Abstract:
Micro-electromechanical display device (“MDD”)-based multimedia projectors ( 90, 120 ) of this invention employ an arc lamp ( 32 ), a color modulator ( 42 ), and anamorphic illumination systems ( 94, 121 ) for optimally illuminating a MDD ( 50, 76 ) to improve projected image brightness. MDDs employ off-axis illumination wherein incident and reflected light bundles are angularly separated about a hinge axis ( 78, 110 ) and the MDD is illuminated by the anamorphic illumination systems of this invention having a slow f/# parallel to the hinge axis and a faster f/# perpendicular to the hinge axis. The resulting anamorphic light bundles ( 86, 88, 112, 114 ) illuminate and reflect more light into and off the MDD and through a fast f/# projection lens.

Description:
RELATED APPLICATION(S) 
     Not Applicable 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     TECHNICAL FIELD 
     This invention relates to video and multimedia projectors and more particularly to improving the projected brightness of such projectors by employing anamorphic illumination of micro-electromechanical display devices employed therein. 
     BACKGROUND OF THE INVENTION 
     Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. Such multimedia projection systems typically receive from a personal computer (“PC”) analog video signals representing still, partial-, or full-motion display images that are converted into digital video signals for controlling a digitally driven image-forming device, such as a micro-electromechanical display device (“MDD ”), a common type of which is a digital micromirror device. An example of a popular MDD-based multimedia projector is the model LP420 manufactured by In Focus Corporation, Wilsonville, Ore., the assignee of this application. 
     Significant effort has been invested into developing multimedia projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected images having adequate brightness are difficult to achieve, especially when using compact portable color projectors in a well-lighted room. 
     FIG. 1, shows a typical prior art multimedia projector  30  including a light source  32  that propagates polychromatic light along an optical path  34 . Light source  32  generates intense light by employing an arc lamp  36  and an elliptical reflector  38 . Optical path  34  includes a condenser lens  40 , a color wheel  42 , a light integrating tunnel  44 , a fold mirror  46 , a relay lens  48 , an MDD  50 , and a projection lens  52 . One or two field lenses (not shown) typically follow light integrating tunnel  72 . The optical components are held together by an optical frame  54  that is enclosed within a projector housing (not shown). A display controller  56  receives color image data from a PC  58  and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to MDD  50  in proper synchronism with the rotating angular position of color wheel  42 . A power supply  60  is electrically connected to light source  32  and display controller  56  and also powers a cooling fan  62  and a free running DC motor  64  that rotates color wheel  42 . Display controller  56  controls MDD  50  such that light propagating from relay lens  48  is selectively reflected by MDD pixel mirrors either toward projection lens  52  or toward a light-absorbing surface  66  mounted on or near optical frame  54 . Color wheel synchronization is achieved by an appropriate sensor coupled to color wheel  42  or by employing a color selective light sensor  68  to detect the time period during which a predetermined color filter segment is in optical path  34 . 
     To increase projected image brightness and uniformity, an input aperture  70  of light integrating tunnel  44  collects light exiting color wheel  42  and homogenizes the light during propagation through tunnel  44  to an output aperture  72 . The uniformly bright rectangular light bundle exiting output aperture  72  propagates through the field lenses, reflects off fold mirror  46 , and is imaged by relay lens  48  onto MDD  50 . Unfortunately, because of the oblique illumination angle of MDD  50 , the bright image of output aperture  72  typically overfills at least a portion of MDD  50  resulting in reduced brightness of the projected image. 
     Brightness-reducing overfill of light valves, such as MDDs is a common problem that prior workers have toiled to solve. For example, U.S. Pat. No. 5,159,485 for SYSTEM AND METHOD FOR UNIFORMITY OF ILLUMINATION FOR TUNGSTEN LIGHT describes employing a tungsten lamp and an anamorphic optical system to illuminate an elongated, linear MDD array used for line-scanning a photo-sensitive drum in a printer. (Conventional anamorphic optical systems employ a lens or lenses having different focal lengths or magnification factors in perpendicular planes to the optical axis.) The anamorphic optical system receives a substantially rectangular light bundle from the tungsten lamp and squashes the light bundle in one axis so that the resulting squashed light bundle illuminates the elongated, linear MDD without substantial overfill. Unfortunately, employing a tungsten lamp without an integrator tunnel results in insufficient illumination uniformity and brightness for use in a multimedia projector. 
