METHOD AND APPARATUS FOR OPTICAL SCANNING FOR SINGLE PANEL COLOUR PROJECTION VIDEO DISPLAY

Abstract: A method and apparatus for a single panel projector, for projecting images having a plurality of colour components, include a mechanism for deflecting light into a plurality of directions, and a light panel for receiving the light deflected by the deflecting mechanism. The deflecting mechanism includes a transparent optical medium having a nonuniform thickness, in which light beams enter through a centre opening of the medium and exit at areas along a side of the medium.

Description: Method and apparatus for optical scanning for single panel colour projection video display. BACKGROUND OF THE INVENTION Field of the Invention The present invention generally relates to an optical scanning method and apparatus, and more particularly to a method and apparatus for sequential scanning of a plurality (e.g., three) of colour bars across the surface of a single light panel (valve). Description of the Related Art With a typical colour projection display television receiver, three separate cathode ray tubes (CRT) are utilized which have respective red, green and blue phosphors, for projecting respective red, green and blue television pictures. These three CRT pictures are projected onto a single display screen by projection lenses. The red, green and blue television pictures are thereby superimposed on the screen to form a single projected colour television picture. In another conventional colour projection display system termed a "three- valve light panel colour projection display" , three light valves are used instead of the three CRTs. White light is split into three beams carrying one of the three primary colours (Red, Green and Blue), each of which is then modulated by one of three light valves, before the three beams are re-combined into a single projected image. The light valve panels are typically thin-film transistor (TFT) array liquid crystal display (LCD) panels. However, the three-valve light panel is expensive, and has a complicated optical path. In a third conventional system, as described, for example, in the U.S. Patent US-A 5,532,763, assigned to the assignee of the present invention and incoφorated herein by reference, a single light valve panel has been developed which employs a rotating prism for scanning the red, green and blue bands of light. In the conventional projection systems, the rotating prism is used to achieve a deflection of three light rays from their original axis for scanning the above- mentioned light rays across the light valve device, the pixel elements of which are programmed with the proper video information (red, green or blue) for the passing light ray of the respective colour. The human eye integrates this "colour sequential" display of three separate images into a "single" image. However, the above-described conventional prism approach suffers from two major drawbacks. First, the scanning motion of the output rays is nonlinear, while the light valve requires constant speed of the scanning light band. Because of this incongruity, less than the full scanning range is employed, resulting in a large overscan and therefore loss of light. Thus, the system's efficiency is degraded and it is difficult to obtain a linear scanning. Secondly, the glass used for the prism has a relatively high refractive index which increases cost for the material. It is therefore an object of the present invention to provide a method and apparatus for overcoming the above problems of the conventional projection systems and methods. Another object of the invention is to provide a method and apparatus for a single panel projection device which does not employ a rotating prism for deflecting a plurality (e.g., three) of light rays from their original axis for scanning the light rays across a light valve device. Yet another object is to provide a method and apparatus which prevents a large overscan and light loss. A further object is to provide a system which is relatively low cost. In a first aspect of the invention, a scanning method and apparatus are provided for use in a single panel projector, for projecting images having a plurality of colour components, which includes a mechanism for deflecting light into a plurality of directions, and a light panel for receiving the light deflected by the deflecting mechanism. The deflecting mechanism includes a transparent optical medium having a nonuniform thickness, in which light beams enter through a centre opening of the medium and exit at areas along a side of the medium. Thus, with the structure and method of the present invention, instead of using the conventional rotating prism, the present invention uses a transparent optical medium having a changing thickness in which the beams enter through the centre opening of the medium and exit at various points along the side of the medium. The medium may advantageously have a cylindrical, egg- or spiral shape, but may have any shape suitable to meet