Abstract:
A hyperspectral imager that achieves accurate spectral and spatial resolution by using a micro-lens array as a series of field lenses, with each lens distributing a point in the image scene received through an objective lens across an area of a detector array forming a hyperspectral detector super-pixel. Each sub-pixel within a super-pixel has a bandpass or other type filter to pass a different band of the image spectrum. The micro-lens spatially corrects the focused image point to project the same image scene point onto all sub-pixels within a super-pixel. A color separator can be used to split the image into sub-bands, with each sub-band image projected onto a different spatially corrected detector array. A shaped limiting aperture can be used to isolate the image scene point within each super-pixels and minimize energy coupling to adjacent super-pixels.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional patent application 60/527,252 filed Dec. 5, 2003 entitled “Spatially corrected full-cubed Hyperspectral imager”, incorporated herein by reference. 

   BACKGROUND 
   1. Field of the Invention 
   The invention relates generally to spectrometry and particularly to hyperspectral imaging. 
   2. Prior Art 
   Hyperspectral imagers (HSI) are a class of spectrometers that record energy in many discrete spectral bands or colors simultaneously at a multitude of spatial picture elements, called pixels. Standard broadband imagers record one value at each picture element for all the detected incident energy across a wide spectrum. Hyperspectral imagers differ from standard broadband imagers by creating an additional spectral dimension in addition to the two spatial dimensions created from a two-dimensional array of detectors. Each HSI picture element may have ten to hundreds of wavelength values recorded. 
   Some hyperspectral imagers use a scanning approach where a scan mirror scans an image across the entrance slit of a spectrometer (pushbroom), often employing a prism or grating to disperse the various wavelengths across a detector array. Scanning hyperspectral imagers are complex and large. Such imaging spectrometers are often mounted on aircraft without a scan mirror; in such cases, the motion of the aircraft serves to scan the scene. 
   A class of imagers known as multispectral imagers or multispectral cameras resolve many frequencies of the image into a limited number of bands, providing a coarse measure of energy as a function of frequency. These systems use a color-sorting prism to separate the image into three spectral bands. Each prism output can drive a monochrome imaging array or a color image array. An example of this type of multispectral camera is available from Redlake MASD, Inc. (formerly DuncanTech), which has a camera configured with a 3-way color-sorting prism illuminating two monochrome imagers and one 3-color imager to cover a total of five spectral bands. The color sorting or separating prism provides bandpass filtering of the image into sub-bands. Trim filters in front of each image array can be used to provide additional wavelength selectivity and rejection of out of band energy. 
   One approach to achieve multispectral capability with a single imaging array, such as a complimentary metal oxide semiconductor (CMOS) or charged-coupled device (CCD) array, uses a color filter array (CFA) to limit the color band passing through to each pixel. The wide spectral response of a pixel is limited to the spectral band of the corresponding CFA element applied to that pixel. This technique is common in color video cameras and digital still cameras, using a Bayer filter pattern of interleaved red, blue, and green filters. An interpolation or de-mosaicing algorithm is used to reconstruct a color corrected image of uniform pixels from a mosaic of separate color pixels created by the filter pattern. These algorithms use a weighted combination of pixel values with neighboring pixel values. This approach creates an approximation of the hyperspectral information. Each pixel detects a different color from a slightly different position in the image scene. In a conventional pushbroom scanning system, each wavelength associated with a single spatial pixel in the cube views the exact same region of the scene. In a typical Bayer filter approach, each pixel views a slightly different region of the scene. Mathematical operations performed by image processing cannot reconstruct information lost as a result of the Bayer filter design. The de-mosaicing process produces an aesthetically acceptable result for some consumer applications. However, this loss of data can affect a variety of applications including those where a direct spectral comparison of spatially correlated images is required. 
   Imaging arrays fabricated with integrated circuit semiconductor technology have active and inactive areas. In addition to the light sensitive detector areas, other circuitry used for readout of the detector pixels uses area in the focal plane but does not collect light. This ratio of light sensitive and light insensitive areas is referred to as fill factor. Various technologies have different fill factors. In order to improve light sensitivity, micro-lens arrays (MLA) have been widely used in both CMOS and CCD arrays to increase the proportion of light that is directed to the light sensitive areas. In prior art designs, there is a one-to-one correspondence between a lens in the MLA and a pixel in the detector array. 
   A color filter array allows using a single detector array to resolve more than one color, but reconstructing the image formed from the interleaved filter array impairs accuracy for high precision hyperspectral imaging. 
