. 2A  for the spectrometer  100  illustrated in  FIG. 1 . 
     The spatial and spectral trade-offs for the refractive relay design form illustrated in  FIG. 1  are illustrated in  FIG. 2B , where it can be seen that for very coarse (large) spectral resolutions the optical field radius (Hρ) is dominated by the size of the spatial component (Hx) of the optical field, where changes to the spectral resolution have little effect on the overall size of the spectrometer. For very fine (small) spectral resolutions, however, the optical field radius (Hρ) is dominated by the spectral component (Hy) of the optical field, where changes to the spectral resolution have a great effect on the overall size of the spectrometer  100 . 
     Since the size of the spectrometer  100  is primarily driven by the off-axis spectral field for fine spectral resolutions, the overall size of the spectrometer  100  can be minimized by substantially locating both the slit  40  or slit aperture and detector element  50  on the optical axis  30 . Alternatively, for a given spectrometer size constraint, the location of the slit  40  and detector element  50  substantially to the optical axis  30  can be used to allow for a larger spatial and/or spectral field than would otherwise be possible. This increased field can be used to either increase the number of spatial pixels or increase the spectral resolution of the sensor. In addition, the reduced beam footprint on the optical elements that results from the location of the slit  40  and detector element  50  substantially to the optical axis  30  can be used to increase the optical speed, or throughput, of the sensor. 
     In one embodiment of the optical system or spectrometer  200 , the slit or slit aperture  40 , which can be used interchangeably, and the detector element  50 , which can be in the form of but not limited to a focal plane array, are located substantially on the optical axis  30  by inserting a light bending element or group of elements, such as but not limited to a prism or combination of prisms, in this embodiment made up of prisms  270  and  272 , between the two halves  210  and  220  of the optical system as illustrated in  FIG. 3 . These prisms  270  and  272  can be used to redirect all or some of the angle of incidence to the dispersive element  260  as well as to redirect all or some of the angle of diffraction from the dispersing element  260 . In principle, these prisms  270  and  272  are typically located substantially near the dispersive element  260 , such as but not limited to a diffraction grating, located in the substantially collimated space between the two halves  210  and  220  of the spectrometer  200 . 
     Reference is made to  FIG. 3 , which is a schematic view of an embodiment of the present invention as spectrometer  200  taken along its optical axis  30  in the plane parallel to the direction of dispersion. In operation, electromagnetic radiation, typically in the ultraviolet, visible, and/or infrared bands, hereinafter referred to generally as light, emitted or reflected by a given object, either real or virtual, hereinafter referred to generally as the source, located at the object plane, in this embodiment, but not limited to, a slit or other method of extracting a line image, hereinafter referred to generally as a slit, slit aperture or element  40 , is incident on a first optical subassembly  210  of an imaging optical system or spectrometer  200 , in this embodiment made up of, but not limited to, refractive elements  212 ,  214 ,  216 , and  218  that, in this embodiment, substantially share a common optical axis  30 . The refractive elements are capable of substantially receiving a portion of the light emanating from the slit  40  and substantially collimating the light. The light is then incident on a first light bending element  270 , in the form of, but not limited to a prism, but in general is any component for bending light, hereinafter referred to generally as a light bending element. The light bending element  270  is capable of substantially receiving the light from the first optical subassembly  210  of the optical system and substantially bending the light. The light is then incident on a dispersing element  260 , in the form of, but not limited to a transmission diffraction grating, but in general is any component capable of angularly separating light energy according to its wavelength, hereinafter referred to generally as a dispersing element, which is capable of substantially receiving the light from the first light bending element  270  and dispersing it according to its wavelength. The dispersed light is then incident on a second light bending element  272 , in the form of but not limited to a prism, which is capable of substantially receiving the light from the dispersing element  260  and substantially bending the light. The light is then incident on a second optical subassembly  220  of the imaging optical system, in this embodiment made up of, but not limited to, refractive elements  222 ,  224 ,  226 , and  228  that in this embodiment substantially share the common optical axis  30 . The refractive elements are capable of substantially receiving the light from the second light bending element  272  and substantially focusing the light to a focus position (hereinafter also referred to as an image plane) of a CCD array, phosphorescent screen, photographic film, microbolometer array, or other means of detecting light energy, hereinafter referred to generally as a detecting element  50 . 
     In a further embodiment or spectrometer  300  of the present invention, the slit  40  and detector element  50  are substantially located on the optical axes  30  and  330 , respectively, by inserting a bend in the optical axis  30  at or proximate to the dispersing element  360 , as illustrated in  FIG. 4 , such that the two halves  210  and  220  of the spectrometer  300  are each substantially centered along their own optical axes  30  and  330  respectively. 
     Reference is made to  FIG. 4 , which is a schematic view of an embodiment of the spectrometer  300  of the present invention taken along its optical axis in the plane parallel to the direction of dispersion. In operation, light emanating from a slit element  40  is incident on a first optical subassembly  210  of an imaging optical system, in this embodiment made up of, but not limited to, refractive elements  212 ,  214 ,  216 , and  218  that, in this embodiment, substantially share a common optical axis  30 . The refractive elements are capable of substantially receiving a portion of the light emanating from the slit  40  and substantially collimating the light. The light is then incident on a dispersing element  360 , in this embodiment a transmission diffraction grating, which is capable of substantially receiving the light from the first optical subassembly  210  of the spectrometer  300  and dispersing it according to its wavelength. The dispersed light is then incident on a second optical subassembly  220  of the spectrometer  300 , in this embodiment made up of, but not limited to, refractive elements  222 ,  224 ,  226 , and  228  that in this embodiment substantially share a common optical axis  330 . The refractive elements are capable of substantially receiving the light from the dispersing element  360  and substantially focusing the light to a detecting element  50 . The optical axis  330  is substantially bent relative to the optical axis  30  at a point substantially proximate to the dispersing element  360  such that the light dispersed by the dispersing element  360  is substantially centered along the second optical subassembly  220  of the spectrometer  300 . 
       FIG. 5A  illustrates the impact of the location of the slit  40  and detector element  50  substantially to the optical axes  30  and  330  respectively in the embodiment  300  illustrated in  FIG. 4  on the overall size of the optical field radius (Hρ), where the spectral component (Hy) of the optical field is greatly reduced due to the substantially axial location of the detector  50  along the optical axis  330 . To better understand this effect, the spatial field (Δx) and spectral field (Δλ) of the dispersed image are shown from the view of a plane perpendicular to the optical axis  330  just prior to the detector  50  in  FIG. 5A , where the detector  50  is shown at its substantially axial position. The optical field radius (Hρ) of the spectrometer  300  must be sufficiently large enough to accommodate the furthest extents of both the spatial field (Δx) of the slit  40  and detector  50 , as well as the spectral field (Δλ) that results from the dispersion of the grating  360 . The relationship between the size of the spatial field (Δx), the focal length (f) of the optical subassembly of the system or spectrometer  300  optically disposed between the diffraction grating  60  and the detector  50 , the period (λ) of the diffraction grating  60 , and spectral bandwidth (Δλ) of the operational spectral band of the sensor on the optical field (Hρ) is also provided in  FIG. 5A  for the spectrometer  300  illustrated in  FIG. 4 . 
     As can be seen in  FIG. 5B , the optical field radius (Hρ) of the spectrometer  300  remains dominated by the size of the spatial component (Hx) of the optical field for coarse spectral resolutions, but does not become dominated by the spectral component (Hy) of the optical field until much finer spectral resolutions than for the spectrometer  100  illustrated in  FIG. 2B . 
     Spectral smile is a wavelength-dependent distortion known in the art that represents a measure of departure from linearity in the monochromatic image of the slit aperture  40  at the image plane in an imaging spectrometer. It is typically desirable to limit this distortion to less than a few hundredths of a percent of the spatial field width. The spectral smile distortions of the embodiments of the spectrometers  200  and  300  illustrated in  FIG. 3  and  FIG. 4 , respectively, are shown side by side in  FIG. 6A  and  FIG. 6B , respectively, where it can be seen that a significant amount of spectral smile distortion  280  and  380  is introduced in both embodiments  200  and  300  respectively. However, since the spectral smile distortion  280  and  380  introduced by the embodiments  200  and  300  respectively are opposite in sign, a further embodiment of the present invention that combines both of these approaches can be used to provide an axial configuration with substantially lower spectral smile distortion than either of the embodiments  200  or  300  alone. 