     What is needed, therefore, is an improved way of capturing as much of the light propagated through a color modulator as possible and uniformly imaging the light on a MDD without significant overfill. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide an apparatus and a method for improving the illumination brightness and uniformity of a MDD employed in a multimedia projector. 
     MDD-based multimedia projectors of this invention employ an arc lamp, a color modulator, and anamorphic illumination systems for optimally illuminating the MDD to improve projected image brightness. MDDs typically employ off-axis illumination wherein incident and reflected light bundles are angularly separated by an amount determined by the mirror tilt angle about a mirror hinge axis. Thus, in an MDD in which the mirrors have a ±10-degree tilt angle, the incident and reflected light bundles would be angularly separated about the hinge axis by 20-degrees. This angular separation limits the conical angles available for the incident and reflected light bundles and, therefore, limits the illumination f/# and defines a practical entrance pupil f/# for the projection lens (hereinafter “f/#” may also be referred to as “f/number”). In this invention, however, the MDD is illuminated by an anamorphic illumination source having a tilt angle limited f/# perpendicular to the MDD hinge axis and a faster f/# parallel to the MDD hinge axis. This causes the incident and reflected light bundles to have elliptical, rather than circular cross-sections, resulting in more total light illuminating and reflecting off the MDD. A projection lens having an enlarged entrance pupil (faster f/# lens) gathers and projects the reflected light bundle to produce a brighter projected image. 
     A first embodiment of the anamorphic illumination system is suitable for use with MDDs in which an array of micromirrors each pivot parallel to a hinge axis that is parallel to an edge margin of the MDD. Light rays exiting the color modulator enter an input aperture of an anamorphic integrator tunnel, propagate through the anamorphic integrator tunnel, and exit through an output aperture. The anamorphic light tunnel has orthogonal length, width, and height axes. The output aperture has a width to height ratio that matches a width to height ratio of the MDD, whereas the input aperture has the same height, but twice the width of the output aperture. The anamorphic integrator tunnel functions as a non-imaging light concentrator and emits the light rays from the output aperture with f/0.4 illumination angles in the width axis and f/1 illumination angles in the height axis. At least one field lens images the output aperture onto the MDD, which receives an anamorphic incident light bundle corresponding to the f/0.4 and f/1 illumination angles. 
     A second embodiment of the anamorphic illumination system is suitable for use with MDDs in which an array of micromirrors each pivot parallel to a hinge axis that extends diagonally across the MDD. In this embodiment the anamorphic optical system comprises a collimating lens system, first and second flyeye lenslet arrays, and an imaging lens system. The first flyseye lenslet array includes a 3-by-5 array of first lenslets having as aspect ratio substantially the same as the MDD aspect ratio. The projection along the optical axis of the centers of curvature of the first flyseye lenslets corresponds to the centers of the of the lenslets of the second flyseye lenslet array. The first lenslets have a slightly rhomboid, but not necessarily equalateral, shape to compensate for the oblique illumination angle on the MDD, and their centers are offset to steer the light rays toward corresponding lenslets in the second flyseye lenslet array. The second flyseye lenslet array includes a 3-by-5 array of second lenslets having centers of curvature that are centered in each lenslet for best light transmission efficiency. The first and second lenslet arrays are arranged into their respective arrays to form an anamorphic illumination ellipse having a major axis that is aligned with diagonal hinge axis of the MDD. The imaging lens system images the first flyseye lenslet array onto the MDD, thereby producing an anamorphic incident light bundle of this invention. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified pictorial and electrical block diagram of a prior art multimedia projector showing a light path employing a color wheel, a light integrating tunnel, a MDD, and a projection lens. 
     FIG. 2 is an isometric pictorial representation of a prior art reflective display device illumination technique. 
     FIG. 3 is an isometric pictorial representation of a reflective display device anamorphic illumination technique of this invention. 
     FIGS. 4A and 4B are respective top and side simplified pictorial views representing a first embodiment of a MDD-based projector optical system employing an anamorphic light tunnel of this invention. 
     FIG. 5 is a frontal view representing anamorphic illumination of a diagonally hinged MDD. 
     FIG. 6 is a simplified pictorial top view representing a second, preferred embodiment, of a MDD-based projector optical system employing anamorphic flyseye lenses of this invention. 
