Abstract:
A system and method for projecting light including an arrangement for illuminating an array of digital micro-mirror devices with light; projecting light reflected by the array elements in an ‘on’ state thereof; and trapping light reflected by the array elements in an ‘off’ state thereof. The light trap is implemented with an anodized metal foam matrix. The matrix is secured relative to the array. The invention may be used in a variety of applications. In a digital light projector, a light source is included along with a prism and a projection lens. The metal foam matrix acts not only as a light trap but also as a heat-sink.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to optics and optical systems. More specifically, the present invention relates to digital micro-mirror devices, digital light modulators, and methods and systems for projecting light using such devices. 
         [0003]    2. Description of the Related Art 
         [0004]    A Digital Micromirror Device (DMD) is an optical semiconductor that is the core of Digital Light Projection (DLP) projection technology. A DMD is a device which has an array of micro-mirrors monolithically integrated onto a memory chip. Often used in projectors, a DMD chip has on its surface several hundred thousand microscopic mirrors. The mirrors are typically arranged in a rectangular array in which each mirror element corresponds to a pixel in the image to be displayed. See Wikipedia at http://en.wikipedia.org/wiki/Digital_Micromirror Device as of Oct. 30, 2006. 
         [0005]    The mirrors are individually tilted ±10-12°, to an ‘on’ or ‘off’ state through an electrostatic attraction. In the ‘on’ state, light from a source is reflected into the lens making the pixel appear bright on the screen. (See U.S. Pat. No. 6,997,564 issued Feb. 14, 2006 to B. Robitaille and entitled DIGITAL PROJECTION 
         [0006]    In the ‘off’ state, the light is directed elsewhere (usually onto a heatsink), making the pixel appear dark. However, during the off-state condition, it is often difficult to prevent the off-state light from re-entering the exit pupil of the projection lens due to retro-reflections, which in turn adversely affects the optical performance of the system. Off-state light is typically allowed to exit the projector assembly freely onto a darkened surface of the subassembly or a heat sink or an adjacent surface inside the device. These surfaces are typically either painted with a light absorbing black paint or simply anodized black. With higher intensity displays, simply dumping the light into the unit may not be an option and light-absorbing paint may not withstand elevated temperatures, therefore limiting options. 
         [0007]    Hence, a need remains in the art for a system or method for disposing of light energy, particularly off-state light in DMD/DLP projectors and displays. 
       SUMMARY OF THE INVENTION 
       [0008]    The need in the art is addressed by the system and method for projecting light of the present invention. The inventive system provides an arrangement for illuminating an array of digital micro-mirror devices with light; projecting light reflected by the array elements in an ‘on’ state thereof; and trapping light reflected by the array elements in an ‘off’ state thereof. 
         [0009]    In more specific embodiments, the invention includes a light trap implemented with an anodized metal foam matrix. The matrix is secured relative to the array with a bonding agent, clip or other mechanism. The invention may be used in a variety of applications. In a digital light projector, a light source is included along with a prism and a projection lens. The metal foam matrix acts not only as a light trap but also as a heat sink. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  depicts a digital projection display according to an illustrative embodiment of the present teachings. 
           [0011]      FIG. 2  is a simplified sectional side view of a portion of the projector of  FIG. 1 . 
           [0012]      FIG. 3  is a diagram showing an illustrative embodiment of the optical light trap of the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0013]    Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
         [0014]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
         [0015]      FIG. 1  depicts a digital projection display  20  according to an illustrative embodiment of the present teachings. This design is substantially similar to that of the above-referenced U.S. Pat. No. 6,997,564 issued Feb. 14, 2006 to B. Robitaille and entitled DIGITAL PROJECTION DISPLAY, the teachings of which are incorporated herein by reference. However, as discussed more fully below, a significant difference between the design of the referenced patent and the embodiment of  FIG. 1  is the inclusion in  FIG. 1  of an optical light trap  60  for minimizing the effects of ‘off’ state stray light on the contrast performance of the system. 
         [0016]    The structure of the digital projection display  20  is most readily discussed in terms of the order of the components along the light path from the light source to the display screen. 
