Abstract:
A color projection display system, comprising a rotatable color drum with transmissive segments on the surface of the drum, the segments composed of dichroic filters having different color transmission bands, a cylindrical lens mounted co-axially inside the drum, and means for focussing images of the boundaries between segments onto an electro-optical light modulator panel, whereby in operation the segment images are scrolled across the surface of the light modulator panel, synchronously with the application of color components of a display signal corresponding to the colors of the scrolling segment images, to produce a full color display image for projection to a viewing screen.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     U.S. patent application Ser. No. 09/127,003 filed Jul. 31, 1998, and assigned to the assignee of the present application, relates to a color projection system incorporating an electro-optic light modulator and a rotating light-reflective element. The entire specification of Ser. No. 09/127,003 is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to color projection systems, and more particularly relates to such projection systems incorporating a single electro-optical light modulator. 
     Color projection display systems are known in which a white light source is separated into red, blue and green sub-beams for separate modulation by corresponding color components of an incoming display signal, and then the modulated sub-beams are recombined into a full color display for projection onto a viewing screen. Modulation of the sub-beams is commonly carried out using three separate electro-optical light modulators such as liquid crystal display (LCD) panels, one for each of the three sub-beams. 
     However, in one type of color projection system, described for example in U.S. Pat. No. 5,532,763, the three sub-beams are all modulated by a single LCD panel. This is accomplished by shaping the sub-beams into band-shaped cross-sections, and scrolling the bands sequentially across the LCD panel, while synchronously addressing those portions of the panel which are illuminated by the bands with the corresponding display signal information. Such projection systems are referred to herein as single panel scrolling raster (SPSR) projectors. 
     A disadvantage of all such SPSR systems is the requirement for a relatively large LCD (having at least an approximately 1.3 inch diagonal for the current lamp technology), in order to accept the light from the three separate primary color beams. Such large LCDs are expensive, and require the use of comparatively large, expensive optical components. 
     Another disadvantage of such SPSR systems is that scrolling of the beams across the panel is generally accomplished using either a single, relatively large, rotating glass prism, or three separate, smaller prisms, either of which alternatives results in a bulky arrangement, the latter having several moving parts. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved SPSR color projection system. 
     It is another object of the invention to provide such an improved system in which the optical architecture is simple and compact. 
     It is another object of the invention to provide such an improved system having only one rotating element to accomplish scrolling. 
     It is yet another object of the invention to provide such an improved system in which the illumination beam is not separated into separate color beams prior to scrolling. 
     In accordance with the invention, a SPSR color display system includes a single, relatively small rotatable optical element with transmissive surface portions having different color transmission bands, and a single electro-optical light modulator. 
     In operation, the element rotates about its axis while broad spectrum (e.g., white) light from a source is incident on the proximal side of the transmissive surface, and is separated into color components by the surface portions which transmit the desired color bands. Light transmitted by the proximal side of the surface is focussed on to the distal side of the surface by a toroidal (e.g., cylindrical) lens mounted co-axially inside the hollow of the drum. The transmitted light from the distal side is focussed onto the light modulator panel. The color bands transmitted from the element are scrolled across the panel by virtue of its rotation. 
     Driver electronics synchronously address the panel with the corresponding color components of the display information during scrolling, whereby a full color image is produced for projection to a viewing screen. 
     The use of such a transmissive element as the scrolling means in a SPSR system eliminates the need to separate the white light into sub-beams prior to scrolling, and thereby enables a smaller (less than 1.3 inch) light modulator panel, and correspondingly smaller optical components. For example, such a transmissive element, e.g., a drum, can have a cross-section of 30 mm diameter or smaller. Thus, a simple and compact optical architecture is possible, leading to a compact projector with a small component count. 
     In accordance with a preferred embodiment, at least one opposing side of a transmissive filter segment of the surface of the element comprises a clear element or an air space, whereby the occurrence of a color artifact in the display image is eliminated. In accordance with another preferred embodiment of the invention, the rotatable optical element has a polyhedral surface, and the transmissive portions comprise dichroic filters having a flat surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in terms of a limited number of preferred embodiments, with reference to the drawings, in which: 
     FIGS. 1 a  and  1   b  are side and top views, respectively, of a schematic layout of one embodiment of a color projection system of the invention; 
     FIGS. 2 a  and  2   b  are cross-sections of one embodiment of a transmissive color drum of the invention, in different degrees of rotation about its axis, together with a co-axial cylindrical lens; 
     FIGS. 3 a  through  3   c  are representations of the spot sizes of light beams at first and second surfaces of the drum, and at the light modulator panel, respectively; 
     FIGS. 4 a  through  4   g  are cross-sections similar to those of FIGS. 2 a  and  2   b  for another embodiment of a transmissive color drum of the invention, in different degrees of rotation about its axis, together with a co-axial cylindrical lens; 
     FIG. 5 is a cross section of a transmissive color drum of the invention, showing nominal and actual light paths through the color filters; 
     FIG. 6 is a cross-section of yet another embodiment of a transmissive color drum of the invention, with a folded light path; 
     FIGS. 7 a  and  7   b  are cross-sections similar to those of FIGS. 2 a  and  2   b,  showing the transmissive color drum at 0 and 35 degrees of rotation about its axis; 
     FIG. 8 is a plot of transmission versus wavelength for typical red, green and blue dichroic filters, and for a color artifact; 
     FIGS. 9 a - 9   d  are cross-sections similar to those of FIGS. 7 a  and  7   b,  showing additional embodiments of a transmissive color drum of the invention with one of a filter pair replaced by a clear segment; 
     FIG. 10 is a cross-section of a transmissive color drum of the invention in which one of a filter pair is replaced by air; 
     FIG. 11 is a cross-section of another embodiment of a transmissive color drum of the invention for use in a red, green, blue and white projection system; and 
     FIG. 12 is a cross-section of yet another embodiment of a transmissive color drum of the invention having interior light shields. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 a  and  1   b  are side and top views, respectively, of a schematic diagram of one embodiment of a color projection system of the invention, incorporating a transmissive color drum. Illumination from a source, preferably a high intensity source such as a high intensity discharge (HID) lamp, is integrated by an integrating lenslet array  12 , and converted to polarized light of P type polarization by PCS (polarization conversion system)  14 . This P beam is focussed by spherical lenses L 1 , L 2  and L 3 , and cylindrical lens L 4   a  onto the surface of hollow drum  16 . Light transmitted by the proximal side of drum  16  into the interior of the drum is focussed onto the distal side of drum  16  by cylindrical lens L 7 . Light transmitted from the distal side of the drum  16  is focussed by cylindrical lens L 4   b  and aspheric lenses L 5  and L 6 , and passes through polarization beam splitter (PBS)  18  and wide band quarter wave plate  20  onto the surface of light modulator panel  22 , which in this embodiment is a reflective liquid crystal display (RLCD). RLCD  22  modifies the light in accordance with a display signal (not shown), and reflects it out of the system for projection to a viewer, via an internal reflective surface of PBS  18 . Quarter wave plate  20  corrects the skew angle problem associated with the use of PBS  18  at a non-zero cone angle, as described in U.S. Pat. No. 5,453,859. 
     FIGS. 2 a  and  2   b  show in cross-section one embodiment of a transmissive color drum  16  of the invention, having a cylindrical surface  24  composed of six transmissive filter elements  26 ,  28 ,  30 ,  32 ,  34  and  36 , arranged in opposing pairs of three different colors, red, green and blue. The drum  16  rotates counterclockwise about its rotational axis, so that the drum filters encounter incoming light Li in the sequence: red, green, blue. Mounted co-axially inside the hollow interior  36  of the drum is cylindrical lens L 7 . 
     FIG. 2 a  shows the drum  16  in the 0 degree position. Note that there are two cycles of red-green-blue filters around the periphery of the drum, with each filter covering 60 degrees of arc. The input light to the drum is imaged at the proximal side of the drum surface  24  by lens elements L 1 , L 2 , L 3  and L 4   a,  shown in FIG.  1 . The input light covers an arc of ±25 degrees, not the full ±30 degree filter height. This allows use of the pulsed lamp color correction scheme described in co-pending U.S. patent application Ser. No. 09/177,014 (filed Oct. 22, 1998) in the projection system of this invention. 
     The input light Li as it comes from the integrator/PCS combination ( 12 , 14 ) is white light. It passes through the first surface  24  of the transmissive drum  16 , which is made of transmissive filter material. While it is anticipated that for a projection application dichroic material will be used, absorptive filters may also be used. After passing through the filter  26 , the light is now red. Cylindrical lens L 7  on the axis A of the drum images this red light from the proximal side of surface  24  onto the distal side of surface  24 . This lens L 7  could alternatively be a nearly-circular toroidal lens, the exact shape being determined by the desired system optimization. Note that the imaging by this lens L 7  in only in the direction perpendicular to the drum axis A. Note that all light that encountered the red filter  26  on the first passage through the drum again encounters a red filter  32  on the second passage through the drum  16 . Therefore the transmitted light Lt remains red and is passed through the drum with a high degree of efficiency and color purity. 
     FIG. 2 b  shows the position of the drum  16  after it has rotated 17 degrees. In a 60 Hz system, with two red-green-blue (RGB) cycles around the periphery of the drum  16 , this would occur after 1.6 ms. Notice that the topmost light bundle of Li now encounters green filter  28 , and is imaged by the cylindrical lens L 7  onto green filter  34 . As the drum  16  rotates further, the transmitted beam will change color in the sequence RGB. Focussing the boundary lines between adjacent colors onto the light modulator panel  22  will result in a sharp demarcation between the colors as they scroll across the panel. The lens L 7  does not image parallel to the drum axis A, as may be seen by comparing the side and top views of FIGS. 1 a  and  1   b.  Since the line between two adjacent colors is always horizontal, such horizontal imaging is not important to the operation of the drum projector. 
     The quality of the imaging in the vertical direction by lens L 7  is indicated by the image spot sizes and shapes shown in FIGS. 3 a - 3   c.  These spot sizes all correspond to a point source on the integrator lenslet  12  at the top, center and bottom of the lenslet both horizontally and vertically. FIG. 3 a  is at the input side of the drum  16 ,  3   b  is at the output side of the drum  16  and  3   c  is at the RLCD  22 . Note that all the spot sizes are elongated horizontally, parallel to the division between two colors. Vertical spot size has been minimized, and a fully optimized system would minimize this vertical size still further. Even so, the spot quality shown in FIGS. 3 a - 3   c  is believed to be adequate for proper color drum operation. The main goal of further optimization would be to reduce the number of optical elements and to reduce the total path length of the projector. 
     FIGS. 4 a - 4   g  show another embodiment of a transmissive drum  40  of the invention, in which the filter elements  41 ,  42 ,  43 ,  44 ,  45  and  46  are flat dichroic segments. 
     There are several potential problems with a flat filter segment drum, especially when the segments each cover as large an angle as a 60 degree segment of the drum. Three of these problems, on closer examination, turn out to be non-issues. First, as the drum rotates the light encounters the filters at a variety of angles from +30 degrees through 0 to −30 degrees. This ±30 degree range is enough to cause a noticeable variation in color of the light passing through the filter. If this color shift with angle resulted in a top to bottom or center to top and bottom color gradient, the color shift would be unacceptable. As can been seem in FIGS. 4 a - 4   g,  this will not happen. The light going to any particular position on the panel, for instance the top of the panel, sees all possible angles of incidence during one color field time. For example, follow the rays that encounter the bottom of the red filter in FIG. 4 a.  These rays have an angle of incidence at the drum dichroic of +25 degrees, with a cone angle of ±12 degrees. As the drum rotates the nominal angle of incidence passes through 0 degrees between FIGS. 4 c  and  4   d.  When the drum rotates into the position shown in FIG. 4 f,  the light now has an angle of −25 degrees on the drum. The effect of this variation in angle is that the effective cut-off slope of the filter is reduced, and the effective 50% point is displaced slightly toward a shorter wavelength compared to the filter at normal incidence. For 60 Hz operation, one color field time is 5.6 ms. The eye will integrate the color variation over this field time and not perceive it. The filter design can easily accommodate the slight 50% wavelength shift introduced by the rotating drum. If the reduced slope is a colorimetry problem, low angular shift filters such as the ones described in U.S. Pat. No. 5,999,321, can be designed to reduce the effect. 
     The second potential problem is that the flat segments  41 - 46  follow chords of a circle and the surfaces of the filters moves toward and away from the focal point as the drum  40  rotates. This is not a problem because the only position on the drum where spot size is an issue is at the boundary between two different colors. This boundary always occurs at the nominal drum diameter and therefore the nominal focal position. 
     There is another potential problem associated with the flat filter segments  41 - 46 . These segments have a finite thickness and thus when tilted in the optical path, they displace the beam slightly. For glass plates having a refractive index n g =1.50, a thickness of 1.0 mm, and a tilt of 30 degrees, the displacement is approximately 0.22 mm. However, the displacement caused by the first passage through the surface of the drum is counteracted by the displacement caused by second passage through the surface of the drum. While opposing segments are parallel and one might expect the displacements to add, this is not the case due to the imaging of the cylindrical lens in the center of the drum effectively reversing the direction of the displacement caused by the first passage through the surface. This effect is shown in FIG. 5, with two rays through drum  40 , tracing the nominal light path with no filter offset effect (dashed lines) and the actual light path with compensating offsets (solid lines). 
     FIG. 6 shows another embodiment of a drum  60  of the invention. In this embodiment, instead of an imaging cylindrical lens at the center of the drum, an imaging cylindrical mirror or semi-cylindrical lens/mirror combination L 8  is employed. This allows the light path to be folded at the drum. FIG. 6 shows this configuration for a drum  60  with 12 flat filter segments  61 - 72 , and thus 4 RGB cycles around the periphery. This results in a 90 degree fold of the optical path at the drum. The rays Li experience total internal reflection (TIR) at the flat surface  74  of the lens L 8  so a separate mirror coating may be unnecessary. 
     When employing a drum with dichroic filter segments, it may be advantageous to replace one of an opposing pair of same-color filter segments with a clear segment or an air gap, in order to avoid a color artifact which may occur under certain conditions. 
     FIG. 7 a  shows a drum  76  of hexagonal cross-section at a 0 degrees rotation angle, with the red dichroics ( 77 , 80 ) in the optical path. Light Li from the illumination source is focused, for example, on point A. Red light passes through the dichroic filter  77  and the blue and green light is reflected. The red light is focused in the vertical direction by the cylindrical lens L 7  on the axis of the drum and imaged at point A′. After passing through the second red dichroic  80 , it is re-imaged on the horizontal centerline of the panel  22 . As the drum  76  rotates, the center of the panel  22  continues to be illuminated with red light until the edge of the red dichroics ( 77 , 80 ) pass focal points (A,A′); and the center line of the panel  22  is illuminated with green light through green dichroics ( 78 , 81 ), as shown in FIG. 7 b.    
     Most of the light in the system behaves as just described. This produces a set of colored bars that scroll vertically down the light valve. Unfortunately, some light behaves differently, due to the fact that the pass bands of dichroic filters are defined by a transmission curves of finite slope. FIG. 8 shows transmission curves R, G, B for typical red, green and blue dichroic filters. 
     Thus, for example, in the case of the red filter, curve R, at wavelengths longer than about 596 nm, substantially all of the light is transmitted, while at wavelengths shorter than 572 nm, substantially all of the light is reflected at point A and never reach point A′. However, for intermediate wavelengths of 572-596 nm, particularly at 585 nm, where the transmission is exactly 50%, the transmitted light is imaged by the cylinder on axis onto point A′. 50% of this light, which contains 25% of the original energy, is reflected back to the cylindrical lens and is re-imaged onto point A. Once again, 50% is transmitted and 50% is reflected. This reflected light represents 12.5% of the original energy, and is imaged a second time at point A′, and so on. As the drum rotates, this light at 585 nm, which is distinctly yellow, is spread in the vertical direction into a band of light which scans vertically at a different rate than the primary color bands, contaminating them and producing non-uniform, desaturated colors. 
