Abstract:
A low-cost high-resolution staring infrared imaging sensor for viewing a large Field Of Regard (FOR) while integrating over a small IFOV to detect small dim targets by subdividing the FOR into a plurality of internal optical paths without the use of mechanically-moveable parts. Each of the plurality of internal optical paths may be further subdivided by a plurality of steerable micro-mirrors to reduce the IFOV and enhance long-range target acquisition capability. The sensor includes a primary lens for accepting infrared radiation from a Field Of Regard (FOR), a plurality of primary mirrors each disposed to reflect a portion of an FOR image along a different optical path, a secondary lens in each optical path to focus the FOR image portion onto a secondary mirror for reflection along a preselected direction, and a tertiary lens in each optical path to focus the FOR image portion onto an image detector array.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT  
       [0001] The present invention is assigned to the United States Government and is available for licencing for commercial purposes. Licensing and technical inquiries should be directed to the Office of Patent Counsel, Space and Naval Warfare Systems Center, San Diego, Code D0012,San Diego, Calif., 92152; telephone (619)553-3001, facsimile (619)553-3821. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to optical sensors and more particularly to a staring infrared sensor having a large Field Of Regard (FOR) while also having the small Instantaneous Field Of View (IFOV) needed for improved dynamic range at higher resolutions.  
           [0004]    2. Description of the Prior Art  
           [0005]    Infrared sensors with relatively large Fields Of Regard (FORs) are needed for viewing a large portion of the horizon. Such sensors that also have a relatively small Instantaneous Field Of View (IFOV) are generally classified as Infrared Search and Track (IRST) sensors. With appropriate signal processing, these IRST sensors can detect and track targets at ranges otherwise too large to permit resolution of the target. The target detection range is highly dependent on the signal to noise ratio at the IRST sensor, which depends on the available IFOV and integration time as well as other factors. IRST sensors are known in the art to include the scanning sensor, the step-stare sensor and the staring sensor.  
           [0006]    The scanning sensor has a small IFOV that is mechanically scanned through the desired Field Of Regard (FOR). A well-known disadvantage of the scanning sensor is the requirement for a large, heavy and expensive electromechanical scanning apparatus. Another well-known disadvantage is the necessarily short sensor integration time, which reduces the sensitivity of the sensor.  
           [0007]    The step-stare sensor has a small IFOV that is mechanically scanned through the desired FOR, but uses a mirror that rotates in the opposite direction in steps to momentarily keep the image stationary. This effectively increases the available integration time. A well-known disadvantage of the step-stare sensor is the complexity and cost of the requisite mechanically-scanned mirror.  
           [0008]    The staring infrared sensor has a relatively small FOR because of the narrow FOV lens needed to keep the IFOV to an acceptably small value. A well-known disadvantage of the staring sensor is the number of individual sensors required to cover the desired FOR. The disadvantage of using an array of many staring sensors is the increased weight, volume and cost.  
           [0009]    In view of these deficiencies, there is accordingly a well-known need for an inexpensive infrared sensor that provides a large Field Of Regard (FOR) and a small IFOV for long-range surveillance applications. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention solves the problem of viewing a large Field Of Regard (FOR) while also integrating over a small IFOV by subdividing the FOR into a plurality of internal optical paths without the use of mechanically-moveable parts.  
           [0011]    It is a purpose of this invention to provide a low cost staring infrared imaging sensor capable of viewing a large portion of the horizon, and detecting small dim targets at long standoff ranges. It is a feature of this invention that each of the plurality of internal optical paths may be further subdivided by a plurality of steerable micro-mirrors to further reduce the IFOV of the invention.  
           [0012]    In one aspect, the invention is a staring infrared imaging sensor including a primary lens disposed to accept infrared radiation and to project therefrom a Field Of Regard (FOR) image onto a primary image plane, a plurality of primary mirrors disposed at the primary image plane each for reflecting a corresponding portion of the FOR image along a corresponding one of a plurality of optical paths, a secondary lens disposed within each optical path to focus the corresponding FOR image portion onto a corresponding secondary image plane within the corresponding optical path, a secondary mirror disposed at the corresponding secondary image plane within each optical path to reflect the corresponding FOR image portion along one of a corresponding plurality of preselected directions, an image detector disposed at a tertiary image plane for generating an electronic signal representing an image projected onto the tertiary image plane, and a tertiary lens disposed within each optical path to focus the corresponding FOR image portion onto the tertiary image plane.  