the designer's needs. As a result, a large overscan and light loss are prevented. Further, the system is relatively inexpensive to manufacture as opposed to the conventional prism-based deflection systems. Another advantage is that the material for the transparent optical medium need not be glass, but can be commonly available plastic or the like, and there is a high degree of freedom in designing the transparent optical medium to increase the scanning area. Further, the optical medium may be easily and advantageously shaped to match the optical system and can compensate for other nonlinearities in the system's optical path. The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings, in which: Figure 1 illustrates a basic principle of the present invention, and more specifically illustrates an optical medium 1 through which a light ray passes. Figure 2 illustrates a cross-section and a side view of a deflector 2 according to the present invention. Figure 3 illustrates a ray entering a body through the centre opening at a predetermined angle . Figures 4a, 4b, and 4c illustrate respective scanning patterns 6, 7, and 8, and the cross-sections of deflectors 9, 10, and 1 1 , respectively. Figures 5A and 5B illustrate a side view and a cross-section, respectively, of a deflector 15 and illustrates the deflection of three light beams according to the present invention and more specifically illustrate the relationship of the change in the outer radius of the deflector to the amount of deflection of the three light beams. Figure 6 is a schematic diagram of a light path arrangement utilizing a deflector according to the present invention. Referring now to the drawings, and more particularly to Figure 1 , the physical principle upon which the present invention is based, is illustrated. As mentioned above, in a typical single panel light valve projection system, a rotating prism is typically used. While the scanning method relates to the conventionally used rotating prism in the single panel projector, the present invention uses, instead of a rotating prism, a transparent optical medium which has a changing thickness in which the beams enter through the centre opening of the cylindrical medium and exit at various points along the side of the medium. The medium advantageously has a cylindrical-, egg- or spiral-shape. Figure 1 depicts the basic principle of the invention. Specifically, the invention is based on the knowledge that a light ray passing through an optical medium 1 (e.g., formed of glass or the like) with two parallel transition surfaces does not change its direction. Instead, the light ray exits the medium 1 at a parallel offset .v which is a function of the thickness d of the material its refractive index n and the entrance angle a. Specifically, the changing value of s is equal to A S = d ■ sin(a) f I - cos(a) (Iff/in 2 - sin 2 (a)))] (1) The above equation makes clear that the change in s is directly proportional to d. Thus, a method of achieving a varying s is to change the thickness d in a controlled fashion. One suitable arrangement is to use a body made of, for example, glass (hereafter referred to as a "deflector") with the following characteristics (see also Fig. 2, described below, for linear deflection). In lieu of glass, commonly-available transparent plastic may also be employed. Turning now to the details of the inventive structure and method, Figures 2-4c are diagram of a scanning mechanism 2 (e.g. , a deflector 2) for use, for example, in a projection television system. The scanning mechanism is for deflecting one or more light beam(s) parallel to its/their respective axes such that the light beam scans (e.g., "moves across"), for example, a light valve. The resulting scanning motion can be either linear or non-linear. First, the cross-section of the deflector 2 is preferably cylindrical, egg- or spiral-shaped with a circular cut-out at the centre, the dimensions being defined as r(o) and r(i), respectively. The height h is not critical so long as the basic function of the deflector is not impaired. Thus, the scanning mechanism is scalable according to the designer's constraints and requirements. r(i) is a constant, while r(o) is a function of the rotation angle. The mathematical function describing r(o) can be linear, non-linear, periodic, monotonous, discontinuous, etc. The refractive index of the deflector material may be any value greater than 1.0, even though a higher number will result in smaller overall dimensions and thus may be more preferable. The deflector may have any thickness depending on the designer's requirements and constraints. However, there is generally a trade-off between the overall system's optical path, the overall dimensions and that the light throughput will be higher the smaller the refractive index is. As shown in Figure 3, a light ray 3 entering the body 4 through the centre opening at an angle a is subjected to a deflection 5. The amount of deflection 5 depends on the effective thickness of the deflector