Abstract:
A color sequential video projector ( 10 ) employs a color modulating device ( 22 ) and a segmented light pipe ( 30 ) that coact to provide a high luminous efficiency. The color modulating device splits polychromatic light into three different colored light beams ( 24, 26, 28 ) that are positionally stationary, but alternate mutually exclusive colors sequentially with time such that each pixel of a light valve ( 48 ) is exposed to all three colors during an image frame time. The segmented light pipe receives the three light beams and forms three adjacent color bands that are precisely aligned on the light valve. Because all three colors of light are constantly illuminating the light valve, light losses are substantially eliminated. The color modulating device employs multiple color wheels ( 82, 84, 86 ) each having filter segments ( 90, 92, 94 ) that form the three light beams by reflecting the alternating, mutual exclusive colors as the wheels rotate. Alternative embodiments employ one, two, or three color wheels having flat or conically-shaped filter segment surfaces. The segmented light pipe includes three rectangular glass cores ( 70, 72, 74 ) surrounded by a low index cladding ( 78 ) and having their abutting faces ( 76 ) coated with a thin, low refraction index coating. The light pipe input apertures ( 32, 34, 36 ) receive three roughly shaped light beams, homogenize them, and provide at three output apertures ( 38, 40, 42 ) uniformly bright rectangular light beams. A precision achromatic lens ( 44 ) images the uniform light beams onto the light valve such that seams between the beams are imaged to within one micron of accuracy and are parallel to the light valve pixel rows.

Description:
RELATED APPLICATIONS 
     Not Applicable 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     TECHNICAL FIELD 
     This invention relates to color video projection displays and more particularly to employing multiple color wheels and a segmented light pipe to increase the luminous efficiency of single light valve color video projection displays. 
     BACKGROUND OF THE INVENTION 
     1,200 lumens is an accepted brightness goal for color video projectors. A contrary goal is to employ the lowest projection lamp power possible to reduce cooling fan noise and power supply size and cost. Both goals can be achieved by substantially increasing the lumens per watt efficiency (hereafter “luminous efficiency”) of the projection display. 
     A commonly employed way of improving the luminous efficiency of color video projectors is by optically dividing the projection lamp illumination into three separate pathways, one for each primary color, providing a light valve in each pathway, modulating each light valve with its respective color data, and recombining the three pathways into a converged projected color image. Such three-path color video projectors have suitable luminous efficiency, but are generally costly, large, heavy, optically complex, and require precision alignment. 
     Prior workers have addressed the above-described problems by employing frame sequential illumination through a single light path that color timeshares a single light valve. Current single light valve projectors are relatively inexpensive, light weight, and compact. Such projectors provide color sequential illumination of the light valve by typically employing a color wheel which, unfortunately, transmits only about 30 percent of the projection lamp illumination at any time. Such projectors typically employ about a 120 watt projection lamp, which results in a brightness of only about 600-800 lumens. 
     U.S. Pat. No. 5,548,347 for SINGLE PANEL COLOR PROJECTION VIDEO DISPLAY HAVING IMPROVED SCANNING, and U.S. Pat. No. 5,845,981 for MULTI-COLOR-BAND SCROLLING ACROSS SINGLE-PANEL LIGHT VALVE describe systems for increasing the luminous efficiency of single path color video projectors by dividing the projection lamp illumination into three primary colors and passing the primary colors through rotating prisms to scroll the resulting color bands across a single light valve. Color data driving the light valve is scrolled in synchronism with the prism rotation to project a color image. Unfortunately, such projectors are are relatively costly, heavy, and optically complex. Moreover, color purity depends on scrolling the color bands across the light valve in precise synchronism with the prism rotation. Accordingly, color purity is ensured by optically separating the scrolling color bands with dark “guard bands” which, unfortunately, reduces the luminous efficiency. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide an apparatus and a method for increasing the luminous efficiency of a single-path color video projector. 
     Another object of this invention is to provide a segmented light pipe for enabling three-color area-division multiplexing of a single light valve. 
     A further object of this invention is to provide multiple color wheel embodiments for illuminating the light pipe segments with alternating primary colors. 
     Still another object of this invention is to provide a color video projector that projects a 1,200 lumen image with a 120 watt projection lamp. 
