Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of pending U.S. patent application Ser. No. 11/782,547 filed on Jul. 24, 2007 now U.S. Pat. No. 7,576,308. The aforementioned patent application claims the benefit of the filing date of Jul. 28, 2006 for U.S. Provisional Application No. 60/833,970. Thus, this application also claims the benefit of the filing date of Jul. 28, 2006 for the aforementioned provisional application. 
    
    
     RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to adaptive optics and, more particularly, to the application of wave front control technology to image sensing and recording. 
     To optically sense or record an image, an object scene is imaged onto a single sensor. This is typically accomplished by collecting light reflected from the object scene and focusing that light onto the sensor, such as a charged coupled device or complementary metal oxide semiconductor imaging array. The image sharpness, technically known as image resolution, and the field of view are limited by the number of pixels in the sensor. The common method for overcoming this drawback is to piece together a composite image from separate images of parts of the whole image taken by changing the position of the imaging system relative to the object scene, and recording the image at each position. Although this method will increase the total number of pixels used to image the object scene, it can be time consuming, prone to operator error, and can also lead to gaps and seams appearing in between the separate images. 
     A common approach to obtaining a wider field of view is to use a wide angle lens to reduce the dimensions of an image to those compatible with the optical sensor. However, this degrades the resolution of objects within the field of view because the light reflected from the wider field of view is imaged onto the same number of pixels contained in the single imaging sensor. 
     In addition to the limit in resolution due to the limited number of pixels, a wide angle lens can introduce angle-dependent wave front errors, such as distortion, which is a variation in image magnification. The image may be in focus, but the scale is distorted at the extremities of the image. Other wave front errors such as field curvature, astigmatism, and coma can also increase with angulation and cause blurring of the image. Although, the optical aberrations caused by wave front errors can be minimized by designs well known to those skilled in the optical art, such modifications are complicated and expensive. 
     There is a need in the optical art for an image sensing and recording apparatus that provides high image resolution with a wide field of view, without seams and gaps, while also resolving the problems of wave front aberrations. The present invention addresses this need in the art. 
     SUMMARY OF THE INVENTION 
     A wave front control system (“WFCS”) organizes an object scene into a mosaic comprised of a grid of segments and transmits each segment in a temporal sequence. The WFCS steers the light emanating from each segment, one segment at a time, through a series of optical components, and ultimately onto a digital imaging sensor. An optical recording device records each sensed segment, and the object scene is then composed by assembling the recorded segments. 
     The steering is accomplished without moving the imaging sensor. The WFCS also corrects wave front aberrations such as tilt, focus, coma and astigmatism that are unique to the angle between each segment and the optical axis of the imaging sensor. 
     In addition to utilizing a WFCS to correct aberrations caused by wave front errors, digital image processing techniques may be employed. In using the WFCS to remove wavefront tilt errors associated with an image segment, the aberration known in the art as distortion will be eliminated for one point—nominally the center point—within the image segment. Since the distorted image within the segment is in focus but scaled incorrectly, the digital image can be scaled, or morphed, to correct the distortion using digital processing techniques. For image recording, this can be accomplished by digitally morphing the recorded image to remove the distortion. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an electromagnetic wave. 
         FIG. 2  is a schematic illustration of electromagnetic waves in phase with one another and shows a planar wave front. 
         FIG. 3  is an abbreviated schematic drawing of a propagating planar wave front. 
         FIG. 4  is a schematic illustration of a planar wave front having its direction changed by propagating through a prism. 
         FIG. 5  is a schematic drawing showing an image distorted by errors introduced by a change in the wave front direction. 
         FIG. 6  is a schematic drawing of a planar wave front propagating through a convex lens and emerging as spherical wave front converging to a focus. 
         FIG. 7  is a schematic diagram illustrating the blurring effect of image field curvature occurring when an object lying in the object plane is imaged by a convex lens and projected onto a flat viewing screen. 
         FIG. 8  is a schematic drawing of a planar wave front incident to a convex lens at an angle with respect to the optical axis, and emerging as an aberrant wave front. 