which in turn is defined by its rotation angle. Maintaining the ray 3 in a fixed position and rotating the deflector causes a periodic deflection of the ray. If r(o) is chosen to be directly proportional to the rotation angle, a linear scanning pattern results, thereby overcoming the shortcomings of the conventional prism approach. However, the principle of the invention is not limited to such a narrow function, and numerous modifications and extensions can be envisioned. For example, Figures 4a-4c respectively illustrate examples of the scanning patterns 6, 7, and 8 and the respective cross-sections 9, 10, and 1 1 of deflectors according to the present invention. The design of the outer and inner radius given a large degree of freedom in adapting the scanning motion to the light valve's optical path and the overall system. Thus, changes in the system may be easily compensated for. In the above-mentioned projection system, three light rays (e.g., red, green and blue (R,G,B)), as opposed to simply one light ray, are to be deflected. One approach would be to use three deflectors, synchronized by, for example, known mechanical means such as a gearbox. However, a more compact approach would be to utilize only a single deflector 15, as shown in Figures 5a and 5b. As shown in Figures 5a and 5b, three light rays 12, 13, and 14 representing red, green and blue, respectively, enter the core of the deflector 15 from different sides so that each colour has its own respective rotation angle. Each of the rays has a different offset from its respective incoming beam angle. Thus, for example, in Figures 5a and 5b, it is shown that light beam 14 (e.g.. Blue) is deflected the most, followed by beam 12 (e.g., Red), and finally beam 13 (e.g. , Green) which requires the least amount of deflection. The diverging exit rays 16, 17, and 18, respectively, have to be collimated with a suitable mirror arrangement (e.g. , mirrors 27, 29) and a recombination optical system 30. The recombination optical system 30 includes, for example, intertwined dichroic mirrors (e.g., a so-called "3D dichroic combiner"). Fig. 6 illustrates a light path arrangement utilizing the inventive arrangement of Figures 5a and 5b. Specifically, white light 19 is applied to a three-dimensional dichroic splitter 20 including a plurality of dichroic mirrors (or filters) which cause the red 21 , green 22 and blue 23 portions of the incoming light to exit the splitter perpendicular to the axis of the incoming ray and having a predetermined angle (e.g. , an angle of 120 degrees) between each other. Each ray is reflected by one of three dichroic mirrors. For brevity and clarity, Figure 6 simply illustrates two mirrors 24, 25 of the three mirrors. The third mirror (e.g. , not shown) is positioned behind the splitter 20 relative to the reader's direction. Each ray enters the deflector 26 core at the same angle relative to the rotation axis. The rays exiting the deflector 26 undergo a deflection different for each of the three rays because each of the rays goes through a different thickness of the optical medium 26. Thus, the thickness of the optical medium results in a different deflection for each of the rays. Mirrors 27, 28, and 29 redirect the beams towards the three-dimensional dichroic combiner 30. Three separate rays 31 , 32, and 33 exit the setup perpendicular to the axis of the incoming ray 19. The rotation of the deflector 26 causes the z position of the respective rays to change according to the desired scanning motion. It is noted that specialized coatings could be used for the mirrors such as infrared (IR) or ultraviolet (UV) coatings for respectively filtering out IR and UV rays. The above structure is advantageous in that the structure is relatively easy to construct and that the problems of the conventional rotating prism design are overcome. Thus, with the structure and method of the present invention, a rotating prism for deflecting the three light rays from their original axis with the puφose of scanning the light rays across a light valve device, is not used. The inventive method and apparatus - prevent a large overscan and light loss. Further, a system is provided which is relatively low cost. Hence, instead of using the conventional rotating prism, the present invention uses a transparent optical medium having a changing thickness in which the beams enter through a centre opening of the medium and exit at various points along a side of the medium. Further, a simpler optical path and a less complicated arrangement of components result with the structure of the present invention. While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practised with modification within the spirit and scope of the appended claims. For example, while the drawings show the optical medium as having a cylindrical, egg- or a spiral shape, other shapes such as a polygonal shape, would also be advantageous depending upon the system configuration, system performance requirements and dimensional constraints.

