Abstract:
A light engine for a single panel scrolling color projection display system employs a light guide to guide light from a source through unequal path lengths to an output lens. The light guide is characterized by little or no light loss or increase in etendue (angular extent) regardless of path length, resulting in a compact arrangement having high performance and low cost.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    U.S. patent application Ser. No. ______, filed ______, 2002, and assigned to the same Assignee as the present application, Attorney Docket No. PH ______ (ID 702697), relates to loss-less etendue-preserving light guides, which are used in the light engines of the present application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to light engines for projection display systems, and more particularly relates to a compact light engine employing light guides which is particularly suitable for use in a single panel scrolling color projection display system.  
           [0003]    A single panel scrolling color projection display system is characterized by a single light modulator panel such as a liquid crystal display (LCD) panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while the illuminated rows of pixels are synchronously addressed with display information corresponding to the color of the then incident stripe. See, for example, U.S. Pat. No. 5,410,370, “Single panel color projection video display improved scanning” issued to P. Janssen on Mar. 25, 1994, and U.S. Pat. No. 5,416,514, “Single panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valve” issued to P. Janssen et al. on May 16, 1995.  
           [0004]    Such single panel systems are to be distinguished from the more conventional three-panel systems, in which separate red, green and blue beams each fully illuminate and are modulated by a separate light modulator panel. The modulated beams are then superimposed on a display screen to produce a full color display. See, for example, U.S. Pat. No. 5,917,561, “Liquid-crystal image projecting apparatus having a color purity correction filter” issued to Hatanaka on Jun. 29, 1999.  
           [0005]    Light engines for both single-panel and three-panel color projection display systems commonly utilize high intensity arc lamps to provide the level of intensity needed for a bright display, as well as dichroic filters to split the lamp light into red, green and blue components for modulation, and then to recombine the modulated components for projection display.  
           [0006]    In both the single panel and the three panel systems of the prior art, the desire for high light efficiency has dictated that the optical path lengths of the red, green and blue beams are approximately equal. Otherwise, those beams which must travel farther from the light source to the display panel have a greater etendue (angular extent), and some of light from those beams is lost. See, for example, U.S. Pat. No. B1 4,864,390, “Display System with Equal Path Lengths”, issued to McKechnie et al. on Sep. 5, 1989.  
           [0007]    Unfortunately, such systems, while efficient in terms of light utilization, require multiple relays of relatively high optical quality to create equivalent images for the three colors. In addition, thorough integration (mixing) of light in the preceding light collection stages is necessary. The large number of optical components contributes significantly to the size and overall cost of the system.  
           [0008]    The illumination architecture for a presently used light engine I for a scrolling color projector is shown schematically in FIG. 1. White light from source S is split into a blue component B and a green/red component G/R by dichroic element  2 . The B component is directed by lens  3  and mirror  4  to prism scanner  5 . The G/R component is passed by dichroic element  2  through lens  6  to dichroic element  7 , which splits the GIR component into a green component G and a red component R. The G component is reflected by element  7  to prism scanner  8 , while the red component is passed through dichroic element  7  to prism scanner  9 . The scanned R, G, B components are then directed to recombination dichroic elements  10  and  11  by mirror  12  and relay lenses  13  through  17 .  
           [0009]    Relay lenses  13  through  17  are designed to limit the light expansion over the long recombination path from the prism scanners  5 ,  8  and  9  to the output lens  18 . Consequently, light that is telecentric at the prism scanners  5 ,  8  and  9  is not telecentric at the recombination dichroic elements  10  and  11 . As a result, color shading is introduced over the scan (from the top to the bottom of the display) unless (expensive) shaded dichroics are used.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the invention, at least the light engine portion of a projection display system employs loss-less etendue-preserving light guides, enabling a compact arrangement through the use of unequal path lengths for the separate light beams, while eliminating the need for many high quality optical lenses, and preserving the light efficiency of the equal path length designs of the prior art.  
           [0011]    In accordance with one aspect of the invention, a light engine for a projection display system comprises: a beam splitter for splitting light from a source into two or more light components; and a light guide comprising: at least a source branch for guiding light from the source to the beam splitter; and at least two component branches for guiding the light components away from the beam splitter.  