   SUMMARY OF THE INVENTION 
   A spatially corrected hyperspectral imager according to the present invention achieves accurate spectral and spatial resolution by using a micro-lens array (MLA) as a series of field lenses, with each lens distributing a point in the image scene received through an objective lens across an area of a detector array forming a hyperspectral detector super-pixel. Each sub-pixel within a super-pixel has a filter used to spectrally characterize the object. The micro-lens spatially corrects the focused image point to project the same image scene point onto all sub-pixels within a super-pixel. 
   The array of micro-lens is positioned between the objective lens and the detector array to distribute light for a super-pixel to all of its sub-pixels. One lens of the micro-lens array corresponds to one super-pixel of the detector array. A super-pixel consists of several sub-pixels. Each sub-pixel within a super-pixel collects energy at different wavelengths by use of a color filter array in the optical path between the MLA and the detector array. Since each sub-pixel in any given super-pixel views the same portion of the image, the errors in conventional Bayer image sensors and de-mosiacing algorithms are no longer a limitation. In the most straightforward application, a super-pixel consists of an array of sub-pixels within the detector array where each sub-pixel has a unique narrow-band filter and is thus responsive to a unique band of wavelengths. The color filter matrix pattern, for example, employing 16 unique narrow-band filters in a 4×4 array, is repeated across the entire detector array. In this example, a super-pixel consisting of 16 sub-pixels represents one point in the image and measures 16 different wavelengths. 
   A conventional MLA with one lens per sub-pixel may be used for increasing fill factor, which may be an integral part of the detector array. The super-pixel MLA is an entirely separate element used in addition to the MLA that may be used for increasing fill factor. 
   A color filter array of various discrete bandpass filters forms a filter matrix. The bandpass filters can cover a contiguous range of wavelengths or non-contiguous wavelengths. The filters in the array may be narrow band, broad band, or complex in spectral response. 
   Each lens of the micro-lens array covers one super-pixel. Each super-pixel is an array of individual sub-pixels and forms a multiband detector. The array of multiband detectors forms a two-dimensional imager with a third spectral dimension, resulting in a full-cubed hyperspectral imager. 
   The super-pixel MLA may be used with a sensor that includes a fill factor improving MLA. If it is, then an additional MLA is required to preserve the field of view of the super-pixel. 
   In another embodiment of the invention, which further increases the number of colors resolved by the hyperspectral imager, a color separator is used to split the full band being imaged into sub-bands. Each sub-band is then imaged with one hyperspectral imaging array. For example, a 3-way color separator projecting the three sub-bands onto three hyperspectral imagers, using detector arrays with super-pixels capable of detecting 16 colors, will achieve 48-color resolution. 
   In one particular embodiment of the invention, the color separator uses three-color separating prisms to direct three broad spectral bands to three CMOS imagers located on the output faces of each prism. Two dichroic filters located at the prism interfaces act as band splitters. The dichroic filters can be deposited directly on the surface of the prisms. Other methods of color separation are well known, including layered cubes, layered filters, 45 deg dichroic mirrors, and gratings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an image sensor, filter mask, micro-lens, and objective lens according to the present invention. 
       FIG. 2  shows an image sensor with a mask array near the micro-lens array. 
       FIG. 3  shows a micro-lens array positioned between an objective lens and a filter mask with a representative ray path. 
       FIG. 4  shows examples of a segment of a micro-lens array with a hexagonal and square packing. 
       FIG. 5  shows example of photomasks for fabricating a filter matrix for use with the present invention. 
       FIG. 6  shows an example of a filter matrix pattern. 
       FIG. 7  shows an example of an alternate filter matrix pattern to increase isolation between super-pixels. 
       FIG. 8  shows a color separating prism assembly directing light sub-bands onto three imagers. 
       FIG. 9  shows details of a color separating prism assembly suitable for use with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows the side view of the layers of the hyperspectral imager according to the present invention. Detector array  110 , color filter array  120 , and micro-lens array (MLA)  130 , each shown separated for clarity, are rigidly attached to each other. MLA  130  is positioned where objective lens  140  forms an image of the object under observation. Detector  110  is placed where the lenses in MLA  130  form an image of the exit pupil of objective lens  140 . Color filter array  120  should be placed as close as feasible to detector  110  to ensure minimal spectral crosstalk and maximum efficiency. Optional spacing  125  can be filled with glass, other optically transparent material, vacuum, or air. Each lens of MLA  130  distributes light across a super pixel area of filter array  120  and detector array  110 . 