           [0013]    In a preferred embodiment, each secondary mirror includes a plurality of micro-mirrors each moveable from one to another of a plurality of positions, whereby an instantaneous field of view (IFOV) image within the corresponding FOR image portion can be redirected from one to another of the preselected directions.  
           [0014]    In another aspect, the invention is an infrared detection system including a staring infrared imaging sensor having a primary lens disposed to accept infrared radiation and to project therefrom a Field Of Regard (FOR) image onto a primary image plane, a plurality of primary mirrors disposed at the primary image plane each for reflecting a corresponding portion of the FOR image along a corresponding one of a plurality of optical paths, a secondary lens disposed within each optical path to focus the corresponding FOR image portion onto a corresponding secondary image plane within the corresponding optical path, a secondary mirror disposed at the corresponding secondary image plane within each optical path to reflect the corresponding FOR image portion along one of a corresponding plurality of preselected directions, an image detector disposed at a tertiary image plane for generating an electronic signal representing an image projected onto the tertiary image plane, and a tertiary lens disposed within each optical path to focus the corresponding FOR image portion onto the tertiary image plane, and a controller coupled to the secondary mirrors for apportioning the amount of time during which the corresponding FOR image portion is directed along any one of the corresponding plurality of preselected directions. 
       
    
    
       [0015]    The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, in which like reference designations represent like features throughout the several views and wherein:  
         [0017]    [0017]FIG. 1 illustrates a schematic representation of the operation of an exemplary embodiment of the staring infrared imaging sensor of this invention;  
         [0018]    [0018]FIG. 2 shows a facial view of an exemplary section of a micro-mirror array suitable for use with the staring infrared imaging sensor of this invention;  
         [0019]    FIGS.  3 A- 3 B show exemplary dispositions of the micro-mirror array of FIG. 2 from the side;  
         [0020]    [0020]FIG. 4 illustrates a block diagram of an illustrative embodiment of the staring infrared imaging sensor of this invention; and  
         [0021]    FIGS.  5 A- 5 D illustrate selected operational details of the staring infrared imaging sensor of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    [0022]FIG. 1 illustrates a schematic representation of an exemplary embodiment of the staring infrared (IR) imaging sensor  10  of this invention. Sensor  10  provides a larger field or regard (FOR) and smaller instantaneous field of view (IFOV) than is available from sensors known in the art that do not mechanically move a sensor head or reverse step-scan a mirror. In operation, the infrared radiation  12  arrives from the distant FOR (not shown) at the primary lens  14 . Part of the FOR image captured by primary lens  14  is directed along the first optical path  16  to a primary image plane at the first primary mirror  18 . Similarly, an adjacent part of the FOR image is directed along a second optical path  20  to the primary image plane at the second primary mirror  22 . First and second primary mirrors  18 ,  22  are moveably disposed against one another to form a primary mirror assembly  24  of abutted folding mirrors exemplified by primary mirrors  18 ,  22 . The FOV of primary lens  14  has a diameter large enough to project an FOR image on the primary image plane subtending primary mirror assembly  24 . The precise number of primary mirrors in array  24  and the diameter of the image plane depends on the FOR desired for sensor  10 .  
         [0023]    At primary mirror  18 , a first sub-area of the FOR is reflected from the primary image plane along the optical path  26  to a secondary lens  28 , which refocuses the corresponding portion of the FOR image at a secondary image plane occupied by the secondary mirror  30 . Similarly, another sub-area of the FOR is reflected from the primary image plane along the optical path  32  to a corresponding secondary lens  34  that refocuses the corresponding portion of the FOR image at another secondary image plane occupied by the secondary mirror  36 .  