           [0012]    In accordance with a preferred embodiment of the light engine, the beam splitter comprises crossed dichroic elements or splitting the source light into red, green and blue components.  
           [0013]    In accordance with another preferred embodiment of the light engine, a scanning stripe generator is provided for the red, green and blue light components, and each component branch of the light guide guides one of the light components to one of the scanning stripe generators.  
           [0014]    In accordance with another preferred embodiment of the light engine, a beam recombiner of crossed dichroic elements is provided for recombining the red, green and blue components.  
           [0015]    In accordance with another aspect of the invention, a projection display system is provided, the system comprising a light engine of the invention, at least one light modulator panel for modulating light in accordance with a display signal; and a projection lens for projecting the modulated light onto a display screen.  
           [0016]    In accordance with a preferred embodiment of the projection display system, a polarizing beam splitter (PBS) is provided between the light engine and the light modulating panel for transmitting light of a first polarization state and reflecting light of a second polarization state transverse to the first polarization state.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    [0017]FIG. 1 is a schematic layout for a prior art scrolling color projector having equal path lengths from the light source to the display panel for the red, green and blue beams;  
         [0018]    [0018]FIGS. 2A through 2C are top, side, front views, respectively, of a compact light engine for a scrolling color projector employing light guides in accordance with one embodiment of the invention;  
         [0019]    [0019]FIG. 2D is a detail view of the beam splitting portion of the light engine of FIG. 2A;  
         [0020]    [0020]FIG. 3 is a schematic illustration of another embodiment of the beam splitting portion of the light engine of FIG. 2A;  
         [0021]    [0021]FIG. 4 is a schematic illustration of a projection display system employing the compact light engine of FIG. 2A;  
         [0022]    [0022]FIG. 5 is a rear view of the portion of the light guide of FIG. 2B between an illumination source and the beam splitter; and  
         [0023]    [0023]FIG. 6 is a schematic illustration of a polarization conversion system useful in a projection display system of the type illustrated in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Referring now to FIG. 2A, a top view of one embodiment  20  of the illumination architecture for a light engine of the invention, white light from a source S, made telecentric by collection optics (not shown), is carried by a source branch  21   a  of a light guide  21  to a beam splitter  25  that is incorporated in the light guide path. Source branch  21   a  includes straight light guide sections  22  and  24  of rectangular cross section, and light coupling element  23 , having an internal reflecting surface  23   a . The beam splitter  25  splits the white light into red, green and blue (R, G, B) components, each of which is directed to a rotating prism ( 38 ,  39 ,  40 ). The G and R components are piped to prisms  39  and  40  by component branches  21   b  and  21   c , respectively, of light guide  21 . Component branches  21   b  and  21   c  each include three straight light guide sections of rectangular cross section ( 26 ,  28 ,  30  and  32 ,  34 ,  36 , respectively) and three light coupling elements ( 27 ,  29 ,  31  and  33 ,  35 ,  37 , respectively).  
         [0025]    Light is guided through the various branches with little or no loss or increase in etendue (angular extent). A more detailed description of the structure and operation of the light guide may be found in copending U.S. patent application Ser. No. ______ (Attorney Docket No. PHID702697), filed concurrently herewith and incorporated herein by reference.  
         [0026]    Due to multiple reflections within the different branches of the light guide, the light at the exit apertures  25   a ,  31   a  and  37   a  is thoroughly mixed, hence very uniform. The exit apertures  25   a ,  31   a  and  37   a  are imaged onto the rotating prisms  38 ,  39  and  40 , respectively, forming rectangular color stripes thereon. Parallax of the rotating prisms causes a continuous scrolling motion of the color stripes. The scrolling color stripes are combined by the beam recombiner  41 , which includes dichroic elements  41   a  and  41   b , crossed at angle α. A combined image of the R, G and B scrolling stripes is thus delivered to output lens  43 .  
