Abstract:
A system for projecting digital images of ultraviolet, visible, and infrared scenes in which two or more digital micromirror devices (DMDs) are employed to create projected images which have greater contrast and bit depth than can be achieved with comparable prior art devices. A source DMD creates an image of the digital scene which is used to illuminate a projector DMD, so that the projector DMD is illuminated to an extent proportionate to the corresponding scene brightness, thereby reducing the amount of undesired radiation which would otherwise illuminate the projected scene via scattering, diffraction, and/or spurious reflections from within the projection system. By including dichroic beam-splitters and additional projector DMDs it is possible to project multi-spectral-band images having enhanced contrast and bit depth relative to prior art digital projection systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the following pending provisional patent application: 
         [0002]    Title: Structured Illumination of a Digital Micromirror Device 
         [0003]    Application No.: 61/688,259 
         [0004]    Filing Date: May 10, 2012 
         [0005]    This provisional patent application described the basic concepts described in the present patent application. During the past year, these concepts have been implemented and demonstrated to be successful in obtaining significantly improved contrast in projected images of visible and infra-red scenes. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0006]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract N68335-11-C-0182 awarded by the U.S. Naval Air Warfare Center. 
     
    
     BACKGROUND OF THE INVENTION 
       [0007]    This invention relates generally to the field of apparatus for the projection of images in the ultraviolet, visible, and infrared portions of the spectrum, particularly such apparatus utilizing digital micro-mirror devices (DMDs). Prior art DMD projectors are limited in the scene contrast and dynamic range which they can achieve. This invention provides a means for greatly increasing the achievable contrast and dynamic range of images projected using digital micromirror devices. 
         [0008]    Prior art in the area of increasing contrast in images created by DMD projectors includes:
   1. METHOD AND APPARATUS FOR INCREASING THE DYNAMIC RANGE OF A PROJECTION SYSTEM; EUROPEAN PATENT 1300008 (INVENTOR: SEAN ADKINS)   2. PROJECTION SYSTEM AND MIRROR ELEMENTS FOR IMPROVED CONTRAST RATIO IN SPATIAL LIGHT MODULATORS U.S. Pat. No. 6,523,961 (INVENTOR: FEDOR A. ILKOV)   3. REDUCED MICROMIRROR MIRROR GAPS FOR IMPROVED CONTRAST RATIO U.S. Pat. No. 6,028,690 A (INVENTOR: ANTHONY DICARLO)   4. DMD EQUIPPED PROJECTOR U.S. Pat. No. 6,874,894 (INVENTOR: TAKATOYO KITAMURA)   5. METHOD AND APPARATUS FOR INCREASING THE DYNAMIC RANGE OF A PROJECTION SYSTEM EUROPEAN PATENT 1300008 (INVENTOR: SEAN ADKINS)   6. PRISM FOR HIGH CONTRAST PROJECTION U.S. Patent 20070252957 (INVENTOR: PENN, STEVEN M.)   7. DIGITAL MICROMIRROR BASED IMAGE SIMULATION SYSTEM U.S. Pat. No. 5,457,493 (INVENTOR MICHAEL LEDDY)   8. PROJECTION SYSTEM AND MIRROR ELEMENTS FOR IMPROVED CONTRAST RATIO IN SPATIAL LIGHT MODULATORS U.S. Pat. No. 6,523,961 (INVENTOR: ILKOV, FEDORA.)   9. Contrast-Improving Methods for Digital Micromirror Device Projectors, Youri Meuret; Patrick De Visschere, Opt. Eng. 42(3), 840-845 (Mar. 01, 2003).   10. Contrast Analysis for DMD-Based IR Scene Projector, Julia Rentz Dupuis, David J. Mansur, Samuel Grant, and Scott P. Newbry. Copyright 2012 Society of Photo-Optical Instrumentation Engineers. This paper was published in The Proceedings of Technologies for Synthetic Environments: Hardware-in-the-Loop Testing XVII.   11. High Dynamic Range DMD-Based IR Scene Projector, Julia Rentz Dupuis, David J. Mansur, Robert Vaillancourt, Ryan Benedict-Gill and Scott P. Newbry; paper presented at Photonics West, San Francisco, Feb. 4- Feb. 7, 2013.   
 
       BRIEF SUMMARY OF THE INVENTION 
       [0020]    In view of the limitations of the prior art in providing DMD projectors capable of projecting high contrast images, particularly in the infrared portion of the spectrum, the present invention provides a new and useful means for providing projected images from a DMD-based projector that have higher-contrast than that which is available from prior art devices. 