         [0024]    Secondary mirrors  30  and  36  are each preferably embodied as a relatively inexpensive micro-machined mirror array such as the Digital Micro-mirror Device™ (DMD™) made and sold by Texas Instruments Incorporated. The DMD™ is a reflective array of fast, digital light switches that are monolithically integrated onto a silicon address chip for digital image display system applications. For instance, Texas Instruments Incorporated also make and sell the Digital Light Processing™ (DLP™) projection display systems based on their DMD™, which is adapted to provide high-quality, seamless, all-digital images with exceptional stability and minimal image lag. Digital projection display systems based on the DMD™ use its silicon addressing circuitry and monolithic aluminum mirrors to achieve unique addressing modes, bit resolutions, pixel resolutions, aperture ratios, and color spaces in both three-chip and single-chip configurations. Until now, however, there is no teaching nor suggestion in the art for any use of the DMD™ in infrared sensor applications. One version of the DMD™ is known for use in projector systems and large screen displays available from several vendors. In these devices lamps are used to illuminate the micro-mirror arrays, which reflect the light through a projection lens. Other useful micro-mirror devices are described by practitioners in the art. For instance, Gleb Vdovin describes the micro-mirror art in “Micromachined Adaptive Mirrors,” Laboratory of Electronic Instrumentation, Delft University of Technology, P. O. Box 5031, 2600 GA, Delft, The Netherlands, Phone: 31-15-2785756, Fax: 31-15-2785755, email: gleb@ei.et.tudelft.nl.  
         [0025]    [0025]FIG. 2 shows a facial view of an exemplary section of a micro-mirror array suitable for the secondary mirrors  30  and  36 , which each include hundreds of thousands of individual micro-mirrors exemplified by the single micro-mirror  38 . For example, secondary mirror  30  or  36  (FIG.  1 ) may include 1.3 million individual micro-mirrors exemplified by micro-mirror  38  (FIG. 2), each measuring 16 by 16 microns, arranged in a 1280 by 1024 array, suitable for mapping the 1.3 million pixels defining an SVGA digital image. Each micro-mirror in such an array acts as a reflective digital light switch that precisely controls the light intensity for one pixel of a projected image.  
         [0026]    As shown in FIGS.  3 A- 3 B, each individual micro-mirror  38  is attached to the substrate  40  by a hinge  42  that allows each micro-mirror  38  to tilt independently in one of two directions by a predetermined angle; for example, precisely ±10 degrees. The predetermined angles are established by mechanical stops (not shown) disposed between substrate  40  and micro-mirror  38 . In this description, one of the angles (e.g. +10 degrees, shown for micro-mirror  38 A) is arbitrarily designated as “on” and the other (e.g. −10 degrees, shown for micro-mirror  38 C) is arbitrarily designated as “off.” The relaxed angle (e.g., zero degrees, shown for micro-mirrors  38 B and  38 D) is not normally used because of the imprecision in position arising from absence of a predetermined mechanical stop. As used in the digital video projection arts, the intensity of each pixel is adjusted by using a suitable pulse-width modulated signal to flip the corresponding micro-mirror on and off so rapidly that the flicker is invisible to the human eye. The average illumination reflected by the corresponding micro-mirror then determines the visual intensity of the pixel, which can be adjusted by changing the duty-cycle of the pulse-width modulated control signal.  
         [0027]    Returning to FIG. 1, the use in accordance with this invention of a plurality of micro-mirror arrays, disposed as secondary mirrors  30  and  36 , to sequentially sample sub areas of the image plane is now described. In FIG. 1, the two exemplary optical channels (one comprising paths  16  &amp;  26  and the other comprising paths  20  &amp;  32 ) operate to effectively double the FOV of staring IR imaging sensor  10 . Following the teachings set forth herein, more optical channels may be added to further enhance sensor FOV, limited only by the size of the primary image plane and FOV of the input lens. When the micro-mirrors in secondary mirror  30  are tilted “on,” and the micro-mirrors in secondary mirror  36  are tilted “off,” the first optical channel comprising paths  16  &amp;  26  is active and the image in the first sub-area of the FOR is reflected along the optical path  44  and focused by the tertiary lens  46  onto the IR detector array  48  disposed in a tertiary image plane. These positions are maintained for a predetermined integration time to allow the accumulation of each of the electronic pixel signals (not shown) produced by detector array  48  responsive to the pixel intensities of the first sub-area image projected onto the tertiary image plane. During this integration time, the second FOV sub-area is reflected by the micro-mirrors in secondary mirror  36  along the optical path  50  onto the housing wall (not shown). During the next integration time period, the micro-mirrors in secondary mirror  30  are tilted “off” and the micro-mirrors in secondary mirror  36  are tilted “on.” The second optical channel comprising paths  20  &amp;  32  is active and the image in the second sub-area of the FOR is reflected along the optical path  52  and focused by the tertiary lens  54  onto the IR detector array  48  disposed in the tertiary image plane. These positions are maintained for a predetermined integration time to allow the accumulation of each of the electronic pixel signals (not shown) produced by detector array  48  responsive to the pixel intensities of the second sub-area image projected onto the tertiary image plane. During this integration time, the first FOV sub-area is reflected by the micro-mirrors in secondary mirror  30  along the optical path  56  onto the housing wall (not shown).  
