Abstract:
Systems and methods for multispectral imaging are disclosed. The optical system includes 1) an array of optical elements, each optical element optically disposed to receive incident electromagnetic radiation; 2) a filter capable of substantially operating as a filter array, each filter element spectrally filtering electromagnetic radiation substantially into a spectral band having a predetermined central wavelength; and 3) a detector system capable of substantiality operating as a detector array of detector elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/760,133, COMPACT SNAPSHOT MULTISPECTRAL IMAGING SYSTEM, filed on Jun. 8, 2007, which claims priority of U.S. Provisional Application 60/811,889, SNAPSHOT HYPERSPECTRAL IMAGER, filed on Jun. 8, 2006, which is incorporated by reference herein in its entirety and for all purposes. 
    
    
     BACKGROUND 
     These teachings relate to compact snapshot multispectral imaging systems. 
     A multispectral or hyperspectral imaging system (the terms multispectral and hyperspectral are used interchangeably here) is commonly used to observe objects or scenes, whereby light emitted or reflected by a given object or scene is imaged by some means onto a detecting element or array of detecting elements, where multiple images with different spectral content can readily be observed or recorded. 
     Due to the short temporal duration of many events, it is necessary to capture multispectral data in a short amount of time. A snapshot multispectral imaging system is a multispectral imaging system that captures all desired spectral images at a single moment, rather than relying on either spatial or spectral scanning of the object or scene. 
     In many designs incorporating snapshot multispectral imaging systems, there is a need for the overall system to be compact. Such needs stem from weight and space constraints in the application in which the system is used. Conventional snapshot multispectral imaging systems are typically large in size due to their large single aperture optics or optical relay subsystems. 
     Recent advances in snapshot multispectral or hyperspectral imaging systems have been made using Computed Tomography Imaging Spectrometer (CTIS) devices. In these devices, the image is dispersed across multiple dispersive orders using a computer generated hologram and onto a single detector or detector array, analogous to integrated slices through the 3D data cube at various angles across the two spatial dimensions. Using reconstructive techniques similar to those used in CT scans in the medical field, they used computed tomography to build up the 3D data cube from the two dimensional detector. In this manner, spatial and spectral information is captured in a single integration time. Unfortunately, this technique has limitations on the spatial and spectral resolutions that can be captured due to the limited number of dispersive orders that can be generated. Furthermore, these systems tend to be fairly large. 
     There is therefore a need for a snapshot multispectral imaging system that is more compact in physical size than current multispectral imaging systems. 
     Furthermore, there is also a need for a snapshot multispectral imaging system that has greater spatial and spectral resolution than current imaging systems. 
     Furthermore, there is also a need for a snapshot multispectral imaging system that has a greater degree of image co-registration than current imaging systems. 
     Still further, there is a need for an imaging system that provides a combination of the characteristics described above with superior trade-offs than have been previously attainable. 
     BRIEF SUMMARY 
     The needs for the teachings set forth above as well as further and other needs and advantages of the present teachings are achieved by the embodiments of the teachings described hereinbelow. 
     In one embodiment, an optical system of these teachings includes an array of filters with varying spectral transmission characteristics capable of receiving electromagnetic radiation from a source and transmitting at least a portion of the electromagnetic radiation received from the source, an array of micro-optic imaging subsystems capable of receiving electromagnetic radiation from the array of filters and imaging at least a portion of the received electromagnetic radiation onto an image plane. 
     In another embodiment, the optical system of these teachings includes a filter capable of substantially operating as a filter array. 
     Various other embodiments of the optical system of these teachings and embodiments of the method of these teachings of are also disclosed. 
     In one embodiment of the present teachings the array of micro-optic imaging subsystems are miniaturized and tiled into an array, which is placed behind a filter with an array of spectral transmission characteristics and in front of an image plane, detector, or detector array. These arrays can comprise, but are not limited to, micro-optic elements that are arranged in proximity to one another. In this manner, an array of spectrally varying images can be generated at the image plane. 