         FIG. 9  is a schematic drawing of an aberrant wave front propagating through a wave front control device and emerging restored as planar wave front. 
         FIG. 10  is a schematic illustration of periodic electromagnetic waves in phase with one another and showing planar wave fronts. 
         FIG. 11  is an equivalent abbreviated illustration of the propagating planar wave fronts shown in  FIG. 10 . 
         FIG. 12  is a schematic illustration of a planar wave front being steered by a diffractive optic comprised of an array of micro prisms. 
         FIG. 13  is a schematic drawing of a rotatable steering mirror steering a planar wave front. 
         FIG. 14  is a schematic drawing of a Risley prism steering a planar wave front. 
         FIG. 15  is a schematic drawing of an aberrant wave front being steered and compensated by a deformable mirror comprised of a flexible reflective surface adjustably shaped by electro-mechanical means. 
         FIG. 16  is a schematic drawing of an aberrant wave front being steered and compensated by a segmented deformable mirror array comprised of a plurality of flat reflective segments moveable in one dimension by pistons. 
         FIG. 17  is a schematic drawing of an aberrant wave front being steered and compensated by a segmented deformable mirror array comprised of flat reflective elements that are rotatable and also vertically moveable. 
         FIG. 18  is a schematic drawing of an aberrant wave front being compensated by a liquid crystal spatial light modulator comprised of regions having individually varied refractive indexes. 
         FIG. 19  is a schematic diagram of a mosaic imager of the present invention, which places a wave front control system at the entrance aperture to transmit a mosaic image from an object scene to a digital imaging sensor. 
         FIG. 20  is a schematic diagram of another embodiment of the present invention, which places an imaging objective at the entrance aperture and before a wave front control system, to transmit a mosaic image from an object scene to a digital imaging sensor. 
         FIG. 21  is a schematic diagram of a third embodiment of the present invention, which uses two wave front control systems along the optical axis to transmit a mosaic image from an object scene to a digital imaging sensor. 
     
    
    
     DETAILED DESCRIPTION 
     The invention utilizes optical wave front steering and wave front compensation to transmit an image from an object plane to a sensor with minimum distortion. The physical principles and terminology are explained in conjunction with the drawings, as follows. Light is comprised of electromagnetic waves.  FIG. 1  is a graphical representation of an electromagnetic wave  22 . Arrow  23  indicates the direction of wave propagation. A “wave front” is the surface defined by points of identical phase across the cross section of the light. 
       FIG. 2  shows electromagnetic waves  24  that are in phase with one another and propagating in the direction of arrows  26 . When waves  24  are in phase as shown, the wave front is planar as represented in two dimensions by line  28 .  FIG. 3  is an abbreviated drawing of propagating wave front  28  that omits the graphical representation of the electromagnetic waves and shows only planar wave front  28  and arrow  26  representing the propagation direction. Generally speaking, a planar wave front is associated with a non-aberrant image. 
     A variation in optical path, introduced by a tilted or deformed mirror or a refractive medium such as glass, can delay parts of a propagating wave front. Wave front tilt, corresponding to a change in propagation direction, occurs when a planar wave front is linearly retarded across its face.  FIG. 4  shows a planar wavefront  30  propagating through prism  32  and emerging as tilted planar wave front  34 . When intentionally introduced to steer an image, tilt is a desired alteration of the wave front. The aberration known as distortion results when errors in wave front tilt occur across an image. For example,  FIG. 5  shows distorted image  36  of square  38 . The distortion occurring at the extremities of the image distorts the linear sides of square  38 . 
     A curved mirror or lens that becomes thinner towards its edges, i.e., a convex lens, focuses a planar wave front by delaying the central part of the wave front with respect to the edges.  FIG. 6  shows a planar wave front  40  propagating through convex lens  42  and emerging as spherical wave front  44  converging to a focus. Focusing an image is thus a desired alteration of a wave front. 