     FIG. 7 is a frontal pictorial view taken along lines  7 — 7  of FIG. 6 showing a lenslet array layout of a first anamorphic flyseye lens of this invention. 
     FIG. 8 is a frontal pictorial view taken along lines  8 — 8  of FIG. 6 showing a lenslet array layout of a second anamorphic flyseye lens of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 shows the illumination of an exemplary prior art reflective micromirror display device  76  in which the micromirrors pivot about a hinge axis  78  that is parallel to an edge margin of display device  76 . This example is also applicable to other mirror hinge axis orientations as described later with reference to FIGS. 5,  6 , and  7 . Display device  76  receives a conventional conical incident light bundle  80  and reflects a conventional conical reflected light bundle  82 , the centers of which are separated by an angle  84  corresponding to the mirror tilt angle range of display device  76 . In this example, incident light bundle  80  and reflected light bundle  82  each have at an f/3 illumination number. Therefore, a projection lens having an f/3 entry pupil is the fastest practical lens usable in this example. 
     In contrast, FIG. 3 shows reflective micromirror display device  76  employing anamorphic illumination in an embodiment of this invention. In this example, an incident light bundle  86  and a reflected light bundle  88  each have a first f/number of f/3 in the direction perpendicular to hinge axis  78 . However, incident light bundle  86  and reflected light bundle  88  each have a substantially faster second f/number of f/1.4 in the direction parallel to hinge axis  78 . Light bundles  86  and  88  are, therefore, anamorphic. In this example, a projection lens having an f/1.4 entry pupil (shown in dashed lines) is the fastest practical lens usable and would produce a brighter projected image than the f/3 projection lens described with reference to FIG.  2 . 
     This invention increases the total illumination of display device  76  by increasing the illumination f/# in the direction parallel to hinge axis  78 . The individual mirrors in display device  76  are isotropic, which circumvents any decreased reflectance performance that might otherwise result from the anamorphic f/# illumination of this invention. 
     FIGS. 4A and 4B show respective top and side views of a first embodiment of this invention that is suitable for use in a multimedia projector  90 , which is architecturally similar to prior art multimedia projector  30  of FIG.  1 . However, projector  90  does not include fold mirror  46 , and MDD  50  is replaced by display device  76 , which has an array of micromirrors that each pivot parallel to hinge axis  78  to control pixels in on- and off-states. Hinge axis  78  is parallel to an edge margin of display device  76 . Display controller  56  (FIG. 1) controls the states of the array of micromirrors (pixels) to selectively reflect a projected image through projection lens  52 . 
     Multimedia projector  90  preferably includes conventional light source  32  for propagating intense illumination through a color modulator, such as color wheel  42 . Light exiting the color modulator enters an input aperture  92  of an anamorphic integrator tunnel  94 . Light rays  96  propagate by multiple reflection through anamorphic integrator tunnel  94  and exit through an output aperture  98 . Anamorphic integrator tunnel  94  has orthogonal length, width, and height axes. 
     Output aperture  98  has a width to height ratio that matches a width to height ratio of the display device  76 , whereas input aperture  92  has the same height, but twice the width of output aperture  98 . Anamorphic integrator tunnel  94  functions as a non-imaging light concentrator and emits the light rays from the output aperture with f/0.4 illumination angles in the width axis and f/1 illumination angles in the height axis. At least one field lens  100  images output aperture  98  onto the display device  76 , which receives an anamorphic incident light bundle  86  corresponding to the f/0.4 and f/1 illumination angles. Because display devices, such as display device  76 , typically have a 4:3 aspect ratio, output aperture  96  preferably has a 4:3 aspect ratio and input aperture  92  preferably has an 8:3 aspect ratio. 
     As shown in FIGS. 4A and 4B, light source  32  presents an f/1 illumination pupil to input aperture  92 . However, because input aperture  92  is stretched in one axis, anamorphic integrator tunnel functions as a non-imaging light concentrator and emits light rays  96  from output aperture  98  with f/0.4 illumination angles in one axis (FIG. 4A) and f/1 illumination angles in the perpendicular axis (FIG.  4 B). 
     Field lens or lenses  100  forms incident light bundle  86  by imaging output aperture  98  onto display device  78 . Lens or lenses  100  have an aperture that accepts the maximum f/0.4 illumination angle of light rays  96  exiting anamorphic integrator tunnel  94 . 