         [0028]    Since the several optical paths for each FOV sub-area image are slightly off of the detector axis, the cold shield aperture must be opened to avoid blocking some of the incoming IR radiation. The thermoelectric (TE) cooler surfaces  58  and  60  and the associated optics operate to prevent detector array  48  from measuring heat from the warm housing. TE cooler surface  60  is disposed for imaging onto detector array  48  through the inactive channel along the optical path  62  through the quaternary lens  64  onto the tilted-off micro-mirrors in secondary mirror  36  and along optical path  52  when the first sub-area image is directed onto detector array  48  through the active first channel. Thus, while the first channel is active, the only photons impinging onto detector array  48  are the photons from the first sub-area image of the scene and the photons from TE cooler surface  60 . Similarly, TE cooler surface  58  is disposed for imaging onto detector array  48  through the inactive channel along the optical path  66  through the quaternary lens  68  onto the tilted-off micro-mirrors in secondary mirror  30  and along optical path  44  when the second sub-area image is directed onto detector array  48  through the active second channel. Thus, while the second channel is active, the only photons impinging onto detector array  48  are the photons from the second sub-area image of the scene and the photons from TE cooler  58 .  
         [0029]    TE cooler surfaces  58  and  60  are required to prevent detector array  48  from seeing the warm walls of the housing. If TE cooler surfaces  58  and  60  are cooled much cooler than the FOR scene background, the contribution of TE cooler surface noise photons should be negligible. Other benefits of the TE coolers are discussed herein below. In operation, the positions of all micro-mirrors in the secondary mirror arrays are changed in azimuth by ±10 degrees. Secondary mirrors  30  and  36  are disposed such that when the individual micro-mirrors of secondary mirror array  30  are flipped to one side, to +10 degrees for example, the sub-area image is deflected off of secondary mirror  30  and re-imaged onto detector array  48 . At the same time, the individual micro-mirrors in secondary mirror  36  are flipped to the other side, at −10 degrees, and therefore direct the other sub-area image away from detector array  48  while directing an image of TE cooler surface  60  onto detector array  48 . After holding these positions for the desired integration time, the micro-mirrors in both secondary mirrors  30  and  36  are flipped to opposite sides so the sub-area image from the first optical channel now misses detector array  48  and the sub-area image from secondary mirror  36  is now directed onto detector array  48  along with the image of TE cooler surface  58 . Simple signal processing is required to reassemble the sequential electronic image pixel signals into a single seamless wide-FOV image.  
         [0030]    Except for oscillation of the individual micro-mirrors exemplified by micro-mirror  38  in FIG. 2, there are no moving parts in the staring IR imaging sensor of this invention. The increase in FOR is achieved over the prior art without panning or adding other sensors. In operation, the FOR image area is effectively broken up into a plurality of IFOV image sub-areas that are sequentially transmitted from lens  14  onto a single detector array  48  (FIG. 1). Generally, given an IFOV image sub-area specification, the monitored FOR image area can be expanded by a factor equal to the number of available optical channels exemplified by the two optical channels discussed above with reference to FIG. 1. For the staring IR sensor of the prior art having a 1280 by 1024 pixel detector array and a 200 micro-radian IFOV requirement, the monitored FOR in azimuth is about 15 degrees. Four of these prior art staring sensors are required to monitor a 60-degree FOR. Using the same 1280 by 1024 pixel detector array and 200 micro-radian IFOV requirement, the same 60-degree field of regard can be monitored using the same detector array in a single staring IR sensor of this invention with four optical channels disposed similarly to the two optical channels illustrated in FIG. 1.  
         [0031]    Using an alternative embodiment of this invention, the FOR may be doubled again with no additional optical channels. Although the 1280 by 1024 micro-mirror array is currently preferred, a staring IR sensor of this invention employing a larger array of 1000 by 2000 micro-mirrors for the secondary mirrors  30  and  36  (FIG. 1) permits a doubling of the monitored FOR for the same IFOV image sub-area requirement. The micro-mirror array area is subdivided into two sub-areas within one of which the micro-mirrors tilted to one position, +10 degrees for example, while all the micro-mirrors within the other are tilted to the other position, −10 degrees. Only that portion of the secondary image plane subtended by the +10 degree micro-mirrors is directed through the tertiary lens onto the detector array. After an integration time, the two sub-areas of the secondary mirror are tilted to opposite positions. In this manner, a secondary mirror array with 2000 micro-mirrors in azimuth monitors twice the azimuth of a secondary mirror array with 1000 mirrors in azimuth.  
         [0032]    As used in the image projection arts, the individual pixel micro-mirrors flip back and forth according to a pulse-width modulated control signal of as much as 1000 Hz to vary the average illumination projected for any particular pixel. In accordance with the operation of the staring IR imaging sensor described above, the individual micro-mirrors are held stationary during a detection integration time period. In a useful alternative embodiment of this invention, selected micro-mirrors in the secondary mirror array are caused to oscillate during the integration interval. For expository simplicity, FIG. 4 illustrates this concept for a single pixel transmitted through the active image channel  70  comprising detector array  48  and secondary micro-mirror array  30  (FIG. 1). When a single pixel detector  76  in detector array  48  approaches a saturation condition caused by the IR level arriving from a hot spot  78  in FOR image  12  (FIG. 1), the signal processor  80  sends a signal  82  to the micro-mirror controller  84  responsive to the impending saturation condition of pixel detector  76 . Controller  80  responsively sends a signal  86  to secondary micro-mirror array  30  that causes the corresponding micro-mirror element  88  to oscillate at a predetermined duty cycle selected to reduce the effective integration time for pixel detector  76 , thereby prevent pixel detector  76  from saturating. The actual output of pixel detector  76  is rescaled in signal processor  80  by the predetermined duty cycle to recover the actual IR pixel intensity value for hot spot  78  before sending an output signal  90  to a display device  92 .  
         [0033]    In the above description, only the two extreme micro-mirror tilt angles (e.g., ±10°) are considered because nothing more is needed for single band imaging. When one of the micro-mirror arrays is tilted to +10 degrees to reflect an image sub-area onto detector array  48 , all other micro-mirror arrays are tilted to the −10 degree position. But a third micro-mirror position is useful for dual-band imaging. This third “zero-bias” position is available from the micro-mirror array by removing the tilt bias voltage, which allows the micro-mirror to relax to a default position roughly midway between the positive and negative extremes. The uniformity of this “zero-bias” position over the micro-mirror array is insufficient for imaging but is still useful for dual-band applications because the image sub-area reflected by the “zero-biased” micro-mirrors need not be imaged. Dual-band imaging can be achieved by adding a second detector array  94  (for detecting an image in a second optical band) to staring infrared (IR) imaging sensor  10  from FIG. 1, as shown in FIGS.  5 A- 5 D. The photons that are otherwise deflected by “off-tilted” micro-mirrors in secondary mirror arrays  36  and  30  (FIG. 1) into the surrounding housing along optical paths  50  and  56  are now deflected and imaged onto second detector array  94 . As discussed above with reference to FIG. 1, photons are deflected by “on-tilted” micro-mirrors in secondary mirrors  36  and  30  are deflected along optical paths  52  and  46  and imaged onto detector array  48 , which is used for detecting an image in a first IR band. The spectral band of the second detector array can be either visible or IR.  
         [0034]    FIGS.  5 A- 5 D illustrate the four different dispositions of these several elements over one complete integration cycle during operation of a dual-band detector of this invention. The illustrations shown in FIGS.  5 A- 5 D are limited for simplicity of exposition to the principle rays in each optical path, with the various secondary and tertiary lenses omitted. During the first of four dispositions, as shown in FIG. 5A, the image sub-area  96  is reflected from the primary image plane  98  along the ray  99  onto first-band detector array  48  by the micro-mirror  100  when tilted “on” (+10 degrees). Simultaneously, the image sub-area  102  is reflected from primary image plane  98  along the ray  103  onto second-band detector array  94  by the micro-mirror  104  when tilted “off” (−10 degrees). During the first prescribed integration interval, all tilt bias is removed from the micro-mirrors  106  and  108  so they assume the “zero-bias” default position somewhere midway between the “on” and “off” tilt positions. When in the “zero-tilt” position, micro-mirrors  106  and  108  reflect the image sub-areas  110  and  112  along the rays  114  and  116  into the sensor housing region (not shown) somewhere between detector arrays  48  and  94  substantially as shown.  