   Objective lens  140  is preferably a telecentric lens. The chief ray for each field point of the image formed by a telecentric lens is perpendicular to the image plane. The chief ray is the ray for each field point (angle) that passes through the center of the aperture stop in the optical system. Alternatively, another field lens can be placed at or near the image plane to direct the light into the MLA as a telecentric lens would, thus causing a non-telecentric lens to function as a telecentric lens. 
   Aperture  150  represents the entrance pupil of the optical system and aperture  152  represents the exit pupil of the optical system. These pupils are images of the aperture stop, the aperture that limits the axial cone of energy from the object. The entrance  150  and exit  152  pupils may be actual objects or merely images of the aperture stop. To maximize light throughput and minimize crosstalk from adjacent super-pixels, the shape of the exit pupil  150  should match the shape (outline) of the layout of the detector pixels used to form the super-pixel. The focal length of MLA  130  should be designed to produce a demagnified image of exit pupil  152  that matches the layout of the detector pixels. For lenses whose aperture stop shape does not match the shape of the layout of the detectors (e.g. square, rectangular, hexagonal, or circular), a stop can be placed at the location of aperture  150  or  152  to change the effective shape of the aperture stop. 
   Each lens in the MLA images the exit pupil of the optics path, which has the effect of imaging the limiting aperture, onto the detector array. The image is out of focus at each detector sub-pixel because the MLA is at the focus of the objective lens, however, at the super-pixel level the scene is in focus. While not necessary for the invention to operate, forming the limiting aperture shape to match the shape of the super-pixel allows optimum performance. 
     FIG. 2  shows an alternative approach to achieving energy separation by using an aperture mask positioned in front the MLA or in front of the color filter. The aperture mask has an array of openings that restrict light to an area that is the shape of the super-pixel. Aperture masks  135  and  137  are shown in two alternative positions to act as a field stop. Aperture masks  135  and  137  can be constructed at tubular baffles. 
     FIG. 3  shows ray tracing of the optical path for one super-pixel. Pixel A, Pixel B, and Pixel C represent super-pixels composed of an array of sub-pixels, for example a 4×4 array in this illustration. Color filter array  120  is used to create different wavelength responses in each of the sub-pixels within each super-pixel. Micro-lens array  130  is used as an array of field lenses with each lens corresponding to a super-pixel. Limiting aperture  150  defines the shape of the light incident on filter array  120 . 
   A complete hyperspectral detector array, for example, uses a color filter array with a repeating matrix of 4×4 narrow band filters with 10 nm bandwidth in conjunction with a 1936×1086 detector array produces a spatial resolution of 484×271 points with 16-wavelength detection at 10 nm spectral resolution. A suitable CMOS array is available from Rockwell Scientific, which has intrinsic response from less than 400 nm to greater than 1000 nm. 
   Micro-Lens Array 
     FIG. 4  shows two examples of micro-lens array packing techniques. Circular lenses can be arranged in a staggered pattern. The lenses can touch or have a gap between each lens. Square lenses can be arranged in a regular pattern as shown or have a slight stagger (not shown) to allow denser packing that fills the space at the corners. Lenses can alternatively be rectangular shaped. 
   Regardless of the shape of the MLA lens, a single micro lens spreads the light incident onto the lens across the area of a super-pixel. The MLA aperture should roughly match the super-pixel shape and the shape of the aperture stop. Deviations from this will cause light loss. 
   The micro lens is placed at the image formed by the objective lens and each micro lens functions as a field lens. Such a field lens in the optical path reduces the divergence spread of the light rays, making the light more collimated, thus the position of the optics behind the image plane is less critical. The direction of each cone of light projected from the objective lens is changed so that the cone is bent toward the centerline. Light focused to a point at the micro lens is spread across the super-pixel area. An image formed at the focus of the objective lens is refocused at the super-pixel resolution instead of diffraction-limited resolution of the objective. 
   Each lens has an effective collection aperture and the light received at any point in the aperture is distributed evenly across the underlying super-pixel. 
   The pitch of the micro-lenses is adapted to the size of the super-pixel. For example, with pixels of 5 um square forming a super-pixel of 20 um square, the center-to-center spacing of each micro-lens would be 20 um. 