         [0021]    The purpose of the present invention is to provide a new structured illumination of a DMD that serves to enhance the contrast and dynamic range of the final projected images. 
         [0022]    In prior art DMD projectors, the DMD is uniformly illuminated by a source of radiation and associated optical elements. The individual mirrors of the DMD are tilted by ±12 degrees in order to steer reflected radiation either towards a projection lens (the ON direction) or towards a radiation absorber (the OFF direction). A multitude of gray levels is achieved by varying the duty cycle with which individual mirrors are tilted between the ON and OFF directions. Individual micro-mirrors in commercially available DMDs are generally square with a linear dimension of 13.7 microns. Individual micro-mirrors diffract light into a cone with a cone-angle of approximately λ/d where λ is the wavelength of the light, d is the linear dimension of the micro-mirror, and where the angle is measured in radians. For visible light with a mean wavelength of 0.55 microns, this cone angle is approximately 0.04 radians or 2.3 degrees. However, for infrared radiation having a wavelength on the order of 10 microns, the diffraction cone angle becomes 10/13.7 radians or 41.8 degrees. As a consequence of this large diffraction angle, a significant portion of the radiation incident on a micro-mirror that is tilted in the OFF direction, is diffracted into the ON direction, thereby increasing the background level of illumination in the projected image. Other factors which reduce the contrast in DMD projected images include reflection of radiation by the DMD windows and other optical elements, scattered energy from DMD components other than the micromirrors, reflection and/or emission of radiation by the surface(s) irradiated when the micro-mirrors are in the OFF state, and emission of radiation by the DMD components. 
         [0023]    The present invention serves to greatly reduce this background level of illumination by illuminating the projection DMD only in those areas where micro-mirrors are tilted in the ON direction. This selective or structured illumination of the projection DMD is achieved by using a second DMD, preceding the projection DMD, that is controlled to create the same images and which is imaged onto the projection DMD. In this manner, the projection DMD is illuminated only in those areas where radiation is desired in the final image, thus eliminating OFF pixels from being illuminated and subsequently contributing, by means of diffraction or scattering, to an undesired background illumination of the image plane. 
         [0024]    Even in the absence of undesired radiation, this use of dual DMDs serves to increase the contrast in the final image plane. If the maximum and minimum values of the transmission T are T max  and T min  then the contrast C in the single DMD system is given by C 1 =T max /T min , while the contrast for the dual DMD system is C 2 =C 1   2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Drawing  1  illustrates the manner in which an individual micromirror of a DMD can be tilted to either their nominal OFF state or their ON state. Incoming radiation,  1 , is reflected by micromirror  8 , either in direction  2  to the projection lens, or in direction  4  to an absorbing surface. 
           [0026]    Drawing  2  illustrates the basic layout of the invention. The radiation source  8  is imaged by the first source lens  9  onto the second source lens  10  which forms an image of uniformly illuminated source lens  9  onto the surface of the source DMD  11 . Just ahead of DMD  11  is relay lens  12  which forms a 1:1 image of the source DMD  11  onto the projection DMD  13 . Projection lens  14  either forms a real image of DMD  13  on a screen, or a virtual image that can be viewed by a telescope or other viewing optical system. 
           [0027]    Drawing  3  shows the preferred configuration of this invention for dual-spectral band operation. Source  15  is imaged by source lens  16  onto a second source lens  17 . Source lens  17  forms an image of lens  16  onto a third source lens  18 , which produces a collimated beam from each point on the source  15 . This arrangement of source and source lenses comprises a Koehler telecentric system for illuminating the source DMD  19  with a high degree of uniformity. Radiation from the source that is reflected by micromirrors goes to the next optical element, relay lens  21  which, in combination with relay lens  23 , forms an image of DMD  19  onto the surface of projection DMD  24 . A dichroic beam-splitter  22  serves to transmit radiation in a first spectral band A towards projection DMD  24 , while at the same time reflecting radiation in spectral band B towards projection DMD  29 . Relay lenses  23  and  28 , in combination with relay lens  21 , serve to image source DMD  19  onto projection DMDs  24  and  29  respectively. Radiation reflected by ON state micromirrors in projection DMDs  24  and  29  proceed to relay lenses  25  and  30  respectively and are transmitted or reflected, respectively, by beam-combiner  27 . The combined radiation in bands A and B then proceed to projection lens  31 , forming a high-contrast two-band image at infinity (to be viewed by an imaging system under test) or on a projection screen at a finite distance. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The present invention generally comprises a means of illuminating a DMD with a structured illumination whose structure is imposed by a source-conditioning DMD. The purpose of this invention is to improve the overall dynamic range of the total projected image in the form of improved contrast and grayscale bit depth. Contrast refers to the ratio of the projected light level associated with all micromirrors on to the projected light level associated with all micromirrors off; contrast also refers to the ratio of the brightest to darkest light level within a given image. The background contributions to the projected image when the micromirrors are in the off position are due mainly to scattering and/or diffraction of the optical source energy by the micromirror array and by whatever structural elements are accessible between the micromirrors; it is this energy that limits the contrast in a traditional single DMD optical system. The structured source discussed here is dynamically programmable for each image displayed by the projector DMD. For any given image, high optical intensity can therefore be directed to areas of the projector DMD which will display bright objects, and low or zero optical intensity can be directed to areas of the projector DMD which will display dark objects. The result is improved overall contrast, achieved by the reducing or eliminating scattered and diffracted radiation levels in the darker regions of the image. 