         [0027]    The telecentricity of the white light at the input is preserved throughout by the light guide branches as well as the recombination dichroics, despite the lack of expensive relay optics. Moreover, this non-imaging architecture is very compact, enabling a light engine for a color projection display which has high performance and low cost.  
         [0028]    [0028]FIG. 2B, a side view of the light engine of FIG. 2A, shows another possible arrangement for the input of white light from source S. In this arrangement, white light is guided in from the top by straight light guide sections  44  and  46  and light coupling element  45 . FIG. 5, which is a rear view of this arrangement, shows that light guide section  46  is connected to light guide section  22  by light coupling element  47 , having an internal reflecting surface  47   a.    
         [0029]    [0029]FIG. 2C, a front view of the light engine of FIG. 2A, showing the “lens pupil”  44 . The shape of the lens pupil  44  is determined by the rectangular aperture of output lens  43  and the telecentricity (circular cone) of the recombined light. As the beam emerges, it spreads out from the rectangular output aperture and forms a rounded shape.  
         [0030]    [0030]FIG. 2D, a detail view of the beam splitter and the adjacent light guide sections, shows that white light entering the beam splitter  25  from straight light guide section  24 , is split by the buried dichroic elements into three component beams. Dichroic element  25   a  reflects blue light, sending component B along light guide section  26 , and passes green light, sending component G along light guide section  48 . Dichroic element  25   b  reflects red light, sending component R along light guide section  32 , and passes green light, sending component G along light guide section  48 . It will be noted that light guide section  48  has been dispensed with in the arrangement of FIG. 2A, due to the proximity of rotating prism  38  to the output face  25   c  of beam splitter  25 .  
         [0031]    [0031]FIG. 3 shows another beam splitting arrangement in which beam splitter  25  with crossed dichroic elements has been replaced by two separate beam splitting elements  54  and  57 , each with a buried dichroic surface ( 54   a ,  57   a ). White light from a source S enters beam splitter  54  from a source guide including straight light guide elements  51  and  53 , and light coupling element  52  with internal reflecting surface  52   a . Blue light is reflected from surface  52   a  into straight light guide section  55  as component B. Red and green light is passed into beam splitter  57  where red light is reflected by surface  57   a  into straight light guide section  60  as component R. Green light is passed into straight light guide section  58  as component G. Adhesive layers  56 ,  59  and  61  secure straight light guide sections  55 ,  58  and  60  to beam splitters  54  and  57 .  
         [0032]    [0032]FIG. 4 is a schematic layout for one embodiment of a projection display system  70  employing a light engine  71  of the present invention. Output light from the light engine, which constitutes an image of scrolling bands of R, G, and B light, is directed through polarizer  72  to a polarizing beam splitter (PBS)  73 , having an internal polarized beam splitting surface  73   a . Polarizer  72  converts the unpolarized light to light of one polarization state, eg. S. Surface  73   a  passes this S light to light modulator panel  74 , which modulates the light in accordance with a display signal  75 , and reflects the modulated light back to the PBS  73 . In the process of modulation and reflection, panel  74  changes the polarization state of the light from S to P, and surface  73   a  reflects the light out of the PBS  73  to a projection lens  76  for display.  
         [0033]    [0033]FIG. 6 is a schematic illustration of a polarization conversion system (PCS)  80  which is useful in a scrolling color projection display system of the type shown in FIG. 4. PCS  80  replaces polarizer  72 , and is advantageous in that it doubles the width of the scrolling color bands. Light guide section  82  guides color band  81  (seen in cross section, which corresponds to the cross section of light guide section  82 ). Unpolarized color band  81  enters light coupling element  83  having an internal polarizing layer  83   a , where components in the S state of polarization are reflected by layer  83   a  through output surface  83   b  into light guide section  86 . Components in the P state of polarization are passed through to light coupling element  84 , and are reflected from reflecting surface  84   a . The reflected P components pass through output surface  84   b  into light guide section  86  through a half wave plate  85 , where they are converted to S components. PCS  80  has thus converted unpolarized color band  81  into an S polarized color band  87  having double the width of color band  81 . This wider aspect ratio is advantageous, for example, in certain higher frequency scanning systems.  
         [0034]    The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims.