Abstract:
System and method for improving color convergence in projection display systems employs one or more mirror units disposed for reflecting light from the projection lens to a display screen, each mirror unit including one or more reflective layers adjustably positioned for changing the path length of selected one or more color components of the color image. Changing the path length of the selected one or more color components of the color image effectively changes the magnification of those selected color components according to:            δ                 v     v     =       δ                 m     m                             
     where v is path length traversal of the light from the projection lens to the display screen, δv is change in path length effected by the positioning of the reflective layers, m is the magnification factor, and δm is the change in magnification factor of a color component required to correct color convergence of a displayed image. Once a desired change in path length is calculated to adjust magnification of the color component to improve color convergence, the relative spacing of the one or more reflective layers in each mirror unit may be determined.

Description:
FIELD OF THE INVENTION 
     This invention relates to projection display technology and to a system for improving the quality of a color image projected by a projection display. 
     BACKGROUND OF THE INVENTION 
     In a projection display, a full color image on the screen is achieved by overlaying separate images containing red, green and blue components. These component images are projected by a common projection lens onto a distant screen for viewing. As described in the reference “Applied Photographic Optics,” pages 103-111, Sidney F. Ray, Focal Press, 1994, it is a feature of glass that its refractive index changes with the wavelength of light, and a lens may, therefore, have a different magnification for red, green and blue images. Therefore, it is commonly seen that the red, green and blue component image do not exactly overlap especially at the edges of the screen. This is a serious problem for some applications and it is necessary to construct a projection lens of considerable complexity to attempt to remedy this (color convergence) problem. 
     To minimize this effect, a lens may consist of a number of elements employing different glasses so as to color correct the lens at two or more specific wavelengths. As displays of higher resolution are developed, the demands on the projection lens become more severe. 
     In applications such as rear projection monitor or TV, the viewer may approach closely to the screen and misconvergence detracts noticeably from the quality of the image. In electronic theater applications, misconvergence at the edges of the screen is objectionable since some viewers may sit significantly to the side of the screen. 
     FIG. 1 illustrates the imperfect overlapping of, for example, red and blue images at a screen. The dots may be made to overlap perfectly at the center of the screen by adjustment of the position of the objects being imaged, however, there is a displacement (misconvergence) between the different colored dots toward the edge of the screen. 
     FIG. 2 shows a highly simplified projection system  50  where a target or panel A,  100 , is imaged in red light by a projection lens  200  onto a distant screen  300 . The outline of the imaging panel  100  is shown as a square  400  on the screen. A second panel B,  140 , is imaged in blue light via a dichroic mirror  180  that reflects blue light, but is transparent to red light. In this way, an image  440  of the panel is produced on the screen in proximity to the image  400 . 
     In a projector, the panels  100 ,  140  are variously described as light valves or spatial light modulators. The panel may be transmissive in type, that is, light is transmitted through the panel or reflective in type where light is reflected from the panel. The light valves are designed to produce a pattern or image within the light valves from an electronic or other source, that is small in size, e.g., one inch in width. This image is projected onto a distant screen  300  with a magnification of order 20 to 200 times or more. 
     It can be seen that in the example shown in FIG. 2, the size of the images  400  and  440  are slightly different due to the variation in lens strength with color of light. It is the case that the magnifications cannot be equalized by movement of one of the panels  100  or  140  since there is only one position of correct focus for the panels. To vary the magnification, it is necessary to vary the distance from lens  200  to the screen  300  separately for the two panels, however this is not possible. In a projector, red, green and blue images are required and the optical system may include more than two panels that are imaged onto the screen. 