     A color sequential video projector of this invention employs a color modulating device and a segmented light pipe that coact to at least double the lumens per watt efficiency of the projector relative to projectors employing conventional color wheels. This invention employs a color modulating device that splits polychromatic light into three different colored light beams that are positionally stationary, but alternate mutually exclusive colors sequentially with time such that each pixel of a light valve is exposed to all three colors over an image frame time. A segmented light pipe receives the three light beams and forms three adjacent color bands that are precisely aligned on the light valve. Because all three colors of light are constantly illuminating the light valve, albeit in a rapidly alternating manner, the approximately 60 percent color wheel light attenuation is substantially eliminated. 
     The color modulating device employs multiple color wheels each having filter segments that form the three light beams by reflecting the alternating, mutual exclusive colors as the wheels rotate. Alternative embodiments employ one, two, or three color wheels having flat or conically-shaped filter segment surfaces. 
     The segmented light pipe includes three rectangular glass slabs surrounded by a low index cladding and having their abutting surfaces coated with a very thin, low refraction index coating. The light pipe input apertures receive three roughly shaped light beams, homogenize them, and provide at three output apertures uniformly bright rectangular light beams. A precision achromatic lens images the uniform light beams onto the light valve such that seams between the beams are imaged to within one micron of accuracy and are parallel to the light valve pixel rows. 
     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 overall schematic block diagram showing a color video projector of this invention. 
     FIGS. 2A,  2 B, and  2 C are simplified plan views of horizontally arranged aperture segments of a light pipe propagating a color illumination sequence of this invention. 
     FIGS. 3A,  3 B, and  3 C are simplified plan views of preferred vertically arranged aperture segments of a light pipe showing a color illumination sequence of this invention. 
     FIG. 4 is a cross-sectional view of a segmented light pipe of this invention taken across lines  4 — 4  of FIG.  1 . 
     FIG. 5 is a simplified oblique pictorial view of a segmented light pipe of this invention. 
     FIG. 6 is a simplified oblique pictorial view of a segmented light pipe having flared input apertures of this invention. 
     FIGS. 7A and 7B are respective side and plan pictorial views of a three color wheel embodiment of a color modulating device of this invention. 
     FIGS. 8A,  8 B, and  8 C graphically represent reflectance versus wavelength response characteristics for color wheel filter segments of this invention. 
     FIGS. 9A and 9B are respective side and plan pictorial views of a two color wheel embodiment of a color modulating device of this invention. 
     FIGS. 10A and 10B are respective side and plan pictorial views of a single color wheel embodiment of a color modulating device of this invention. 
     FIGS. 11A and 11B are respective side and plan pictorial views of a double conical-shaped color wheel embodiment of a color modulating device of this invention. 
     FIGS. 12A and 12B are respective side and plan pictorial views of a single conical-shaped color wheel embodiment of a color modulating device of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a single light valve color video projector  10  of this invention that achieves a high luminous efficiency by reducing illumination losses associated with color modulating devices, such as a color wheel. Projector  10  provides a projected polychromatic image having a brightness of at least about 1,200 lumens. 
     Projector  10  includes a light source  12  having a lamp  14  and a reflector  16  that produce intense polychromatic light  18  that is focused by a relay lens  20  on a color modulating device  22 , embodiments of which are described with reference to FIGS. 7-12. Lamp  14  is preferably a metal halide arc lamp or high-intensity discharge lamp having a power dissipation of about 120 watts. 
     Color modulating device  22  receives polychromatic light  18  and divides it into first, second, and third light beams  24 ,  26 , and  28  each comprising alternating and mutually exclusive first, second, and third colors, preferably red (“R”), green (“G”), and blue (“B”). First, second, and third light beams  24 ,  26 , and  28  are directed to a segmented light pipe  30  having first, second, and third input apertures  32 ,  34 , and  36  that are positioned to receive respective first, second, and third light beams  24 ,  26 , and  28 . 