     For a better understanding of the present teachings, together with other and further needs thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with an embodiment of the present teachings taken along a plane containing a column of optical axes; 
         FIG. 2   a  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings taken along a plane containing a column of optical axes; 
         FIG. 2   b  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings taken along a plane containing a column of optical axes; 
         FIG. 3  is an isometric quarter cutaway view of the embodiment of the present teachings illustrated in  FIG. 2   a;    
         FIG. 4   a  is a schematic sectional view of a portion of the embodiment of the present teachings illustrated in  FIG. 2   a  taken along the optical axis; 
         FIG. 4   b  is schematic sectional view of the present teachings illustrated in  FIG. 2   a  taken along a plane containing a column of optical axes; 
         FIG. 5  is a front facing view of the filter array component of the embodiment of the present teachings illustrated in  FIG. 2   a;    
         FIG. 6  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings, taken along a plane containing a column of optical axes; 
         FIG. 7   a  is a front facing view of the filter array component of the embodiment of the present teachings illustrated in  FIG. 6 ; 
         FIG. 7   b  is a further front facing view of the filter array component of the embodiment of the present teachings illustrated in  FIG. 6 ; 
         FIG. 8   a  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings, taken along a plane containing a column of optical axes; 
         FIG. 8   b  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings, taken along a plane containing a column of optical axes; 
         FIG. 9  is an isometric view of the embodiment of the present teachings illustrated in  FIG. 8   b;    
         FIG. 10  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings, taken along a plane containing a column of optical axes. 
         FIG. 11  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a further embodiment of the present teachings, taken along a plane containing a column of optical axes. 
         FIG. 12  is a schematic sectional view of a compact snapshot multispectral imaging system in accordance with a still further embodiment of the present teachings, taken along a plane containing a column of optical axes. 
     
    
    
     DETAILED DESCRIPTION 
     Compact snapshot multispectral optical systems are disclosed hereinbelow. 
     The terms “micro-optics” and “micro-optical component” as used herein, refer to optical components having apertures substantially smaller than the entrance pupil of the conventional optical imaging subsystems discussed herein. The micro-optical components can be refractive, diffractive or reflective or any combination thereof. Exemplary micro-optical components include, but are not limited to, diffractive, refractive, and hybrid micro-lenses, GRIN rod lenses, micro-mirrors and micro-prisms. 
     The term “gradient index rod lens” as used herein, refers to radial gradient index optical components. 
     Reference is made to  FIG. 1 , which is a schematic sectional view of an embodiment of these teachings  100 , taken along a plane containing a column of optical axes. Electromagnetic radiation, typically in, but not restricted to, 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 also referred to as a source, is incident upon a filter capable of substantially operating as a spectral filter array  110  (hereinafter referred to as spectral filter array  110 ) which substantially transmits various portions of the light to an array of lenses  120 , in this embodiment consisting of the refractive microlens elements  122 , and imaged onto an image plane  150 . In this manner, an array of images with varying spectral characteristics is generated at the image plane  150  that are highly co-registered with one another due to the monolithic design of the imaging optics. 
     The individual filters of the spectral filter array  110  can be, but are not limited to, colored glass or gelatin filters, a substantially bandpass filter, a substantially low-pass filter (also referred to as a long pass filter), a substantially high-pass filter. (also referred to as a short pass filter), or an interference filter. 
     In some applications, although not a limitation of these teachings, a CCD array, CMOS imager, phosphorescent screen, photographic film, microbolometer array, or other means of detecting light energy, hereinafter referred to generally as a detector or detector array, is substantially located at the image plane. The detector arrays typically consist of many individually readable light detecting pixels or elements. In one embodiment, a detector or detector system, capable of substantially operating as a detector array of the detector elements, is substantially located at the image plane. 
     In another embodiment, the detector system comprises an array of detector subsystems, each of a detector subsystem being, for example, but not limited to, one of the detectors described above. In one instance, detector system comprises a number of detector subsystems wherein a spectral sensitivity of at least some of detector subsystems is different from a spectral sensitivity of at least some of the other detector subsystems. In this detector system, the multiple detector arrays or subsystems can be different, for example, to cover different spectral bands. In another type of detector system, the detector arrays or subsystems can be similar or identical and used to increase the number of pixels in the imager. 