         [0017]    The digital projection display  20  includes a light source  22  producing as an output a light beam  24 . The light source  22  may be of any operable type, but is typically a polychromatic light source such as a compact arc lamp. 
         [0018]    Optionally but preferably, a color wheel  26  having red, blue, and green segments receives the light beam  24  from the light source  22 . The color wheel  26  spins in the light beam  24  so that the light beam  24  is sequentially colored with the primary red-blue-green colors that are combined to produce a full-color image after subsequent modulation. A white fourth segment may also be present. The color wheel  26  is not required if the projected image is to be a black-white image or if using a multi-color (e.g. a blue, red and green) LED light source. 
         [0019]    An integrator  28  receives the light beam  24  from the color wheel  26  (or directly from the light source in the absence of the color wheel  26 ). The integrator  28  is preferably a solid transparent rod or a hollow pipe that provides multiple internal reflections of the light beam  24 . The integrator has two effects. It makes the light beam  24  more uniform over its cross section and prevents the formation of an image of the filament of the light source  22 . Second, the integrator  28  shapes the light beam  24  to have the desired peripheral shape of the final projected image. In the usual case, the final projected image is rectangular, so that the output of the integrator  28  is rectangular with an aspect ratio that matches that of the spatial light modulator. The integrator  28  does not spatially modulate the light beam  24 . 
         [0020]    An illumination lens  30  receives the light beam  24  from the integrator  28 . The illumination lens  30  may include one lens element or more than one lens element. In the illustration, the illumination lens has two illumination lens elements  70  and  72 . The illumination lens  30  images the exit end of the integrator  28  onto the digital light modulator to be discussed subsequently. 
         [0021]    An illumination fold mirror  32  receives the light beam  24  from the illumination lens element  70  and reflects the light beam  24 . The illumination fold mirror  32  changes the direction of the light beam  24 . In combination with other fold mirrors in the digital projection display  20 , the illumination fold mirror  32  allows the optics of the digital projection display  20  to fit within a compact envelope. In the preferred embodiment, the illumination fold mirror  32  reflects the light beam  24  through an angle A of about 80 degrees. After the light beam  24  reflects from the illumination fold mirror  32 , it passes through the illumination lens element  72 . 
         [0022]    A light director  33 , preferably a total internal reflection (TIR) prism  34 , receives the light beam  24  from the illumination fold mirror  32 . An internal reflective surface (not shown) of the TIR prism  34  is oriented such that the light beam  24  that enters the TIR prism  34  is totally reflected. 
         [0023]    In an alternative approach, termed an offset approach, a lens directs the light beam  24  to and from the light modulator (discussed next), and there is no TIR prism. A digital light modulator  36  receives the light beam  24  from the light director  33  (which is preferably the TIR prism  34 ) and spatially modulates the light beam  24 . The digital light modulator  36  receives image information in electronic form from an image source (not shown). The digital light modulator  36  then spatially modulates the light beam  24  with that electronic image information. The digital light modulator  36  is preferably a digital micromirror device  38 . The digital micromirror device  38  is an array of movable small mirrors, each of which small mirrors serves as the modulator for one pixel of the resulting image. However, the present invention is not limited thereto. That is, the present teachings may be used in any system in which stray energy is problematic. 
         [0024]    By controlling the orientations of the individual small mirrors, each pixel of the incident light beam  24  may be selectively reflected in the proper direction to eventually form part of the reflected image (an illuminated pixel), or selectively reflected in another direction so that it does not form part of the reflected image (a dark pixel). The result is that the light beam  24  is spatially modulated. 
         [0025]    In the preferred embodiment wherein the light director  33  is the TIR prism  34 , the light beam  24  is sent back to the TIR prism  34  in its spatially modulated form. The internal reflective surface of the TIR prism  34  is oriented such that the incident light beam  24  that is received back from the digital light modulator  36  is not reflected by the internal reflective surface and passes straight through the TIR prism  34 . This is illustrated more clearly in  FIG. 2 . 