   Micro-lenses are described generally and manufacturing techniques are disclosed in the following references, each is incorporated herein by reference: 
   Nakama et al., U.S. Pat. No. 5,867,321, “Planar microlens array and method of making same”; Li, U.S. Pat. No. 6,587,147, “Microlens array”; Teshima et al., U.S. Pat. No. 6,506,623, “Microstructure array, mold for forming a microstructure array, and method of fabricating the same”; Hamanaka et al., U.S. Pat. No. 6,432,328, “Method for forming planar microlens and microlens obtained thereby”; Border et al., U.S. Pat. No. 6,402,996, “Method of manufacturing a microlens and a microlens array”; Iijima, U.S. Pat. No. 6,374,024, “Image sensor and method of manufacturing the same”; Marchant et al., U.S. Pat. No. 6,353,502, “Vcsel field construction”; Assadi et al., U.S. Pat. No. 6,540,946, “Microlens for surface mount products”. 
   Filter Matrix 
     FIG. 5  shows three masks in a series of masks used to construct the filter matrix. A 4×4 filter matrix requires a series of 16 masks; each mask has openings, represented by the small square regions, corresponding to one of the pixel positions at each super-pixel. Photolithography techniques are used to construct the masks and deposit the wavelength-determining coatings at each pixel position. The process of creating masks and depositing coating for bandpass filters is well known. Pixel sizes of 5×5 um and smaller are well within the state of the art in photolithography. In addition to non-overlapping mask, absorptive Filters may be made in an overlapping fashion. 
     FIG. 6  shows an example of a filter matrix pattern. In this example, a 4×4 array forms one super-pixel. Sub-pixels correspond to filter matrix elements number 1 through 16. The 16-element filter matrix pattern is repeated across the image array. A regular packed pattern of super-pixels fills the image array area. Light passing through one micro lens fills one super-pixel area. 
     FIG. 7  shows an alternative filter matrix pattern with unused pixel positions between lenses. One or more pixels between each super-pixel can be sacrificed to insure that no light from adjacent super-pixels is received. Additionally, this filter matrix pattern can be used with a circular or alternately shaped limiting aperture or a limiting aperture that is created by the finite size of the objective lens itself. The 16 colors of the filter matrix corresponding to 16 pixels on the image array are circumscribed by the circular region. A perimeter around the active filter matrix area is not used because a full pixel is not covered by the projected image. Alternatively, other pixels arrangements can be used to approximate an area that more closely corresponds to the projected image area. 
   The boundary between lenses may have excessive optical impairments, thus areas of unused pixels can be assigned to these dark areas. The filter matrix defines which pixels are used. The required size of the detector array increases for a given resolution due to unused pixels, or reduces the resolution available for a given size array. The benefit gained is improved illumination uniformity across the super-pixel, even though detector real estate is not fully used with this alternative filter matrix pattern. 
   Color Separator 
     FIG. 8  shows color separator prism  210  receiving an image from objective lens  220 . Color separator  210  splits the incident image into three sub-bands. Imagers  213 ,  215 , and  217  are positioned on the output face of each prism and further resolve each sub-band into narrower frequency bands. Each imager  213 ,  215 , and  217  comprises a micro-lens array, color filter matrix and imaging detector array, each forming a hyperspectral imager. 
     FIG. 9  shows color separator prism  210  comprising green prism  212 , blue prism  214 , and red prism  216 . Two dichroic filters located at the prism interfaces act as band splitters. One band-splitting filter is located between blue prism  214  and red prism  216 . The other band-splitting filter is located between red prism  216  and green prism  212 . The dichroic filters can be deposited directly on the surface of the prisms. Red/Green/Blue are representative colors, the actual wavelengths of each sub-band can be any wavelength including above and below the visible spectrum used in this example. The technique of using color-separating prisms can be applied to 2 bands or more. Using three imaging arrays with a filter matrix array covering 16 separate narrow bands, 48 wavelength bands in total can be covered. 
   Data Processing 
   The HSI described here can generate large amounts of data. A technique for acquisition and processing of data at real time rates of 30 cubes per second is described in Dombrowski et al., U.S. Pat. No. 5,949,074 “Imaging Spectroradiometer”, incorporated herein by reference. The techniques for reading the electronic signals from CMOS detector arrays are well known. Generally, digital and analog circuitry interfaces with the detector array to control the readout of rows and columns of image induced signal at each pixel. Preferably, an analog to digital converter digitizes the analog signal from the detector and further storage and processing of the image data is done digitally.