       FIG. 7  illustrates the effect of image field curvature. An object lying in object plane  46  is imaged by lens  48  and projected onto flat optical sensor  50 . Lens  48  causes the light emanating from object plane  46  to form an in-focus image along the focal points forming locus  52 . However, at flat optical sensor  50 , the image of the object is only in focus near center  54 , and is otherwise out of focus and thus blurred at viewing screen  50 . The blurring increases with the radial distance from center  54 , i.e., as the angle θ increases. 
     In addition to the angle-dependent wave front errors of distortion and field curvature, other angle-dependent wave front errors, such as coma and astigmatism, are associated with more complex irregularities.  FIG. 8  shows planar wave front  56  incident to convex lens  58  at angle α with respect to optical axis  59 . Wave front  56  emerges from lens  58  as aberrant wave front  60 , and results in a blurred image. By introducing appropriate optical path delays across the wave front, a wave front control device (“WFCD”) can compensate for such complex wave front irregularities.  FIG. 9  shows aberrant wave front  61  propagating through WFCD  62  and emerging restored as planar wave front  63  propagating in direction  64 . 
     Diffractive principles, associated with the periodic nature of optical waves, may also be employed to steer optical wave fronts and compensate for wave front errors.  FIG. 10  graphically depicts periodic electromagnetic waves  65 , and figuratively demonstrates that the “wave front” is actually a series of wave fronts  66  propagating in direction  67  and spaced apart by the wavelength of the light.  FIG. 11  is an equivalent illustration of wave fronts  66  propagating in direction  67 , wherein the graphical representation of the electromagnetic waves is omitted. 
       FIG. 12  illustrates steering or tilting a planar wavefront utilizing diffractive principles. A succession of planar wavefronts  68 ,  70 , and  72 , respectively shown by the dotted, dashed, and solid lines, is incident upon diffractive optic  74 , schematically shown as array of micro prisms  76 . An individual micro prism refracts the wave fronts causing them to tilt, i.e., change their direction of propagation. The discontinuities at the prism boundaries cause discontinuities in each successive transmitted wave front. Diffracted wave fronts  78  propagating in new direction  79  are formed by locally refracted wave fronts being aligned with adjacent succeeding wave fronts. This technique is referred to as “modulo-lambda” (where lambda denotes both the width of the micro prisms and the optical wavelength) wave front control or “diffractive” wave front control. 
     In addition to tilting a wavefront as illustrated in  FIG. 12 , the same modulo-lambda principle may be applied to focus a wavefront and to compensate an aberration. This diffractive approach to wavefront control offers several advantages. Large range wavefront control may be achieved with low-range optical path modulation. This is illustrated in  FIG. 12  where the widths of the micro prisms are approximately equal to an optical wavelength (or approximately 500 nanometers at visible wavelengths). Furthermore, a single high-resolution diffractive optical element can introduce simultaneously wavefront steering, focus, and aberration compensation. The foregoing is discussed in M. T. Gruneisen, R. C. Dymale, J. R. Rotge, and D. L. Lubin, “Near-diffraction-limited compensated imaging and laser wavefront control with programmable diffractive optics,” in  High - Resolution Wavefront Control: Methods, Devices, and Applications IV , John. D. Gonglewski, Mikhail A. Vorontsov, Mark T. Gruneisen, Sergio R. Restaino, Robert K. Tyson, Editors, Proceedings of SHE 4825, 147-157 (2002). 
     Modulo-lambda wave front control can be implemented with programmable diffractive optics technologies, which allow reconfiguration of the diffractive optic in real time. Reconfigurable liquid-crystal diffractive optical devices with over one-million resolution elements are now commercially available at relatively low cost. Implementing diffractive wavefront control with these elements offers several noteworthy advantages, including no mechanical motion and hysteresis-free, wide dynamic range aberration correction, wavefront focusing, and wavefront steering. The foregoing is discussed in M. T. Gruneisen, J. R. Rotge, R. C. Dymale, and D. L. Lubin, “Programmable diffractive optics for wide-dynamic range wavefront control using liquid-crystal spatial light modulators,”  Opt. Eng ., Vol. 43, No. 6, 1387-1393 (2004). 