         [0026]      FIG. 2  is a simplified sectional side view of a portion of the projector of  FIG. 1 . As shown in  FIG. 2 , light from the source  22  enters the prism  34  and reflects off the interface  35  thereof toward the DMD  38 . In its ‘on’ state, each mirror in the DMD reflects light back to and through the prisms  34  and  37  to a projection lens  40  as discussed more fully below. ‘Off’ state light from mirror elements in the DMD in the ‘off’ states thereof fails to pass through the prisms  34  and  37 . This light is reflected internally in the second prism  37  and exits the second prism  37  to the novel optical light trap  60  provided in accordance with the present teachings. 
         [0027]      FIG. 3  is a diagram showing an illustrative embodiment of the optical light trap  60  of the present invention. In the illustrative embodiment, the light trap  60  is a metal (e.g. aluminum) foam matrix that is clipped, bonded or otherwise secured in place within the projector  20  of  FIG. 1 . The foam matrix may be purchased from a manufacturer such as ERG Materials &amp; Aerospace as Duocel® foam metal. DUOCE® Metal Foam (aluminum) has a 3-dimensional skeletal structure, duodecahedronal-shaped cells connected by continuous metal ligaments which can be purchased in various porosities ranging from 5 to 40 pores per inch. In the best mode, the foam has a porosity on the higher end of the range, i.e., 40 pores per inch. The foam should be anodized to enhance the absorptive properties thereof. Those of ordinary skill in the art will arrive at a thickness to provide sufficient absorption for a given application. 
         [0028]    Returning to  FIG. 1 , the projection lens  40  receives the light beam  24  in its spatially modulated form from the TIR prism  34  (or directly from the digital light modulator  36  in some embodiments). In the present design, the projection lens  40  has at least a first projection lens element  42 , and a second projection lens element  44  that is spaced apart from the first projection lens element  42 . Taken together, the lens elements of the projection lens  40  focus the light beam  24  onto the display screen that is viewed by the user of the digital projection display  20 , as discussed subsequently. The throw ratio of the projection lens  40  is preferably about 1.1. The throw ratio is defined as the distance along the light path  24  from the display screen to the nodal point closest to the display screen, divided by the width of the display screen. 
         [0029]    A projection lens fold mirror  46  is disposed between the first projection lens element  42  and the second projection lens element  44 . The light beam  24  passes through the first projection lens element  42 , reflects from the projection lens fold mirror  46 , and passes through the second projection lens element  44 . In the preferred embodiment, the projection lens fold mirror  46  reflects the light beam  24  through an angle B of about 90 degrees. 
         [0030]    A projection fold mirror  48  receives the light beam  24  from the projection lens  40  (and specifically from the second projection lens element  44 ) and redirects the light beam to the display screen to be discussed subsequently. In the preferred embodiment, the projection fold mirror  48  reflects the light beam  24  through an angle C of about 72 degrees. 
         [0031]    The digital projection display  20  preferably includes a housing  50  in which the light source  22 , the color wheel  26 , the integrator  28 , the illumination lens  30 , the illumination fold mirror  32 , the light director  33 , the digital light modulator  36 , the projection lens  40 , the projection lens fold mirror  46 , and the projection fold mirror  48  are received. The housing  50  has a housing envelope depth HD, a housing envelope width HW, and a housing envelope height HH. A housing envelope volume V is the product HD times HW times HH, even though the housing  50  may not be a perfectly defined rectangular prism. 
         [0032]    The digital projection display  20  preferably includes a display screen  52  that receives the light beam  24  from the projection fold mirror  48 . The display screen  52  typically forms one face of the housing  50 . The light beam  24  is desirably incident upon the display screen  52  substantially perpendicularly to the display screen  52 . As a result, the display screen  52  need not be holographic in structure, with its associated high cost when produced in relatively small numbers, and the projected image on the display screen  52  is not distorted. The display screen  52  has a display screen (maximum) diagonal dimension DD. The display screen  52  is typically rectangular in shape, as illustrated, and the dimension DD is the diagonal dimension of the rectangular shape. 
         [0033]    Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof. 
         [0034]    It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
         [0035]    Accordingly,