Claims: CLAIMS: 1. A single panel projector system, for projecting images having a plurality of colour components, comprising: means for deflecting light into a plurality of directions; and a light panel for receiving said light deflected by said deflecting means, said deflecting means comprising a transparent optical medium having a nonuniform thickness, wherein beams enter through a centre opening of said medium and exit at areas along a side of said medium. 2. A projector system according to claim 1 , wherein said medium is cylindrical shaped or egg-shaped or spiral -shaped. 3. A projector system according to claim 1 , wherein said deflecting means deflects at least one light beam parallel to its respective axis such that the light beam moves across said light panel, and wherein a scanning motion is one of linear or non-linear. 4. A projector system according to claim 1 , wherein said deflecting means comprises one of a transparent plastic and glass. 5. A projector system according to claim 1 , wherein said deflecting means includes a cross-section which has one of a spiral-, half-spiral, egg- and cylindrical-shape with a cut-out therein. 6. A projector system according to claim 5, wherein the cut-out is circular and at a centre of the deflecting means, the dimensions being defined as r(o) and r(i), respectively, wherein r(i) is a constant and r(o) is a function of the rotation angle. 7. A projector system according to claim 6, wherein r(o) is chosen to be directly proportional to the rotation angle, such that a linear scanning pattern results. 8. A projector system according to claim 1 , wherein a plurality of light rays are deflected simultaneously by said deflecting means. 9. A projector system according to claim 1 , wherein a single deflector is provided for deflecting a plurality of beams simultaneously, and wherein each beam of said plurality of light beams has a respective rotation angle. 10. A projector system according to claim 1 , further including means for collimating diverging exit rays and a recombination optical system, said recombination optical system including intertwined dichroic mirrors. 1 1. An optical system, comprising: a three-dimensional dichroic splitter including a plurality of dichroic mirrors for receiving an incoming light beam and for causing respective portions of the incoming light to exit the splitter peφendicular to the axis of the incoming ray and to have a predetermined angle between each other; a plurality of mirrors for reflecting respective ones of said portions; a deflector for receiving each of said respective ones of said portions and for deflecting said portions at a different angle; a plurality of second mirrors for redirecting the portions of the beam; and a combiner system for receiving said portion of said beam having been redirected by said plurality of second mirrors, wherein said portions of said beam exit said combiner system peφendicular to an axis of the incoming ray and a rotation of said deflector causes a predetermined position of the respective portions to change according to a desired scanning motion. 12. A method for projecting images having a plurality of colour components and for use with a single panel projector system, said method comprising steps of: providing said single panel projector system with a transparent optical medium having a nonuniform thickness; deflecting, by said transparent optical medium, light into a plurality of directions; and receiving, by a light panel of said single panel projector system, said light having been deflected, wherein beams enter through a centre opening of said medium and exit at areas along a side of said medium. 13. A method according to claim 12, wherein said step of providing a transparent optical medium comprises a step of providing one of a cylindrical medium, an egg-shaped medium, and a spiral-shaped medium. 14. A method according to claim 12, wherein said step of deflecting includes a step of deflecting at least one light beam parallel to its respective axis such that the light beam moves across said light panel, and wherein a scanning motion is one of linear or non-linear. 15. A method according to claim 12, wherein said step of providing a transparent optical medium comprises a step of providing one of a transparent plastic and glass. 16. A method according to claim 12, wherein said step of providing a transparent optical medium comprises a step of providing a transparent optical medium including a cross-section which has one of a spiral-, egg- and cylindrical-shape with a circular cut-out at a centre thereof, the dimensions being defined as r(o) and r(i), respectively, wherein r(i) is a constant and r(o) is a function of the rotation angle. 17. A method according to claim 16, wherein r(o) is chosen to be directly proportional to the rotation angle, such that a linear scanning pattern results. 18. A method according to claim 12, wherein said step of deflecting comprises a step of deflecting simultaneously a plurality of light rays by said transparent optical medium. 19. A method according to claim 12, wherein said step of deflecting comprises a step of deflecting simultaneously a plurality of light rays by a single one of said transparent optical medium, and wherein each beam of said plurality of light beams has a respective rotation angle, said method further including a step of collimating diverging exit rays.

Inventors: OTTO DETLEV, OTTO, DETLEV
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H04N9
IPC codes: G02B5, G02B26, G03B21, G03B33, G09F9, H04N9