     FIGS. 2A,  2 B, and  2 C show a first arrangement of first, second, and third input apertures  32 ,  34 , and  36 . FIGS. 2A,  2 B, and  2 C show the colors of light generated by color modulating device  22  and propagated by first, second, and third light beams  24 ,  26 , and  28  during respective first, second, and third sequential time periods. First input aperture  32  is sequentially illuminated by R, G, and B light while second input aperture  34  is illuminated by G, B, and R light and third input aperture  36  is illuminated by B, R, and G light. Accordingly, segmented light pipe  30  is illuminated by all three light colors during all three time periods, but the colors illuminating first, second, and third input apertures  32 ,  34 , and  36  are mutually exclusive during the three time periods. Color modulating device continually repeats the color sequence shown in FIGS. 2A,  2 B, and  2 C at a rate above a human viewer&#39;s flicker fusion frequency. 
     FIGS. 3A,  3 B, and  3 C show a second arrangement of first, second, and third input apertures  32 ,  34 , and  36 . The second arrangement is preferred because it provides first, second, and third input apertures  32 ,  34 , and  36  with a more favorable illumination aspect ratio. As before, FIGS. 3A,  3 B, and  3 C show the colors of light generated by color modulating device  22  and propagated by first, second, and third light beams  24 ,  26 , and  28  during respective first, second, and third sequential time periods. However, in this embodiment, first input aperture  32  is sequentially illuminated by R, G, and B light while second input aperture  34  is illuminated by G, R, and B light and third input aperture  36  is illuminated by B, G, and R light. Again, segmented light pipe  30  is illuminated by all three light colors during all three time periods, but the colors illuminating first, second, and third input apertures  32 ,  34 , and  36  are mutually exclusive during the three time periods. Of course, other mutually exclusive color sequences may alternatively be employed. 
     Referring again to FIG. 1, segmented light pipe  30  homogenizes first, second, and third light beams  24 ,  26 , and  28  and propagates them out respective first, second, and third output apertures  38 ,  40 , and  42 . Embodiments of segmented light pipe  30  are described with reference to FIGS. 4-6. 
     An achromatic lens  44  projects images of first, second, and third output apertures  38 ,  40 , and  42  through an optional prism  46  onto a light valve  48 . Light valve  48  has a contiguous addressable area  50  that is sub-addressable as first, second, and third addressable segments  52 ,  54 , and  56  for receiving the images of first, second, and third output apertures  38 ,  40 , and  42  that are brightly illuminated by first, second, and third light beams  24 , 26 , and  28 . Light valve  48  is preferably a DMD or reflective CMOS device. Achromatic lens  44  preferably has six or seven elements of sufficient optical quality to project the images of first, second, and third output apertures  38 ,  40 , and  42  accurately onto respective first, second, and third addressable segments  52 ,  54 , and  56  without overlapping or separation that would cause a visible seam in the projected image. Preferably the optical accuracy should allow adjacent rows or columns of pixels in light valve  48  to lie on either side of the seam. In this regard, FIGS. 2A through 3C could also represent the images of output apertures  38 ,  40 , and  42  as they are projected on addressable area  50  of light valve  48 . 
     A controller  58  coupled to color modulating device  22  determines when the first, second, and third sequential time periods occur and conveys color video data to first, second, and third addressable segments  52 ,  54 , and  56  in synchronous correspondence with the sequentially alternating and mutually exclusive first, second, and third colors propagated by color modulating device  22  such that light valve  48  reflects or propagates polychromatic image forming light rays  60  through prism  46  and a projection lens  62  to a projection screen  64 . The polychromatic image on projection screen  64  preferably has a brightness of at least about 1,200 lumens. 
     FIG. 4 shows a cross-sectional view of segmented light pipe  30 , which includes first, second, and third rectangular, optically conductive cores  70 ,  72 , and  74  that are coated on their abutting faces  76  with a very thin (e.g., 500 Angstroms) metal, preferably aluminum and held together by a low index cladding  78 . Segmented light pipe  30  may alternatively be held together by employing optical flat adhesion. 
     FIG. 5 shows a first embodiment of segmented light pipe  30  in which cores  70 ,  72 , and  74  are preferably formed from optical glass, have a length L of at least 2.0 centimeters, a height H of about 0.1 centimeter, and a width W of about 0.6 centimeter. 