     Display device  76  receives anamorphic incident light bundle  86  over first and second orthogonal ranges of incident light angles f/3 and f/1.4 corresponding respectively to the above-described f/1 and f/0.4 illumination angles. Display device  76  pixels in the pixel on-state reflect reflected light bundle  88  over first and second orthogonal ranges of reflected light angles corresponding respectively to the above-described f/3 and f/1.4 illumination numbers. 
     Projection lens  52  preferably has an f/1.4 pupil to receive the f/1.4 component of reflected light bundle  88  even though the f/3 component underfills the f/1.4 pupil. 
     Referring also to FIG. 5, display device  76  is somewhat idealized because typical reflective display devices, such as MDD  50  (FIG. 1) have micromirror arrays that pivot parallel to a diagonal hinge axis  110 . As in multimedia projector  90 , projected brightness can be increased by illuminating MDD  50  with an anamorphic incident light bundle  112  that is stretched parallel to diagonal hinge axis  110  (the fast f/# axis). A reflected light bundle  114  will then be similarly stretched parallel to diagonal hinge axis  110 . The problem is how to increase the illumination diagonally while accounting for the aspect ratio, oblique illumination angles, and minimizing overfill of MDD  50 . 
     FIGS. 6,  7 , and  8  show a second, more preferred, embodiment of this invention that solves the above-described problem in a multimedia projector  120  that is architecturally similar to multimedia projector  90  of FIG.  4 . However, multimedia projector  120  replaces display device  76  with MDD  50  (FIG.  5 ), which includes an array of micromirrors that each pivot parallel to diagonal hinge axis  110  to control pixels in on- and off-states. Display controller  56  (FIG. 1) controls the states of the array of micromirrors (pixels) to selectively reflect a projected image through a projection lens (not shown). 
     Multimedia projector  120  preferably includes conventional light source  32  for propagating intense illumination through a color modulator, such as color wheel  42 . However, instead of entering an anamorphic integrator tunnel, light rays  96  exiting color wheel  42  propagate through an anamorphic optical system  121  comprising a conventional collimating lens system  122 , a first flyseye lenslet array  124 , a second flyseye lenslet array  126 , and a conventional imaging lens system  128 . 
     FIG. 7 shows first flyseye lenslet array  124 , which includes a 3-by-5 array of first lenslets  130  each having the same aspect ratio as MDD  50 . Preferable the shapes of first lenslets  130  are altered slightly to a rhomboid shape to compensate for the oblique illumination angle on MDD  50 . First lenslets  130  are radially symmetric (spherical or aspherical) and have centers of curvature  132  that are selectively offset to steer light rays  96  toward corresponding lenslets in second flyseye lenslet array  126 . 
     FIG. 8 shows that second flyseye lenslet array  126  includes a 3-by-5 array of second lenslets  140 . Second lenslets  140  are radially symmetric (spherical or aspherical) and have centers of curvature  142  that are centered in each lenslet for best light transmission efficiency. The projection along the optical axis of centers of curvature  132  of first lenslets  130  corresponds to centers of curvature  142  of the of second lenslets  140 . 
     First flyseye lenslet array  124  and second flyseye lenslet array  126  are tilted about the optical axis to form an anamorphic illumination ellipse  144  having a major axis  150  that is aligned with diagonal hinge axis  110  of MDD  50 . Imaging lens system  128  images second flyseye lenslet array  126  on MDD50, thereby producing anamorphic incident light bundle  112  as shown in FIG.  5 . 
     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, anamorphic optical system  121  can be built such that the optical functions of collimating lens system  122  are built into first flyseye lenslet array  124 , and/or the optical functions of imaging lens system  128  are built into second flyseye lenslet array  126 . Also, each of second lenslets  140  may be designed to optically overlap all the images of first lenslets  130 . Refractive optical elements are shown, but reflective optical elements may be used in some applications. Single path, frame sequential color projector embodiments are shown, but the invention is also adaptable to monochrome and multi-path color projection embodiments. Light ray angles, micromirror hinge orientations, and f/#s may differ from those described and shown and need not necessarily match. 2× anamorphic stretching is described and shown, but other stretching factors are also usable. Of course, the invention is adaptable to mirror hinge orientations, display aspect ratios, and display aspect orientations other than those described and shown. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. Accordingly, it will be appreciated that this invention is also applicable to optical light path applications other than those found in MDD-based multimedia projectors. The scope of this invention should, therefore, be determined only by the following claims.