     The present invention affects optical wave fronts by using time-dependent optical wave front steering, time-dependent optical wavefront focusing, and time-dependent optical wave front aberration compensation devices. The invention may include any combination of the foregoing devices and, in addition to the diffractive wave front control shown in  FIG. 12 , a number of them will be discussed below in conjunction with  FIGS. 13-17 . It is to be understood, however, that devices noted herein are not intended to comprise an exhaustive listing. 
       FIG. 13  illustrates planar wave front  80  incident upon a steering mirror  82 , which is rotated about axis  84  in order to steer reflected wave front  86 .  FIG. 14  illustrates planar wave front  88  incident upon Risley prism  90  comprised of prisms  92  and  94  rotating about axis  96 . Rotating prisms  92  and  94  steer transmitted wave front  98  in two dimensions. A detailed discussion of Risley prisms is provided by G. Garcia-Torales, M. Stronjnik and G. Paez, “Risley prisms to control wave-front tilt and displacement in a vectorial shearing interferometer,”  Applied Optics , Vol. 41, No. 7, 1380-1384 (1 Mar. 2002). 
     Presently, there are several technologies available for compensating time-dependent aberrations. The technologies described herein operate in real-time, allowing the wave front compensation to be updated at video frame rates or faster, i.e., at least sixty frames per second. Generally speaking, a wave front compensator can also steer a wave front over a limited range of angles.  FIG. 15  illustrates aberrant wave front  100  incident upon deformable mirror  102  comprised of a flexible reflective surface adjustably shaped by electro-mechanical means. Mirror  102  is shaped to cause appropriate path delays to compensate for the aberration in wave front  100  and produce planar reflected wave front  104 . The industry standard for this technology is based on piezo-ceramic actuated glass mirrors, and is prohibitively expensive for consumer products. Emerging technologies that offer the potential for lower cost mass produced deformable mirrors include electro-static deformable mirrors manufactured from silicon wafers utilizing micro-machined electro-mechanical system (MEMS) technologies and bimorph mirrors. 
     Currently available low-cost wave front control technology includes the segmented mirror arrays illustrated in  FIGS. 16 and 17 . In  FIG. 16 , aberrant wave front  106  is incident upon segmented deformable mirror  108  comprised of a plurality of flat reflective segments  110  moveable in one dimension by means of pistons to approximate the continuous surface of a conventional deformable mirror. Segments  110  are adjusted to produce a compensated nearly planar wave front  112 .  FIG. 17  illustrates aberrant wave front  114  incident upon segmented deformable mirror array  116  comprised of reflective elements  118  that can translate vertically by means of pistons, but are also rotatable, to better approximate the continuous surface of a conventional deformable mirror and thereby more accurately produce compensated planar wave front  120 . 
     While the previous approaches achieve wave front control by changing the optical path length with prisms and reflective mirrors, the same effect can be achieved by modulating the refractive index in electro-optical media, for example, a reconfigurable liquid-crystal spatial light diffractive optical device.  FIG. 18  illustrates aberrant wave front  122  incident upon liquid crystal spatial light modulator  124  comprised of regions  126 . The refractive index is varied for each of regions  126  to introduce the appropriate optical path delays to create transmitted planar wave front  128 . The aforementioned electro-optical apparatus controls optical wavefronts without any moving parts, offering distinct advantages where mechanical motion is problematic. The foregoing is discussed in M. T. Gruneisen, J. R. Rotge, R. C. Dymale, and D. L. Lubin, “Programmable diffractive optics for wide-dynamic range wavefront control using liquid-crystal spatial light modulators,”  Opt. Eng ., Vol. 43, No. 6, 1387-1393 (2004); and G. Love, “Wave-front correction and production of Zernike modes with a liquid-crystal spatial light modulator,”  Applied Optics , Vol. 36, No. 7, 1517-1524 (1 Mar. 1997). 
