IMAGE PROJECTION SYSTEM

Abstract: A high brightness and high contrast image projection system particularly suitable for use with light valves that have an array of pivotable mirror pixels (22) is disclosed. By disposing an asymmetric aperture (92) in the light path with its longitudinal axis located along the pivot axis (24-24) of the pixels (22) maximum brightness is obtained without adversely affecting contrast. The system may be used in either a monochrome or color (RGB) system and in single or multiple light valve systems.

Description: Image projection system. This invention relates an image projection system comprising: a light source; a projection lens for projecting an image onto a viewing surface; a light path extending between said light source and said projection lens; a light valve disposed in said light path between said light source and said projection lens, said light valve having an array of pivotable mirror pixels, said pixels being pivotable about a pivot axis, said pixels reflecting light from the light source impinging thereon either into said projection lens or away from said projection lens to modulate light in accordance with image information input thereto; and aperture means disposed in said light path. The term image projection apparatus should be understood in a wide sense and comprises a device for displaying, for example, a video image, a graphic image, numerical information or a combination thereof. The images may be both monochrome and colour images. Most commercially available projection video systems utilize separate projection paths for each of the three primary colors. The systems thus require red, green and blue light valves and optical paths which must be accurately converged on the screen which adds to complexity and expense. Recently, projection video systems utilizing only a single light valve have been developed. One such system is a so-called color field sequential system, in which the normal video field, for example l/50th or 1/ 60th of a second, is broken into three parts, or color subfields of for example 1/ 150th or 1/ 180th of a second. During the three color sub-fields, the light valve is illuminated with red, green and blue light sequentially. While the light valve is illuminated with any given color, the video data corresponding to that color is displayed on the light valve. The eye then fuses the three color sub-fields into a single, full color field. The eye also fuses successive video fields into full motion, full color video. This system requires a device for sequentially illuminating the surface of the light valve with the three colors. The simplest of such devices is a color wheel which changes the color of a beam from a white projection lamp as it rotates. Recently, improved light valves particularly suitable for use in projection television systems have become available. One such device is a so-called deformable mirror device (sometimes called a digital mirror device or DMD) which is illustrated in U.S. Patent No. 5,079,544, in which the light valve consists of a array of tiny pivotable mirror-like pixels for deflecting a beam of light either through the projection lens to the display screen (on) or away from the projection lens (off). This device is particularly suitable for use in a field sequential system because its pixels are capable of being switched very rapidly. By the possibility of very rapid switching of the pixels even grey scale generation is possible. There is always a desire for greater brightness in projection video systems so that the brightness approaches or exceeds that of direct view (CRT) systems. In addition to a bright image, the image must also have good contrast so that the projected image does not appear "washed out". A usual method of increasing brightness in a projection video system is to increase the aperture of the system. However, increasing the aperture generally increases the complexity and expense of the optical system and moreover may reduce system contrast because the increased aperture permits more spurious light to enter the system. Where high contrast is of paramount importance many projection system designs have decreased the aperture to increase the contrast, but at the expense of reduced brightness. The present invention provides an image projection system that has both increased brightness and increased contrast. This system is characterized in that said aperture means have a long dimension and a short dimension, and being oriented in the light path with said long dimension being disposed parallel to the pivot axis of the pixels of said light valve. An asymmetric aperture is located at one or more places along the light path. The longitudinal axis of the asymmetric aperture is orthogonal to the switching direction of the pixels. If greater contrast enhancement is desired, the aperture can be further arranged so that it is narrower in the direction in which light is diffracted or scattered from the structure of the DMD. The asymmetric aperture can be disposed in the light path between the lamp and the DMD or after the DMD for example, in the projection lens, or at both places. The invention is useable in color or monochrome systems as well as systems using multiple light valves and multiple projection lamps. For a better understanding of the invention, reference is made to the following drawings which are to be taken in conjunction with the detailed specification to follow: Fig. 1 is a schematic plan view of the surface of a DMD type light valve showing the orientation of the hinges of the individual pixels; Fig. 2 is a diagram illustrating the angular separation of the incident beam, the ON beam, the specular reflected beam and the OFF beam in a DMD projection system; Fig. 3 is an embodiment of an image projection system according to the invention. Fig. 4 is a diagram illustrating a comparison between a symmetric aperture and an asymmetric aperture as used in the light path of the illumination system; Fig. 5 illustrates the asymmetric aperture of Fig. 4 viewed along the optical axis of the system in relationship to the light valve; Fig. 6 illustrates a second embodiment of an asymmetric aperture for use with the invention; and Fig. 7 illustrates the asymmetric aperture of Fig. 6 viewed along the optical axis of the system in relationship to the light valve. Figure 1 illustrates a light valve 20 in the form