     FIG. 6 shows an alternative embodiment of segmented light pipe  30 , which has the same cross-sectional construction as the first embodiment, but in which first, second, and third input apertures  32 ,  34 , and  36  are flared apart to receive respective first, second, and third light beams  24 ,  26 , and  28  with a minimum of adjacent beam overlap. 
     Other alternative embodiments (not shown) of segmented light pipe  30  may include sizes, shapes, contours, and angles of first, second, and third input apertures  32 ,  34 , and  36  that are optimized to receive a variety of possible cross-sectional shapes and reception angles of first, second, and third light beams  24 ,  26 , and  28 . 
     FIGS. 7A and 7B show a first embodiment of color modulating device  22  in which a motor  80  co-rotates first, second, and third color wheels  82 ,  84 , and  86  about a rotational axis  88 . 
     First color wheel  82  includes 120-degree dichroic filter segments  90 B,  90 G, and  90 R that sequentially receive polychromatic light  18  from light source  12  and form first light beam  24  by reflecting respective ones of first, second, and third colors, e.g., B, G, and R, while transmitting the other two colors to second color wheel  84 . 
     Second color wheel  84  includes 120-degree dichroic filter segments  92 G,  92 R, and  92 B that are aligned with filter segments  90  to receive the other two colors and form second light beam  26  by reflecting ones of the other two colors, e.g., G, R, and B, while transmitting remaining colors. 
     Third color wheel  86  includes 120-degree dichroic filter segments  94 R,  94 B, and  94 G that are aligned with filter segments  90  and  92  to receive the remaining colors and form third light beam  28  by reflecting selected ones of the remaining colors, e.g., R, B, and G. 
     FIGS. 8A,  8 B, and  8 C show a representative set of reflectance versus wavelength response characteristics for the dichroic color wheel filter segments of this invention. FIGS. 8A,  8 B, and  8 C show filter segment responses occurring when first, second, and third color wheels  82 ,  84 , and  86  are rotationally aligned to receive polychromatic light  18  during the respective first, second, and third sequential time periods. 
     In particular, FIG. 8A shows that during the first time period, filter segment  90 B receives polychromatic light  18 , reflects B light as first light beam  24 , transmits G and R light to second and third color wheels  92  and  94 , second color wheel filter segment  92 G reflects the G light as second light beam  26 , and third color wheel filter segment  94 R reflects the R light as third light beam  28 . 
     FIG. 8B shows that during the second time period, filter segment  90 G receives polychromatic light  18 , reflects G light as first light beam  24 , transmits B and R light to second and third color wheels  92  and  94 , second color wheel filter segment  92 R reflects the R light as second light beam  26 , and third color wheel filter segment  94 B reflects the B light as third light beam  28 . 
     FIG. 8C shows that during the third time period, filter segment  90 R receives polychromatic light  18 , reflects R light as first light beam  24 , transmits B and G light to second and third color wheels  92  and  94 , second color wheel filter segment  92 B reflects the B light as second light beam  26 , and third color wheel filter segment  94 G reflects the G light as third light beam  28 . 
     FIGS. 9A and 9B show a preferred second embodiment of color modulating device  22  in which motor  80  co-rotates only first and second color wheels  82  and  84  about rotational axis  88 , and third color wheel  86  is replaced by a mirror  100 . In this embodiment, the filter segment ordering is on each color wheel is changed merely by way of example. Alternatively, the ordering employed in the first embodiment, or other orderings, could be employed. 
     First color wheel  82  includes 120-degree dichroic filter segments  90 R,  90 G, and  90 B that sequentially receive polychromatic light  18  from light source  12  and form first light beam  24  by reflecting respective ones of first, second, and third colors, e.g., R, G, and B, while transmitting the other two colors to second color wheel  84 . 
     Second color wheel  84  includes 120-degree dichroic filter segments  92 G,  92 B, and  92 R that are aligned with filter segments  90  to receive the other two colors and form second light beam  26  by reflecting ones of the other two colors, e.g., G, B, and R, while transmitting the remaining colors. If filter segments  90  and  92  have well defined color separation, the remaining colors transmitted will be substantially B, R, and G, and no additional dichroic filtering will be necessary. 
     Accordingly, third color wheel  86  may be replaced by mirror  100 , which forms third light beam  28  by reflecting the remaining colors, e.g., B, R, and G. 