       FIG. 19  illustrates imager  130  comprised of WFCS  132 , imaging objective  134 , and digital optical sensor  136 . All of the aforementioned components transmit light reflected from object scene  138  along optical axis  139 . Object scene  138  is comprised of twenty-five mosaic segments designated as A through Y. For each segment, for example, segment P, the light reflected therefrom is first transmitted by WFCS  132 . Although WFCS  132  is shown as a light transmissive element, it could also be a reflective element. 
     WFCS  132  performs three distinct functions: wave front steering, focusing, and aberration correction. Wave front steering changes the direction of the impinging light so that light from different segments arriving from angles defined by the segment locations is directed along the optical axis  139  and through imaging objective  134 . The steering may be accomplished by using any one of a number of devices well known to those skilled in the optical art, e.g., a steering mirror, Risley prism, micromirror array, deformable mirror or reconfigurable liquid crystal diffractive optical device. The wave front steering device is incorporated as a part of WFCS  132 . 
     WFCS  132  also compensates for wave front aberrations specific to each of the object scene segments that would otherwise deleteriously affect the image of segment P received at digital imaging sensor  136 . However, since the components of imager  130  lie along optical axis  139 , there are no aberrations due to angles between the path the light follows in traveling between WFCS  132  and the other components of imager  130 . 
     As the distance from each segment in object scene  138  to WFCS  132  differs, WFCS  132  introduces a sufficient amount of focus to allow imaging objective  134  to focus the light reflected from each of segments A through Y onto the digital imaging sensor  136 , regardless of the magnitude of the respective differences in the distances. This focus is obtained by any of several optical apparatus well known to those skilled in the optical art, for example, a deformable mirror, a liquid crystal diffractive optical element, or a MEMS micromirror array. Such a focusing apparatus is incorporated into WFCS  132 . In the case of the liquid crystal diffractive optical device, wavefront steering, focus, and aberration correction may be obtained with a single device. 
     In the aforementioned manner, each of segments A through Y of object scene  138  is sequentially transmitted onto digital image sensor  136 . Digital image sensor  136  is a sensor including an array of pixels, with each pixel sensing the total amount of light incident upon that pixel, i.e., image detail is not resolved within a pixel. Each of the segments is imaged using all of the pixels of digital image sensor  136 . 
     Each sensed segment is recorded using any one of a number of digital electronic recording devices well known to those skilled in the relevant art, e.g., a hard drive, random access memory or CD ROM. The object scene  138  may then be displayed or reproduced partially or in its entirety by displaying or reproducing some or all of the segments as a mosaic by using any one of a variety of digital display devices or printers known in the art. 
     In imager  130 , the amount of light transmitted through imaging objective  134  is limited by the dimensions of WFCS  132 . This makes WFCS  132  the “aperture stop” for the optical system. Since wavefront control technologies vary considerably in size and can be as small as a few millimeters in some cases, this can pose a limitation on the light gathering ability of imager  130 . 
       FIG. 20  is a schematic drawing of imager  140 , an embodiment of the present invention that avoids the aforementioned limitation. Imager  140  includes all of the components previously discussed with respect to imager  130 , with the addition of relay optics  142  and  144 . However, the components are configured to affect and process the light emitted from the object scene in a different sequence. 
     Light reflected from object scene  148  is first transmitted through imaging objective  146 , which focuses the light onto image arc  150 . The light from the segments is focused on arc  150  rather than a plane, due to the differing distances between imaging objective  146  and the respective mosaic segments comprising object scene  148 . This is the field curvature aberration described previously in conjunction with  FIG. 7 . The light is then transmitted through relay optic  142 , which collimates the light onto WFCS  152 . 
     Relay optic  142  also reduces the size of the image transmitted by imaging objective  146 , to WFCS  152 . This transmits the wave front characteristics incident upon imaging objective  146  while reducing the height and width of the wave front at imaging objective  146  to match the height and width of the WFCS  152 . The reversed positions of the imaging objective and the WFCS in imager  140  relative to imager  130  thus make imaging objective  146  the aperture stop for this embodiment. Since imaging objective  146  can more easily be scaled to larger sizes than is possible for WFCS  152 , more light can be made available to image sensor  156 , relative to image sensor  136  in imager  130 . 