of a deformable mirror device (hereinafter referred to as "DMD") having an array of mirror pixels 22. Each pixel 22 is mounted so as to be pivotable about torsion hinges 24 which are located at two diagonal corners of each pixel 22. In operation when an appropriate ON voltage is applied to pixel 22, the upper lefthand corner 26 of pixel 22 will move upwardly from the plane of the DMD 20 while the lower righthand corner 28 will move downwardly. Similarly, when an OFF voltage is applied, pixel 22 will pivot about torsion hinges 24 so that corner 26 moves downwardly and corner 28 moves upwardly. Thus the ON and OFF positions comprise two distinct movements of each pixel 22. Generally, pixels 22 can be pivoted by 10° to either side of the plane of the DMD 20. In the ON position, the incident radiation is reflected into the aperture of a projection lens. In the OFF position, the incident radiation is reflected outside the aperture of the projection lens and thus does not reach the viewing screen. This operation is illustrated in Figure 2. The present invention is also applicable to DMD's of the so called "hidden hinge" configuration in which the hinges for the individual pixels are disposed underneath the pixels. Figure 2 illustrates the light path resulting from the switching of pixels 22. In Figure 2, the dotted line 30 illustrates the position of pixel 22 when pivoted into its ON position and dotted line 32 illustrates the position of pixel 22 when pivoted into its OFF position. The vertical solid line 31 illustrates the planar position of pixels 22 (neither ON nor OFF). An illumination system schematically illustrated at 34 provides a beam of light 36 that is incident upon the surface of each pixel 22. When pixel 22 is in ON position 30, the incident beam 36 will be reflected as a beam 38 to a projection lens system 40 which will project beam 38 onto the viewing screen (not shown) behind the projection lens system. When pixel 22 is activated and pivoted into its OFF position 32, incident beam 36 will be reflected to an "OFF" beam 42 which is outside of the angle of acceptance of the projection lens system 40, so that no light will reach the projection screen from the OFF position of pixel 22. In actuality, ON beam 38 and OFF beam 42 are not the only beams reflected from DMD 20. Additionally, there is a specular reflected beam 44 which is reflected from the window covering DMD 20 as well as from the interpixel structure. Specular beam 44 has no information content and represents optical system "noise". Projection lens system 40 and illumination system 34 should be designed so as to reduce the effect of specular reflected beam 44 which will raise the black level, thus reducing overall contrast in the projected image. In the projection system the four beams 36, 38, 44 and 42 should be separated in order to provide adequate contrast, thus the angular acceptance of projection system 40 is generally limited to less than + . 10°. If one attempts to limit the angle of acceptance of the projection system to a greater degree, system brightness will be reduced. Figure 3 is a schematic representation of an image, for example a video, projection system 48 in accordance with the present invention. The system illustrated in Figure 3 is a single panel full color (RGB) system. However, the principles of the present invention are not limited to single panel color systems and may be used in monochrome systems and multiple panel color systems as well. As shown in Figure 3, a projection lamp 49 is used to provide a source of white light. Projection lamp 49 may include an integral reflector 50 or be used in conjunction with a separate reflector. The light emitted from lamp 49 and reflector 50 passes to a converging lens, or lens system 52 which is used to focus a beam 54 towards a color wheel 56. Color wheel 56 comprises a series of segments, or spokes, of red, green and blue transmission filters. Color wheel 56 is rotated about its axis, which is parallel to the system axis 53, so as to have its colored segments sequentially intercept light beam 54. After exiting color wheel 56, the now colored light beam 58 is acted upon by an imaging lens 60 and in turn passes to a condenser lens 62 and an integrator 63. After exiting integrator 63 beam 64 is directed to an aperture plate 66 having an aperture 68 which has a configuration and alignment as is described in detail below. Also disposed at aperture plate 66 is a further integrator plate 70. The beam of light is then focussed towards a prism system 72. In prism system 72 light is reflected from a surface 74 and directed to the surface of DMD 20. In turn, prism system 72 is arranged so that the light which is reflected off the surface of DMD 20, and thus contains the modulated video information, is directed to a projection lens 76 for projection onto a viewing screen or surface (not shown). Prism system 72 may be of the configuration shown in U.S. Patent 4,969,730 to van den Brandt. Integrators 63, 70 which form a uniform illumination beam may be one of those shown in U.S. Patent No. 5,098,134 to van den Brandt et al. Figure 4 illustrates an aperture configuration for use in DMD projection systems such as that illustrated in Figure 3. In most projection systems, the optical system is designed to have an angle of acceptance of _ 15° or less. Optical design, particularly that of the projection lens, becomes more difficult as the aperture increases. Thus to simplify the optical system, the aperture is limited to a practical value. However in devices such as DMDs, the DMD itself further limits the aperture in the switching direction, generally to that of +. 