     It would be highly desirable to provide an apparatus for improving the color convergence of projection display systems that is simple and does not require modification of the projection optical system. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method for improving color convergence in projection display systems. The system employs one or more mirror units disposed for reflecting light from a projection lens to a display screen, with each mirror unit including one or more reflective layers adjustably positioned for changing the path length of selected one or more color components of a color image to be displayed. Changing the path length of the selected one or more color components of the color image effectively changes the magnification of those selected color components according to:            δ                 v     v     =       δ                 m     m                            
     where v is path length traversal of the light from the projection lens to the display screen, δv is change in path length effected by the positioning of the reflective layers, m is the magnification factor, and δm is the change in magnification factor of a color component required to correct color convergence of a displayed image. Once a desired change in path length is calculated to adjust magnification of the color component to improve color convergence, the relative spacing of the one or more reflective layers in each mirror unit is determined. 
     It is clear that the deflection of rays by means of one or more plane (i.e., flat) mirrors does not introduce distortion of projected images, but serves only to redirect the rays from the projection lens. 
     Advantageously, the incorporation of the special mirror device does not require modification of the projection optical/display system. That is, for small corrections to the magnification of the different colored images, the deforming effect due to change in distance to the screen is not objectionable. However, for highly critical applications as, for example, in an electronic cinema installation, the imaging panel in a projector may be moved slightly to achieve correct magnification and the sharpest image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an example of an image showing lack of color convergence; 
     FIG. 2 illustrates the origin of the color convergence problem; 
     FIG. 3 illustrates the mirror unit used to improve color convergence in the projection display system; 
     FIG. 4 illustrates a first embodiment implementing a single mirror unit of the invention; 
     FIG. 5 illustrates a second embodiment implementing a two mirror units of the invention for improved image display; 
     FIG. 6 illustrates a third embodiment implementing two mirror units of the invention; 
     FIG. 7 illustrates a fourth embodiment implementing a mirror unit of the invention in a projection display system incorporating polarized light. 
     FIG.  8 ( a ) depicts the change in path length δv introduced by placement of a mirror device between the projection lens and the display screen according to the embodiment of FIG.  4 . 
     FIG.  8 ( b ) depicts the change in path length δv introduced by placement of two mirror devices between the projection lens and the display screen according to the embodiment of FIG.  5 . 
     FIG. 9 depicts the color convergence problem showing a lateral displacement of color pixels at an edge of a screen. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a method and apparatus for eliminating misconvergence phenomena in conventional projection display systems by enabling adjustments to the magnification of light components without modifying the projection system. The problem of adjusting magnification is solved by interposing one or more mirror devices between the projection lens  200  and the screen  300  (FIG.  2 ). 
     FIG. 3 is a schematic view of the mirror device  215 . The mirror device  215  consists of a first mirror  220  that reflects light of one color only, for example, blue, placed parallel to and spaced apart from a second mirror  224  that reflects another color only, for example, green, and a third mirror  228  placed parallel to and spaced apart from the first and second mirrors that reflects the remaining color, for example, red. It should be understood that the mirror device  215  may comprise less than three mirrors, for example, reflecting only two desired color components, e.g., red or blue, of the color image. In this instance, the third mirror device  228  may comprise a normal mirror, i.e., one that reflects all color components of light. Thus, if the path length of only one or two color components need to be changed in order to obtain convergence, the third mirror  228  may comprise a normal mirror and only one or two color reflecting layers need be provided for reflecting only those desired color components. 