     This effect occurs whenever the transmission is at or near 50%. The magnitude of this effect is shown in FIG. 8 as dashed lines r, g and b. In a projection system with a UHP lamp with a yellow emission line at 578 nm, this problem would be particularly severe for the red dichroic. The green dichroic, with a 50% cut-off value of 566 nm could also be affected. Due to the variation from lot-to-lot in dichroic filters, the effect could vary in severity from one projector to another, causing quality control problems. 
     One solution to this problem is shown in FIGS. 9 a - 9   d.  In this design, dichroic filters, shown as red, green and blue filters ( 91 - 93 ), are paired with AR coated clear segments ( 94 - 96 ), shown as dashed lines. Since there is little or no reflection off the AR coated glass, there is little or no possibility of forming an undesired image through multiple reflections. 
     In FIG. 9 a,  the incoming light Li falls first on the red segment  91  at point A. Red light plus 50% of the yellow light at 585 nm passes through the segment  91  and is imaged by the cylindrical lens L 7  onto the matching clear (or missing) segment  94 . If a clear, AR coated, segment is used, one could expect about 1% reflection. Therefore ½% of the original light at 585 nm is reflected at point A′ and re-imaged onto point A. Here 50% of the light is reflected a second time, or a total of 0.25% of the original light, and so on. The total amount of 585 nm light which is imaged at the screen is thereby reduced to only 4% of the light which would have been imaged in the case of paired dichroic filters. 
     In FIG. 9 c,  the drum has rotated 180 degrees and the light encounters the clear segment  94  first at position C. All of the light is imaged onto location C′. Here the red light is transmitted, as well as 50% of the 585 nm yellow light. This transmitted light is imaged onto the light valve  22  and eventually the screen. 50% of the yellow light is reflected at C′, 1% is again reflected at C, 50% is transmitted after re-imaging at C′, for a total of 0.25%. Again, the problem will only be 4% of the problem with paired dichroic filters. 
     While the problem has been described in terms of the cut-on wavelength of the red dichroics at 585 nm, it can also occur at the cut-on and cut-off of the green dichroics (e.g., 482 and 566 nm, respectively), or the cut-off of the blue dichroics (e.g., 512 nm). In each case, the solution to the problem is the same as described above. 
     In the embodiment of FIGS. 9 a - 9   d,  the clear segments are included mainly for reasons of dynamically balancing the rotating drum  90 . An alternative design, shown in FIG. 10, omits these clear segments altogether. With no segment present, there is no possibility of undesired reflections. The drum  100  is balanced by alternating filter segments  101 ,  103  and  105  with air spaces  102 ,  104  and  106 . 
     Other possible solutions to this problem are: 1) replacing one of a dichroic filter pair with an absorptive filter; and 2) inserting a narrow band rejection (e.g., notch) filter into the non-moving portion of the illumination path designed to reject light at the green/red transition wavelengths (to remove yellow light which is prevalent in the spectra of some high pressure discharge lamps, such as the Philips UHP lamp), and optionally to also reject light at the blue/green transition wavelengths. 
     FIG. 11 shows an octahedral drum design  110  suitable for use in a red, green, blue, white projection system. Red, green and blue filter segments  191 ,  192 ,  193  are paired with clear segments  195 ,  196  and  197 , for the R, G, B components, and clear segment  194  is paired with another clear segment  195  for the white component. Such a system is used to enhance the brightness of the image on the screen at the expense of color purity. 
     Another potential problem may be encountered when stray light passes through the color drum  120  and misses the cylindrical lens L 7  on the axis of the drum. As shown in FIG. 12, non-rotating optical shields  121 ,  122 , mounted on either the cylindrical lens L 7  or a lens holder (not shown) prevent this light from reaching the electro-optical light modulator and causing color artifacts. 
     The invention has been described in terms of a limited number of embodiments. Other embodiments, variations of embodiments and art-recognized equivalents will become apparent to those skilled in the art, and are intended to be encompassed within the scope of the invention, as set forth in the appended claims. 
     For example, the projection system could have fewer total optical elements, fewer custom optical elements, improved spot size, and/or shorter total length. The drum, while shown in the preferred embodiments as having an air space between the surface and interiorly mounted toroidal lens, this space could be partially or completely filled with a material having a lower refractive index than that of the toroidal lens.