     As previously discussed with respect to WFCS  132 , WFCS  152  performs three distinct functions: wave front steering, focusing, and aberration correction. Wave front steering changes the direction of incident wave fronts so that light from different segments, e.g., segment P, can be directed and focused onto relay optic  144 . This design differs from that of imager  130 , where the light transmitted by imaging objective  138  travels along optical axis  139  after being steered by WFCS  132 . 
     In imager  140 , the wave fronts associated with off-axis segments are incident upon imaging objective  146  and relay optic  142  at various angles that result in aberrations specific to each segment. Such segment-specific aberrations are in addition to aberrations inherent to the optical system. WFCS  152  incorporates apparatus to compensate for aberrations from both of the aforementioned sources. This is in contrast to imager  130 , where the wave front angle at the location of optic  134  relative to optical axis  139  is minimal. In the case of the liquid crystal diffractive optical device, wavefront steering, focus, and aberration correction may be obtained with a single apparatus. 
     The light is then transmitted through relay optic  144 , which creates and focuses an image of each image segment, in sequence, on digital optical sensor  156 . Each sensed segment may then be recorded using any one of a number of digital electronic recording devices well known to those skilled in the relevant art. Object scene  148  may be subsequently displayed or reproduced partially or in its entirety by displaying or reproducing some or all of the segments as a mosaic by using any one of a variety of digital display devices or printers known in the art. 
     In imager  140 , the maximum path angle relative to optical axis  154  from which object scene light may be imaged with high clarity will be limited by several factors, including angle-dependent aberrations that exceed the corrective capability of WFCS  152 , and vignetting, where light propagating at large angles through the system physically miss impinging the optical components and are ejected from the optical system. Imager  160 , a third embodiment of the present invention, utilizes two wavefront control systems to ameliorate the foregoing limitations. 
     Referring to  FIG. 21 , light emanating from the center of object scene  162  (segment M) and entering WFCS  166  where optical axis  164  intersects WFCS  166 , subtends the angle θ relative to optic axis  164 . WFCS  166  is a large aperture steering mirror that steers the wave front from the large angle θ, to a new direction nominally along optical axis  164 , thereby minimizing aberrations associated with the center of object scene  162 . WFCS  166  is the aperture stop in imager  160 . 
     Imaging objective  168  forms an intermediate image of object scene  162  centered at on-axis focus  170  and transmits the light to relay optic  172 . Relay optic  172  collimates the light and reduces the size of the image transmitted by imaging objective  168  to WFCS  174 . This transmits the wave front characteristics incident upon imaging objective  168  while reducing the height and width of the wave front at imaging objective  168  to match the height and width of the WFCS  174 . This allows the aberration compensation function to be performed with a smaller component, i.e., WFCS  174 , than imaging objective  168 . WFCS  174  can also correct aberrations that might be introduced by imaging objective  168 . 
     WFCS  174  may also introduce wavefront tilt about an axis normal to optical axis  164 , to receive image segments in the vicinity of nominal angle θ and compensate for angle-dependent aberrations such as focus, astigmatism, and coma. In the case where WFCS  174  is a liquid crystal diffractive optical device, wavefront steering, focus, and aberration correction may be obtained with a single apparatus. WFCS  174  then individually transmits each of the image segments to relay optic  176 , which in turn focuses and transmits the image segments, in sequence, onto imaging sensor  178 . If the angular range of WFCS  174  reaches its limit, WFCS  166  can be tilted to a new nominal angle in order to continue the mosaic imaging process. 
     Each sensed segment may then be recorded using any one of a number digital electronic recording devices well known to those skilled in the relevant art. Object scene  162  may be subsequently displayed or reproduced partially or in its entirety by displaying or reproducing any or all of the segments as a mosaic using any one of a variety of digital display devices or printers known in the art. 
     It is to be understood that the preceding is merely a detailed description of several embodiments of this invention and that numerous changes to the disclosed embodiments may be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.

Technology Category: 3