     The color reflecting layers of the mirror  215  may comprise thin glass supporting color selective films, or more conveniently, stretched plastic membranes. Plastic membranes may be obtained as thin as 1 mil inch in thickness and are used when metallized as mirrors in rear projection systems. The individual mirror layers may be coated with color selective dichroic films following known practice. The spacing  230  between each of the individual color selective mirror layers, and particularly, gaps g 1 , g 2  and g 3  between each of the respective mirrors, may be adjusted by micrometers or similar devices, as will be explained in further detail herein. 
     In the projection display system  52 , when an image is reflected from this mirror to a screen as shown in FIG. 4, the path between the projection lens and the screen is different now for light of each color. By adjusting the spacing  230 , and particularly, gaps g 1 , g 2  and g 3  between each of the mirrors, the magnification of each color image component may be adjusted and may be made equal in size. When the mirror device is in place, it may be necessary to move the panels (FIG. 2) into a new focus to produce a sharp image on the screen and to achieve the desired magnification. In most cases, the correction required is small, but the effect on the image quality is appreciable. It is clear that the method of the invention may be implemented in projection systems employing non-polarized light or linearly polarized light and does not depend on details of the projector optical system provided a single projection lens is used. 
     The sideways image displacement between the red, green and blue images may also introduce a lateral shift of the color pixels, which may be corrected by lateral shift of the light valve or by an electronic shift of the image generated within the light valve. To avoid the relative image displacement associated with the use of a single mirror unit, two mirror units may be implemented in the optical system as shown in FIG.  5 . 
     As shown in FIG. 5, in the projection display system  54 , two reflections are used. In FIG. 5, a second mirror  215 ′ is provided for reflective light “a” along a path “b” for reflection “c” of first mirror  215 . In this case, the lateral shift is canceled without hindering the magnification adjustment capability. This is because the angle positioning of said first and second mirrors  215  and  215 ′ is such that incident light that may be laterally shifted due to reflection, at or near edges of the first mirror device  215 ′, are cancelled by the provision of the second mirror device  215  which is at a complementary angle sufficient for re-shifting the light components near the edges of the second mirror unit  215 . 
     FIG. 6 illustrates another embodiment of the projection display system and mirror devices. In the projection display system  56  of this embodiment, a first normal mirror device  315 , i.e., reflecting all color components of light, is positioned to receive light from the projection lens  200  and to reflect light to the mirror device  215  (FIG.  3 ). Mirror device  215 , in turn, reflects the light back to a second mirror device  215 ′ (FIG. 3) which, in turn, reflects the light to a conventional mirror device  315 ′ which further reflects the light to screen  300 . It is apparent that in this embodiment of the projection display system  56 , the projection lens  200  remains in line with the display screen  300 . 
     FIG. 7 illustrates a method embodiment permitting the use of polarized light. In the embodiment of FIG. 7, a polarizing beam splitting mirror  260  or prism is placed between the projection lens  200  and the mirror device  215 . Polarized light, e.g., in a “P” direction, from the projector passes in transmission through the polarizing beam splitter mirror  260  and traverses a quarter wave plate  280  before reaching the mirror device  215 . The beam is reflected by the mirror device  215  as described in FIG.  2  and traverses the quarter wave plate  280  a second time. The polarization direction of the beam is, therefore, rotated by 180 degrees (half-wave length), resulting , e.g., in a color corrected polarized beam, e.g., in the “S” direction, that is reflected by the polarizing beam splitter mirror  260  to produce an image onto a distant screen  300  instead of traversing it. This method has the advantage of no net lateral shift and good compactness. 
     It is the case that in each of the systems  52 ,  54 ,  56 , and  58  (FIGS.  4 - 7 ) the fractional change in magnification (delta m)/m of the color image components on the screen  300  is related to the fractional change in path length (delta v)/v between the projection lens  200  and screen  300  according to equation 1) as follows: 
     