8°. Generally in projection systems, the apertures are circular. A circular aperture having a beam acceptance of _ 8° is shown as dotted line 90 in Figure 4. However, the angular acceptance of a DMD is limited only in the directions in which pixels 22 pivot. Thus, the aperture of the system can be increased in a direction orthogonal to the direction of tilt, so orthogonal to the hinges 24 of pixels 22. In Figure 4, an asymmetric aperture 92 having a 15° acceptance angle along the hinge direction with an 8° angle of acceptance along the mirror pivot (hinge) direction is shown. In Figure 4, arrow 94 is oriented in the direction of pivoting of pixels 22 and arrow 96 shows the direction of the hinges 24 of individual pixels 22. Thus, it is seen that the longitudinal axis of asymmetric aperture 92 is oriented along the hinge axis. Put another way, asymmetric aperture 90 has been elongated along the pivot axis. Figure 5 is a view along the optical axis 53 of the projection system 48 and illustrates the orientation of asymmetric aperture 92 with respect to DMD 20 and pixels 22. As is seen in Figure 5, asymmetric aperture 92 is aligned along the axis of the hinges 24 of each pixel 22 and thus is tilted at an angle of 45° with respect to the horizon of the image formed by DMD 20. In general, the brightness of the image is proportional to the area of the aperture and the area of asymmetric aperture 92 is 2.3 times the area of symmetric aperture 90. The actual increase in brightness realized depends upon the characteristics of the light source and the collection optics. In Figure 4, the system aperture has been extended in the direction of hinges 24 (orthogonal to the mirror tilt direction). In a DMD system, this aperture configuration may lead to a reduced contrast ratio because a larger aperture may accept more spurious light in the OFF state. Furthermore, the distribution of the spurious light may be such that further shaping of the aperture is desirable. One contribution to loss in contrast is light which is diffracted from the pixel structure into the projection lens system. This diffracted light is oriented along the direction of the pixel edges which are at 45° to the mirror tilt direction. Accordingly, in order to improve contrast ratio, the system aperture may be shaped as shown in Figure 6. In Figure 6, another embodiment 98 of an asymmetric aperture is shown. In Figure 6, asymmetric aperture 98 has been narrowed in the direction 100 towards the center of specular reflected beam 44 as shown in Fig. 2. This will reduce the possibility of diffraction of light from the specular beam into the projection lens system thus raising the contrast of the image formed by the projection system. As is shown in Fig. 7 the edge 102 of aperture 98 will be aligned parallel to the side edges of pixels 22, while the edge 104 will be parallel to the upper and lower edges of pixels 22. As is seen in Fig. 6, edges 102, 104 extend at a 45° angle to upper wall 106 of aperture 98. As noted above, the described embodiments are not limited to color systems and are equally applicable to monochrome systems. For example, a monochrome system similar to that shown in Figure 3 would simply omit the color wheel and the circuitry used to drive the light valve for the three colors. Similarly, in color systems, many other methods, other than color wheels, may be used to generate three sequential colors of light. An RGB system may also be formed with three lamps, one for each primary color. The use of an aperture configured in accordance with the invention is also applicable to this type of illumination system as well, with an aperture disposed in each of the three illumination paths. Anamorphic lenses may also be used to shape the illumination beam into an asymmetric shape. In order to control the aperture of an optical system, a physical stop such as aperture plate 66 with an aperture 68 of the desired shape may be placed at the system aperture stop or an image thereof. In addition, projection lens 76 may also have an internal stop shaped to the desired form. As with most projection systems, the aperture stop of the illumination system should be imaged into the internal stop of the projection lens. Although the present invention has been described as applied to a deformable mirror devices, the concept can be applied to any type of light valve. For example, the contrast ratio produced by a liquid crystal display (LCD) varies as a function of the incidence angle. Generally, this function is not circularly symmetric about normal incidents. Thus, an improved system brightness and/or an improved contrast ratio can be obtained by using a system aperture that is not circular symmetric. The above-described arrangement is merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention.

Claims: CLAIMS: 1. An image projection system comprising: a light source; a projection lens for projecting an image onto a viewing surface; a light path extending between said light source and said projection lens; a light valve disposed in said light path between said light source and said projection lens, said light valve having an array of pivotable mirror pixels, said pixels being pivotable about a pivot axis, said pixels reflecting light from the light source impinging thereon either into said lens or away from said projection lens to modulate light in accordance with image information input thereto; and aperture means disposed in said light path, characterized in that said aperture means have a long dimension and a short dimension, and being oriented in the light path with said long dimension being disposed parallel to the pivot axis of the pixels of said light valve. 2. An image projection system as claimed in Claim 1 wherein said aperture means comprise straight edges along the long dimension and curved edges about the short dimension. 3. An image projection system as claimed in Claim 2 wherein said straight edges of said aperture means are of different length so that said aperture means is narrower along one longitudinal edge. 4. An image projection system claimed in Claim 1, 2 or 3 wherein said aperture means are disposed between said lightsource and said light valve. 5. An image projection system as claimed in Claim 1, 2 or 3 wherein said aperture means are disposed in the light path from the light valve up to and including said projection lens. 6. An image projection system as claimed in Claim 1, 2, 3 or 4 further including a second aperture means disposed in the light path behind the light valve said projection lens. 7. An image projection system as claimed in Claim 1 any one of claims 1-6 for projecting a colour image, further including means for sequentially altering the colour of the light emitted by the light source with at least two different colors. 8. An image projection system as claimed in Claim 7 wherein said means for altering the color of the light emitted by the light source comprise a rotating wheel.

Inventors: JANSSEN PETER, SHIMIZU JEFFREY, JANSSEN, PETER, SHIMIZU, JEFFREY
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04N9
IPC codes: G02B26, G09G3, H04N5, H04N9