         [0029]    Grayscale bit depth is typically achieved via pulse width modulation in DMD-based optical systems where the effective duty cycle of micromirrors in their on versus off positions over a frame duration is varied to achieve different grayscale levels as detected by a sensor which is integrating over the frame duration. The grayscale bit depth is therefore limited by the frame rate coupled with the fastest rate at which the micromirrors can switch position. Grayscale can also be realized spatially with a grayscale to black and white conversion algorithm. Because the structured illumination has an associated gray-scaling, its multiplication with the grayscale image displayed by the projector DMD results in a multiplicative total grayscale bit depth which is increased from the grayscale bit depth of either DMD on its own. 
         [0030]    This description focused on structured illumination for a projector system, however, the use of two DMDs in series can also be applied in areas of imaging and spectrometer instruments. 
         [0031]    The optical layout of the structured illumination is as follows. An optical source is used to illuminate the source DMD at normal or off-normal incidence. The optical source may be a lamp, bulb, filament, glow bar, laser, or light emitting diode and may fall within any portion of the spectral range from the ultraviolet through and including the long-wave infrared. The ON-state energy from the source DMD is then collected and an image of the source-conditioning DMD is formed on the projector DMD. The on-state energy from the projector DMD is then collected by a projection lens which typically forms an image of the projector DMD on a screen for viewing or collimates the energy such that an imaging system can form an image of the projector DMD on its focal plane array. Total internal reflection prisms can also be used for illumination and projection at each DMD. 
         [0032]    The simplest realization of this approach is to directly illuminate the source-conditioning DMD with an image of the first source lens and subsequently image the on-state reflected energy from the source-conditioning DMD onto the projector DMD. The on-state energy from the projector DMD is then collimated by a projector lens or imaged onto a screen for viewing. 
         [0033]    In its original conception, the source-conditioning DMD is operated in binary mode where each micromirror is in either the on or off position for the duration of the projector DMD frame. The source DMD is frame synched with the projector DMD, and the image stream displayed by the source-conditioning DMD is a transformed version of the original image stream to be produced by the two DMDs in series. The transformation of the image stream for the source-conditioning DMD converts intensity grayscale of the square root of the original image stream on a per-pixel basis to a spatially grayscaled image composed of binary micromirror values. The square root results in the equal splitting of contributions between the source-conditioning and projector DMDs (each displays an image proportional to the square root of the original image). Ratios other than the square root may also be used to preferentially weight the contributions between the source-conditioning and projector DMDs. The images to be displayed may be brightened to preserve the maximum intensity of the scene since each DMD can only attenuate and not amplify the illumination. Bright areas are realized with a high spatial duty cycle pattern of off and on micromirrors where the duty cycle is proportional to the intensity relative to the maximum intensity. Similarly, dark areas are realized with a low duty cycle pattern of off and on micromirrors where the duty cycle is proportional to the intensity relative to the maximum intensity. A number of black and white conversion schemes may be applied to the (square root of the) original grayscale image, for example, halftone, ordered, Jarvis, Stucki, Floyd Steinberg, or Cardinality-distribution. The relay optics which form an image of the source-conditioning DMD on the projector DMD are designed such that the spatial gray-scaled image displayed by the source-conditioning DMD is converted to an intensity gray-scaled image at the image plane at the projector DMD. The source-conditioning DMD can also be moved out of focus to induce a more complete conversion of the spatial gray-scaled image at the source-conditioning DMD to intensity gray-scaled image at the projector DMD. The relay optics and defocus therefore function as a low pass filter on the binary image projected by the source-conditioning DMD. Because no pulse width modulation is applied at the source-conditioning DMD, the structured illumination in this case is effectively analog with no grayscale digitization. 