     In a further embodiment of the spectrometer  500 , a hybrid axial refractive relay design form with both the introduction of prism elements and a bent optical axis is illustrated in  FIG. 7 . The slit  40  and detector element  50  are substantially located on the optical axes  30  and  530  respectively by inserting a light bending element  570  or group of elements, such as but not limited to a prism or combination of prisms, between the two halves  210  and  220  of the spectrometer  500  and inserting a bend in the optical axis  30  at or proximate to the dispersing element  560 . The spectral distortion  580  for this embodiment  500  is provided in  FIG. 8  and illustrates the significantly reduced spectral smile distortion  580 , in this embodiment by approximately two orders of magnitude, over the embodiments  200  and  300  illustrated in  FIG. 6 a    and  FIG. 6 b    respectively. 
     Reference is made to  FIG. 7 , which is a schematic view of spectrometer  500  taken along its optical axis in the plane parallel to the direction of dispersion. In operation, light emanating from a slit element  40 , is incident on a first optical subassembly  210  of spectrometer  500 , in this embodiment made up of, but not limited to, refractive elements  212 ,  214 ,  216 , and  218  that, in this embodiment, substantially share a common optical axis  30 , which is capable of substantially receiving a portion of the light emanating from the slit  40  and substantially collimating the light. The light is then incident on a first light bending element  570 , which is capable of substantially receiving the light from the first optical subassembly  210  of the spectrometer  500  and substantially bending the light. The light is then incident on a dispersing element  560 , in this embodiment a transmission diffraction grating, which is capable of substantially receiving the light from the first light bending element  570  and dispersing it according to its wavelength. The dispersed light is then incident on a second light bending element  572 , in this embodiment a prism, which is capable of substantially receiving the light from the dispersing element  560  and substantially bending the light. The light is then incident on a second optical subassembly  220  of the imaging optical system, in this embodiment made up of, but not limited to, refractive elements  222 ,  224 ,  226 , and  228  that in this embodiment substantially share a common optical axis  530 , which is capable of substantially receiving the light from the second light bending element  572  and substantially focusing the light to a detecting element  50 . The optical axis  530  is substantially bent relative to the optical axis  30  at a point substantially proximate to the dispersing element such that the light dispersed by the dispersing element  560  is substantially centered along the second optical subassembly  220  of the spectrometer  500 . 
     In a further embodiment or spectrometer  600 , a hybrid axial refractive relay design form with both the introduction of prism elements and a bent optical axis is illustrated in  FIG. 9 , where the dispersing element  560  is combined with one of the prisms  572  in the embodiment  500  illustrated in  FIG. 7  to form what is commonly referred to as a “grism” or grism element  690 , where a grating is fabricated into one or more surfaces of the prism  572 . The slit  40  and detector element  50  are substantially located on the optical axes  30  and  630  respectively by inserting a prism  670  and a grism element  690  between the two halves  610  and  620  of the spectrometer  600  and by inserting a bend in the optical axis  30  at or proximate to the dispersing element  690 . 
     In a still further embodiment of spectrometer  700 , a hybrid axial refractive relay design form with both the introduction of a grism element  790  and a bent optical axis  730  is illustrated in  FIG. 10 , where the dispersing element is a grism element  790 . The slit  40  and detector element  50  are substantially located on the optical axes  30  and  730  respectively by inserting a grism element  790  between the two halves  710  and  720  of the spectrometer  700  and by inserting a bend in the optical axis  30  at or proximate to the dispersing element  790 . In principle, the dispersive element and prisms can be combined into any combination of dispersing elements, prisms, or grisms in accordance with the present invention. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the invention.