     In one embodiment, the detector system is a pixellated detector such as, but not limited to, a CCD array or a CMOS array. In that embodiment, in one instance, each detector element from the detector array includes a number of pixels. 
     It should be noted that, although in the embodiments of the present invention described here, specific numbers of miniaturized (micro-optic) lens systems are shown, this is not a limitation of these teachings and any pre-determined number of miniaturized (micro-optic) lens systems can be utilized in any one-dimensional or two-dimensional pattern. 
     Reference is made to  FIG. 2   a , which is a schematic sectional view of a further embodiment of these teachings  200 , taken along a plane containing a column of optical axes. In operation, light is incident upon a spectral filter array  110  which substantially transmits various portions of the light to an array of lenses  120 , in this embodiment consisting of the refractive microlens elements  122 . The light is then imaged by the array of lenses  120  onto the image plane  150 , passing through an array of apertures, or field stops,  240 . 
     Reference is made to  FIG. 2   b , which is a schematic sectional view of a further embodiment of these teachings  300 , taken along a plane containing a column of optical axes. In operation, light is incident upon a spectral filter array  110  which substantially transmits various portions of the light to an array of lenses  120 , in this embodiment consisting of the refractive microlens elements  122 . The light is then imaged by the array of lenses  120  onto the image plane  150 , passing through an array of baffles  340 , in this embodiment consisting of the baffle elements  342 . 
     Reference is made to  FIG. 3 , which is an isometric quarter cutaway view of the embodiment of the present teachings  200  illustrated in  FIG. 2   a.    
     Reference is made to  FIG. 4   a , which is a schematic sectional view of a portion of the embodiment of the present teachings  200  illustrated in  FIG. 2   a , taken along the optical axis. At each location in the array of lenses  120 , light is incident upon the spectral filter array  110  which substantially transmits a portion of the light to the refractive microlens element  122 . The light is then imaged by the microlens element  122  onto the image plane  150 , passing through the aperture, or field stop,  242 . 
     Reference is made to  FIG. 4   b , which is a schematic sectional view of the embodiment of the present teachings  200  illustrated in  FIG. 2   a.    
     Reference is made to  FIG. 5 , which is a front facing view of the filter array component  110  of the embodiment of the present teachings  200  illustrated in  FIG. 2   a . In this embodiment, the spectral filter array consists of a pre-determined arrangement of tiled smaller filter windows  112 . 
     Reference is made to  FIG. 6 , which is a schematic sectional view of a further embodiment of these teachings  400 , taken along a plane containing a column of optical axes. In operation, light is incident upon a linearly varying spectral filter  410 , adapted so that the linearly varying spectral filter substantially operates as a two-dimensional filter array (in one instance, the orientation of the linearly varying filter is in client with respect to an axis of the array of lenses) which substantially transmits various portions of the light to an array of lenses  120 , in this embodiment consisting of the refractive microlens elements  122 . The light is then imaged by the array of lenses  120  onto the image plane  150 , passing through an array of apertures, or field stops,  240 . 
     Reference is made to  FIG. 7   a , which is a front facing view of the linearly varying spectral filter component  410  of the embodiment of the present teachings  400  illustrated in  FIG. 6 . The direction of variation  412  of the spectral characteristics of the linearly varying filter  410  is oriented relative to the plane containing a column of optical axes  124  in the array of lenses  120  with angle θ. 
     Reference is made to  FIG. 7   b , which is another front facing view of the linearly varying spectral filter component  410  (in one instance, a wedge filter) of the embodiment of the present teachings  400  illustrated in  FIG. 6 . The orientation of the linearly varying filter  410  (in one instance, the orientation of the direction of linear variation) relative to the plane containing a column of optical axes  124  in the array of lenses  120  creates an array of filter regions  416  consisting of individual filter regions  418 . In this embodiment, the angular orientation θ of the linearly varying spectral filter component  410  is determined according to the following equation:
 
θ=arctan [(Δ y )/( nΔx )]
 
where Δx and Δy represent the horizontal and vertical spacing of the refractive microlens elements  122  of the array of lenses  120 , and n represents the number of microlens elements  122  in a single row of the array of lenses  120 . In this orientation, the individual filter regions  418  will have spectral characteristics that vary linearly from element to element by a shift in wavelength Δλ across the first row and continuing onto the next row, one row after the other, such that the first element in each row has a shift in wavelength equal to nΔλ. This implementation provides a very effective and inexpensive method to separate the image data into a series of images with linearly variable spectral characteristics.