     FIGS. 10A and 10B show a third embodiment of color modulating device  22  in which motor  80  rotates a single color wheel  110  about rotational axis  88 . In this embodiment, color wheel  110  is formed from a disk of optically transparent material having a thickness T that separates first and second major surfaces  112  and  114 . Filter segments  90 R,  90 G, and  90 B are formed on first surface  112  and filter segments  92 G,  92 B, and  92 R (not shown) are formed on corresponding portions of surface  114 . Mirror  100  is in the same relative position as shown in the second embodiment. Color wheel  110  further includes a peripheral surface  116  that is beveled to facilitate exiting propagation of second light beam  26 . 
     FIGS. 11A and 11B show a fourth embodiment of color modulating device  22  in which motor  80  rotates a double-conically-shaped color wheel  120  about rotational axis  88 . In this embodiment, color wheel  120  is formed from two cones of optically transparent material each having an elevation E for forming a tapering separation between first, second, and third major surfaces  122 ,  124 , and  126 . Major surface  124  is preferably the flat surface opposite conical surface  122 . Filter segments  90 B,  90 G, and  90 R are formed on first surface  122 , filter segments  92 B,  92 G, and  92 R (not shown) are formed on corresponding portions of second surface  124 , and filter segments  94 B,  94 G, and  94 R (not shown) are formed on corresponding portions of third surface  126 . Alternatively, third surface  126  may include a mirror surface. 
     In this embodiment, filtered colors of polychromatic light  18  are reflected from first surface  122  at an obtuse angle, from second surface  124  at a right angle, and from third surface  126  at an acute angle, causing first, second, and third light beams  24 ,  26 , and  28  to converge and then diverge. This arrangement provides good color isolation by positioning segmented light pipe  30  (FIG. 6) such that input apertures  32 ,  34 , and  36  intersect first, second, and third light beams  24 ,  26 , and  28  at points of substantial divergence. 
     FIGS. 12A and 12B show a fifth embodiment of color modulating device  22  in which motor  80  rotates a single-conically-shaped color wheel  130  about rotational axis  88 . In this embodiment, color wheel  130  is formed a single cone of optically transparent material having an elevation E for forming a tapering separation between first and second major surfaces  132  and  134 . Filter segments  90 B,  90 G, and  90 R are formed on first surface  132 , filter segments  92 B,  92 G, and  92 R (not shown) are formed on corresponding portions of second surface  134 , and a mirror  136  forms a third surface  138 . 
     In this embodiment, filtered colors of polychromatic light  18  are reflected from first surface  132  at an obtuse angle, from second surface  134  at a right angle, and from third surface  138  at an acute angle, causing first, second, and third light beams  24 ,  26 , and  28  to converge and then diverge. As described with reference to FIG. 11, this arrangement also provides good color isolation. 
     There are performance tradeoffs between the flat color wheel embodiments of FIGS. 7-10 and the conical color wheel embodiments of FIGS. 11 and 12. 
     The flat color wheel embodiments of FIGS. 7-10 have less light attenuation because light is transmitted only once through the dichroic filter segments. 
     The conical color wheel embodiments of FIGS. 11 and 12 have greater light attenuation because light is transmitted twice through the dichroic filter segments. 
     Polychromatic light  18  is subject to beam spreading so needs to be focused by relay lens  20  to a spot on the color wheels to reduce light loss and color purity problems when transitioning between filter segments. Beam spreading effects may be minimized by equalizing path length differences between relay lens  20  and input apertures  32 ,  34 , and  36 . In this regard, the conical color wheel embodiments of FIGS. 11 and 12 are preferred. 
     Referring again to FIG. 1, controller  58  is coupled to color modulating device  22  in part to determine when the first, second, and third sequential time periods occur relative to the rotation of the filter segments. Determining precisely when the first, second, and third colors are propagated by color modulating device  22  may be sensed by conventional rotary encoder methods or preferably by sensing color changes as described in allowed U.S. Pat. No. 5,967,636 for COLOR WHEEL SYNCHRONIZATION APPARATUS AND METHOD, which is assigned to the assignee of this application. 
     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 display applications other than those found in color video projectors. The scope of this invention should, therefore, be determined only by the following claims.