       
         ( δm )/ m= ( δv )·( m+ 1)/( mv )  (1) 
       
     
     where “m” is the magnification, “v” is the path length, δm is the change in magnification, and δv is the change in path length. Since “m” is always much greater than 1, equation 2) provides a good approximation of this relationship: 
     
       
         ( δm )/ m= ( δv )/ v   (2) 
       
     
     Referring back to FIG. 3, and making use of the relationship set forth in equation 2, changes in gap lengths g 1 , g 2 , and g 3  may readily determined by determining the respective δv&#39;s (changes in path lengths) for each color component in order to achieve a desired magnification. Thus, as shown in FIG.  8 ( a ), for the case of a single mirror unit  215  oriented at 45° angle with respect to incident light from the projection lens, δv is shown equal to the extra path length labeled B-B′ which, implementing simple geometry, is equal to 1.4 times the gap length g 1  which is the distance between the first reflecting layer  224  and the second reflecting layer  228  (FIG.  3 ). It is this extra path length δv that gives rise to the change in magnification δm for that color component. As further shown in FIGS.  4  and  8 ( a ) for the case of a single mirror unit  215  oriented at 45° angle with respect to incident light from the projection lens, the color component of light reflected by mirror  228  is subject to a lateral displacement shown in FIG.  8 ( a ) as a distance C-C′ which also equals the length B-B′. In the case of a single mirror unit  215 , correction for this lateral displacement of the selected color component may be achieved by displacing the position of the panel slightly, as shown in FIG.  1 . 
     For the case of two mirror components  215  and  215 ′ between the oriented at 45° angles as shown in the embodiment  54  of FIG.  5  and in FIG.  8 ( b ), the change in path length δv is equal to the addition of lengths B-B′ due to the presence of mirror  215 ′ and the distance C-C′ due to the presence of mirror  215 . As shown in FIG.  8 ( b ), implementing simple geometry, this extra path length is approximately equal to 2×1.4 times the gap distance g 1  between the first reflecting layer  224  and the second reflecting layer  228  of each respective mirror units, i.e., δv=2.8×g 1 . As further shown in FIG.  8 ( b ), and as described herein, the use of two mirrors enables the cancellation in lateral shift so that there is no net displacement of the color component on the display screen  300 . 
     It should be understood that the foregoing description relating to the calculation of change in path length δv may be readily derived for the general case of a mirror unit  215  oriented and an arbitrary angle with respect to incident light from the projector lens. 
     Referring further to FIG. 9, a screen  300  is shown having color pixels at locations exemplifying misconvergence problem. In FIG. 9, the change in magnification δm/m required to correct the color convergence is equal to (W 1 −W 2 )/(W 1 ), where W 1  is the distance between pixels of a first color component of the image and W 2  is the distance between corresponding image pixels of a second color component of the image. In order to correct the color convergence problem shown in FIG. 9, for the case of the projection display system  54  of the invention having two mirror units as shown in the embodiment of FIG.  5  and FIG.  8 ( b ), the relation δv=2.8×g 1  applies. Solving for g 1 , the gap length required between the two reflecting mirrors is equal to g 1 =v×(W 1 −W 2 )/(W 1 ×2.8). Thus, for example, if v=50 meters and (W 1 −W 2 )/(W 1 ) is approximately on the order of 1×10 −2 , then the gap lengths g 1  of the convergence correcting mirrors  215 ,  215 ′ is equal to (50/2.8)×10 −2 =0.18 m approximately. If (W 1 −W 2 )/(W 1 ) is approximately on the order of 1×10 −3 , then the gap length g 1  is equal to (50/2.8)×10 −3 , i.e., 1.8 cm approximately. 
     Generally, as seen in FIG. 1, red and blue pixels in a display may be separated by a distance of order 1 pixel at the edge of the screen even with a good lens, indicating a magnification correction needed of order 2/N where N is the number of pixels across the width of the screen. Hence, the change in path length needed to correct for a 1 pixel convergence error at the edge of the screen is given by equation 3) as follows: 
     
       
           δv= 2 v/N   (3) 
       
     
     Thus, using the simple relationships given herein, it is straightforward to calculate the gaps for the mirror device  215  of FIG. 3 from simple geometry as the following examples illustrate. 
     In a first example, a projection system comprises and an auditorium projector where v=50 meters and N=1280 pixels, then δv=8 cm. In another example, a rear projection display includes a projector where v=1.5 meters and N=1280, then δv=2.3 mm. These extremes may be readily achieved with the multilayer mirror according to the invention. One reflection from the convergence correcting mirror at a 45-degree angle of incidence introduces a path difference δv of 1.4×gap between mirrors. Using the arrangement of FIG. 5, the use of two mirrors introduces a path difference of 2.8×gap. The gaps required for the two examples above are calculated to be 2.9 mm and 0.8 mm, respectively. If the gap between the reflecting surfaces is filled with a material of index other than that of air, a trivial adjustment within the purview of skilled artisans would be needed to accommodate for this. 
     While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.