 
     Reference is made to  FIG. 8   a , which is a schematic sectional view of a further embodiment of these teachings  500 , taken along a plane containing a column of optical axes. In operation, light is incident upon a spectral filter array  510  which substantially transmits various portions of the light to an array of lens systems  520 , in this embodiment consisting of the lens arrays  532  and  534 , each consisting of the refractive microlens elements  522  and  524  respectively. The light is then imaged by the array of lens systems  520  onto the image plane  550 , passing through an array of apertures, or field stops,  540 . 
     Reference is made to  FIG. 8   b , which is a schematic sectional view of a further embodiment of these teachings  600 , taken along a plane containing a column of optical axes. In operation, light is incident upon a spectral filter array  610  which substantially transmits various portions of the light to an array of gradient index rod lenses  620 , in this embodiment consisting of the gradient index rod lens elements  622 . The light is then imaged by the array of gradient index rod lenses  620  onto the image plane  650 , passing through an array of apertures, or field stops,  640 . 
     Reference is made to  FIG. 9 , which is an isometric quarter cutaway view of the embodiment of the present teachings  600  illustrated in  FIG. 7   b.    
     Reference is made to  FIG. 10 , which is a schematic sectional view of a further embodiment  700  of these teachings, taken along a plane containing a column of optical axes. In operation, light is incident upon a substantially a focal lens system  720 , in this embodiment consisting of refractive elements  722  and  724 , which provides angular magnification to the incident light, which is substantially transmitted to the previous embodiment of the present teachings  200 . In this manner, the angular resolution can be increased. 
     Reference is made to  FIG. 11 , which is a schematic sectional view of a further embodiment  800  of these teachings, taken along a plane containing a column of optical axes. In operation, light is incident upon a substantially a focal lens system  820 , in this embodiment consisting of refractive elements  822  and  824 , which provides angular de-magnification to the incident light, which is substantially transmitted to the previous embodiment of the present teachings  200 . In this manner, the field of view can be increased. 
     Reference is made to  FIG. 12 , which is a schematic sectional view of a still further embodiment of these teachings  900 , taken along a plane containing a column of optical axes. In operation, light is incident upon a spectral filter array  910  which substantially transmits various portions of the light to an array of gradient index rod lenses  920 , in this embodiment consisting of the gradient index rod lens elements  922 . The light is then imaged by the array of lenses  920  onto the array of image planes  950 , in this embodiment consisting of image plane elements  952 , and passing through an array of apertures, or field stops,  940 . 
     The form of the miniaturized imaging lens systems that make up the array of lenses or imagers can be any combination of refractive, diffractive, gradient index, or other optical element known in the art. These components need only be miniaturized and placed into arrays to form the miniaturized (micro-optic) lens arrays described above. 
     It should be noted that, although the arrays of miniaturized (micro-optical) imaging systems disclosed above comprise one or two planar array elements, the number of planar array elements is not a limitation of these teachings. 
     In one instance, during use of one embodiment of the system of these teachings, electromagnetic radiation incident and to one lens (or optical element) from the array of lenses  120 , or, more generally, the array of optical elements), is spectrally filtered into substantially one spectral band having a predetermined central wavelength and, the filtered electromagnetic radiation is imaged by the lens onto one detector element from a detector array (or a portion of a detector system that substantially operates as a detector element from a detector array). In one instance, the spectral filtering is obtained by means of a filter capable of substantially operating as a filter array. The filtered electromagnetic radiation imaged by the lens is detected. In another instance, crosstalk between the electromagnetic radiation imaged onto one detector by one optical element and electromagnetic radiation image onto another detector by another optical element is substantially limited. In still another instance, the limitation of crosstalk is obtained by providing a spatial array of apertures or baffles. 
     Although the teachings have been described with respect to various embodiments, it should be realized that these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.