         [0034]    The image stream to be displayed by the projector DMD is also transformed. For this, the original image is divided by a de-resolved version of the binary image displayed by the source-conditioning DMD. The de-resolve accounts for the effects of the relay optics&#39; resolution as well as the extent of defocus. Logic is also applied to threshold zero values such that divide by zero is negated. The product of the analog structured illumination and the grayscale digitized projector DMD image produces an analog total projected image with no grayscale digitization. 
         [0035]    A second realization of this concept has both the source-conditioning DMD and the projector DMD operating in grayscale mode via pulse width modulation where the bit display order of the source-conditioning DMD is coordinated with the bit display order of the projector DMD such that the desired intensity grayscale levels are realized in the total projected images by the two DMDs in series. The grayscale bit depth of the total projected image is the product of the grayscale bit depths of the source-conditioning and projector DMDs. This concept can also be realized via spatially gray-scaling both the source and projector DMDs each operating in either binary or grayscale mode via pulse width modulation. 
         [0036]    DRAWING  2  shows one embodiment of the structured illumination of a DMD where the optical source  8  illuminates the source-conditioning DMD  11  by first using source lens  9  to form an image of the source on source lens  10 . Source lens  9 , which is then imaged onto the source DMD  11  by source lens  10 , thus providing moderately uniform illumination of the source DMD  11 . In this configuration the source-conditioning DMD is illuminated at normal incidence (i.e. the incident radiation strikes the plane of the micromirror array at a right angle). A relay lens  12  then forms an image of the ON-state energy reflected by the source-conditioning DMD on the micromirror array of projector DMD  13 . In this configuration the projector DMD is illuminated at a 24° angle of incidence such that the tilt of the source-conditioning DMD plane is compensated at the projector DMD plane. The optical resolution of the second imaging lens  4  will allow for an extent of blurring of the binary spatially gray-scaled image projected by the source-conditioning DMD to ensure that all ON-state pixels in the projection DMD are fully illuminated. The source-conditioning DMD can also be positioned off of conjugate relative to the projector DMD to provide additional blurring. The on-state energy from the projector DMD is then collimated or imaged onto a screen by a projector lens  14 . Similar configurations can also be realized at other angles of incidence and reflection at both the source-conditioning and projector DMDs. 
         [0037]    DRAWING  3  shows another embodiment of the structured illumination of a DMD where a multi-spectral band system is realized. In this configuration, the optical source  15  illuminates the source-conditioning DMD  19  via a telecentric Koehler configuration of source lenses. The first source lens  16  is used to form an image of the optical source on the second source lens  17 . The second source lens in combination with the source lens  18  telecentrically forms an image of the first source lens on the source-conditioning DMD  19 . In this configuration the source-conditioning DMD  19  is illuminated at normal incidence. Relay lenses  21 ,  23 , and  28  are used to form images of the on-state energy reflected by the source-conditioning DMD  19  on the two projector DMDs  24  and  29  while maintaining telecentric operation. A dichroic beamsplitter  22  is used between the two relay lenses 2 l and  23  to spectrally split the illumination, directing the reflected portion through relay lens  28  to projector DMD  29  while the transmitted portion is imaged by relay lens  23  onto projector DMD  24 . The ON-state images from the two projector DMDs  24  and  29  are telecentrically collimated or imaged at infinity or onto a screen by projector lenses  31  and spectrally fused by dichroic beam combiner  27 . In this configuration the projector DMDs are illuminated at a 24° angle of incidence such that the tilt of the source-conditioning DMD plane is compensated for at the projector DMD planes. The optical resolution of the relay lenses will allow for an extent of blurring of the binary spatially gray-scaled image projected by the source-conditioning DMD. The source-conditioning DMD can also be positioned off of conjugate relative to the projector DMD to provide additional blurring. The multi-spectral band realization can alternatively be done with multiple source-conditioning DMDs, and with more than two spectral channels. This configuration can also be realized with non-telecentric and/or non-Koehler illumination and at other angles of incidence and reflection at both the source-conditioning and projector DMDs. 
         [0038]    It is further intended that any other embodiments of the present invention that result from any changes in application or method of use or operation, method of manufacture, shape, size, or material which are not specified within the detailed written description or illustrations contained herein yet are considered apparent or obvious to one skilled in the art are within the scope of the present invention.