         [0035]    During the second of four dispositions, as shown in FIG. 5B, image sub-area  96  is reflected from primary image plane  98  along ray  99  onto second-band detector array  94  by micro-mirror  100  when tilted “off” (−10 degrees). Simultaneously, image sub-area  102  is reflected from primary image plane  98  along ray  103  onto first-band detector array  48  by micro-mirror  104  when tilted “on” (+10 degrees). During the second prescribed integration interval, all tilt bias is removed from micro-mirrors  106  and  108  as discussed above with reference to FIG. 5A.  
         [0036]    During the third of four dispositions, as shown in FIG. 5C, image sub-area  110  is reflected from primary image plane  98  along ray  114  onto first-band detector array  48  by micro-mirror  108  when tilted “on” (+10 degrees). Simultaneously, image sub-area  112  is reflected from primary image plane  98  along the ray  116  onto second-band detector array  94  by micro-mirror  106  when tilted “off” (−10 degrees). During the third prescribed integration interval, all tilt bias is removed from micro-mirrors  100  and  104  so they assume the “zero-bias” default position somewhere midway between the “on” and “off” tilt positions. When in the “zero-tilt” position, micro-mirrors  100  and  104  reflect the image sub-areas  96  and  102  along rays  99  and  103  into the sensor housing region (not shown) somewhere between detector arrays  48  and  94  substantially as shown.  
         [0037]    During the last of four dispositions, as shown in FIG. 5D, image sub-area  110  is reflected from primary image plane  98  along ray  114  onto second-band detector array  94  by micro-mirror  108  when tilted “off” (−10 degrees). Simultaneously, image sub-area  112  is reflected from primary image plane  98  along the ray  116  onto first-band detector array  48  by micro-mirror  106  when tilted “on” (+10 degrees). During the fourth prescribed integration interval, all tilt bias is removed from micro-mirrors  100  and  104  as discussed above with reference to FIG. 5C.  
         [0038]    It may be readily appreciated by those familiar with the art that multi-spectral band imaging for more than two optical bands can be achieved by inserting additional micro-mirror arrays and re-imaging lens to form additional optical channels, based on these teachings. In such a configuration, each micro-mirror array is disposed to reflect its sub-area of the secondary image plane either onto a detector array or onto a successive micro-mirror array.  
         [0039]    As discussed above with reference to FIG. 1, for normal IR imaging, only one image sub-area at a time need be reflected onto detector array  48  by a corresponding (active) secondary micro-mirror array. During the integration time, radiation from the respective TE cooler is imaged onto detector  48  by the other secondary micro-mirror array through the inactive optical channel. In FIG. 1, for example, upon arrival at sensor  48 , the optical signal from active secondary mirror array  30  is added to the optical signal reflected by inactive secondary mirror array  36  from TE cooler  60 . In normal operation, the temperature of TE cooler  60  is low enough so the inactive channel signal has negligible effect on the electrical output from sensor  48 . However, the temperature of TE coolers  60  and  58  may be set as desired and individual micro-mirrors (pixels) of secondary mirror arrays  30  and  36  may be selected and controlled to reflect onto detector array  48 , while adjacent micro-mirrors (pixels) in the same secondary mirror array are disposed to reflect radiation from TE cooler  60  or  58  onto detector array  48 . In this manner, the temperature of the image sub-area reflected from the FOR can be estimated by comparing the signal level of the pixels representing the image sub-area with the pixels representing the TE cooler.  
         [0040]    Although unsuitable for the specific type of micro-mirror array discussed above, a suitable micro-mirror array may be used with the apparatus of this invention to stabilize the image reflected onto the detector array. Such a suitable micro-mirror array must include micron-sized mirrors capable of continuous controlled angular disposition rather than the simple binary disposition (on and off) discussed above. In such an embodiment, the bias voltages that drive the micro-mirrors from one extreme position to the other must include a second analog signal voltage component responsive to an inertial navigation system, or similar attitude-sensing device, for incrementing the micro-mirror positions as necessary to offset the effects of platform motion. To correct for motion in two dimensions (e.g., for roll and pitch), two orthogonally-disposed micro-mirror arrays are required in each optical channel. In operation, the individual micro-mirrors (pixels) are disposed at a fixed angle modified by a position-sensor signal to offset the effects of platform motion.  
         [0041]    Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing.