MXPA99006978A - Projection televisions with holographic screens having center to edge variations - Google Patents

Abstract

A projection screen (22) for a television has a three-dimensional hologram (26) on a film substrate (24). The screen (22) can have a plurality of stacked holograms and/or fresnel lenses having varying optical properties in vertical and horizontal directions. Image projectors (14, 16, 18) for different colors direct images which converge on a projection screen (22) formed by at least one hologram (26) on a light transmissive panel, collecting the converging light from the projectors (14, 16, 18) on the rear and displaying the images on a front with controlled light dispersion so as to orient the images substantially forward. The holographic screen (22) forms an interference array and has optical properties that vary horizontally and vertically across the field of view, in at least one of holographic gain, collimation and centering. Collimation is provided by at least one fresnel element, which can have a focal length that varies from a center point to the edges for improving edge brightness by directing light rays more inwardly toward a center axis. The center point can be displaced from the screen center, especially vertically, and the rate of change in fresnel focal length can be different proceeding in opposite directions or in different planes from the center point. Preferably the fresnel elements are linear lenticular fresnels.

Description

TELEVISIONS OF PROJECTION WITH HOLOGRAFIC SCREENS THAT HAVE VARIATIONS FROM THE CENTER TO THE SHORE BACKGROUND Field of the Invention This invention relates generally to the field of projection television receivers, and in particular to projection television receivers having screens that provide significantly reduced color change and / or significantly cabinet depth. reduced.

Background Information Color change is defined as the change in the ratio of red / blue or green / blue of a white image formed to the center of a projection screen by projected images from red, green and blue projection tubes, when Go to different angles in the horizontal plane, using the observations made to the peak brightness of the vertical angle of view. The problem of color change is caused by the need for at least three image projectors for respective images of different colors, for example, red, blue and green. A projection screen receives images from at least three projectors on a first side and exposes the images on a second side with controlled light scattering of all the exposed images. One of the projectors, usually green and usefully in the center of the array of projectors, has a first optical path in an orientation substantially orthogonal to the screen. At least two of the projectors, usually red and blue, and commonly placed on opposite sides of the central green projector in the array, have respective optical paths converging towards the first optical path in a non-orthogonal orientation defining incident angles. The color change results from a non-orthogonal relation of the red and blue projectors, in relation to the screen and the green projector. As a result of the color change, the color tones may differ in each position on the screen. The condition in which the difference in color tone is large is commonly referred to as poor white uniformity. The smaller the color change, the better the uniformity of white. The color change is denoted by a number scale, in which the lower numbers indicate less color change and better white uniformity. According to a common procedure, the values for the luminance of red, green and blue are measured at the center of the screen from a variety of horizontal viewing angles, usually from at least approximately -40 ° to + 40 °, as much as approximately -60 ° to + 60 °, in increments of 5 ° or 10 °. The positive and negative angles represent horizontal viewing angles to the right and left of the center of the screen, respectively. These measurements are taken at the peak vertical angle of view. The red, green and blue data are normalized to the unit at 0o. In each angle, one or both of the following equations (I) and (II) are evaluated: red (6) C (0) = 20 »log10 (); (I) blue (?) green (?) C (?) = 20 «log10 ();; Blue ID (?) where ? is any angle within a range of horizontal viewing angles, C (0) is the color change in the angle? , red (0) is the red luminance level at an angle? , blue (0) is the level of blue luminance in the angle? and green (?) is the level of green luminance at an angle? . The maximum of these values is the color change of the screen. In general, the color change should not be greater than 5, nominally, in any commercially acceptable screen design. Another engineering and design constraint may sometimes require that the color change be somewhat higher than 5, although said embodiment of color change is undesirable and usefully results in a significantly lower image with poor white uniformity.

The screens for projection television receivers are generally manufactured by an extrusion process using one or more pattern rolls to form the surface of a thermoplastic sheet material. The configuration is usually a set of lenticular elements, also referred to as lenticles and contact lenses. The lenticular elements can be formed on one or both sides of the same sheet material or on one side only of different sheets which can then be combined permanently as a laminated unit or can be otherwise assembled one adjacent and the other to function as a unit laminated In many designs, one of the surfaces of the screen is configured as a Fresnel lens to provide light diffusion. Prior art efforts to reduce color change and improve white uniformity have focused exclusively on two aspects of the screen. One aspect is the shape and arrangement of the lenticular elements. The other aspect is the degree to which the screen material, or portions thereof, is added with light diffusion particles to the control light diffusion. These efforts are exemplified by the following patent documents. In U.S. Patents Nos. 4,432,010 and 4,536,056, a projection screen includes a lenticular sheet of light transmission having an entrance surface and an exit surface. The entrance surface is characterized by horizontal diffusion lenticular profiles that have a ratio of a lenticular depth of Xv to a radius of curvature near the axis Rl (Xv / Rl) which is within the range of 0.5 to 1.8. The profiles are extended along the optical axis and form lenticular lenses with aspheric input. The use of a screen with double-sided lenticular lenses is common. The aforesaid screen has cylindrical input lenticular elements on an input surface of the screen, cylindrical lenticular elements formed on an output surface of the screen and a light absorption layer formed in the non-convergent part of light of the output surface. The lenticular input and output elements each have the shape of a circle, ellipse or hyperbola and are represented by the following equation (III): Cx2 Z (x) = (III) 1+ [1- (K + l) C2x2] where C is a principal curvature and K is a conical constant. In an alternative way, the lenses have a curve to which a term with an order higher than the second order.

In the screens that make use of these double-sided lenticular lenses, it has been proposed to specify the positional relationship between the input lens and the output lens, or the lenticular elements that make up the lenses. It has been taught, for example, in U.S. Patent No. 4,443,814, to place the input lens and the output lens in such a way that the surface of one lens is present at the focal point of the other glasses. It has also been taught, for example in Japanese Patent Number 58-59436, that the eccentricity of the input lens is substantially equal to a reciprocal of the refractive indices of the material constituting the lenticular lens. It has also been taught, for example in U.S. Patent No. 4,502,755, to combine two sheets of double-sided lenticular lenses in such a way that the planes of the optical axis of the respective lenticular lenses are at right angles one to the other. another, and forming these double-sided lenticular lenses such that the input lens and the output lens at the periphery of one of the lenses are asymmetric with respect to the optical axis. It is also taught, in U.S. Patent No. 4,953,948, that the position of the convergence of light only in the valley of an input lens must be compensated toward the viewing side from the surface of an output lens. so that the alignment tolerance of the optical axis and the difference in thickness can be made larger, or it can be made smaller. In addition to the different proposals to decrease the color change or the lack of uniformity of the target, other proposals to improve the performance of the projection screen are aimed at making the images bright and ensuring appropriate visual fields in both horizontal and vertical directions. A summary of many of these proposals can be found in United States Patent Number 5,196,960, which teaches a sheet of double-sided lenticular lenses comprising an input lens layer having an input lens, and an output lens layer having an output lens whose lens surface is formed at or near the point of convergence of input lens light, wherein the input lens layer and the the output lens are each formed of a substantially transparent, thermoplastic resin and at least the output layer contains fine particles of light diffusion, and where there is a difference exists in the properties of light diffusion between the lens layer input and the output lens layer. A plurality of input lenses comprise a cylindrical lens. The output lens is formed of a plurality of output lens layers, each having a lens surface at the point of convergence of the light of each lens of the input lens layer, or in the vicinity of the lens. same A light absorption layer is also formed in the non-converging part of light of the output lens layer. It is said that this screen design provides sufficient horizontal visual field angle, a reduced color change and a brighter image, as well as ease of fabrication through extrusion processes. Several additional brilliance problems arise due to the nature of the projection systems. One of the most common performance issues of a projection television design is the relative difference in brightness between the edges of the screen and the center of the screen under comparable degrees of illumination. Normally, the image in the corners is not as bright as in the center of the image. The difference in relative brightness occurs partially because the optical path is shorter from the projectors to the center of the screen, that from the projectors to the edges of the screen. The difference also occurs partially because the projectors are generally oriented towards the center of the screen, converging their beams normally in the center. The projectors therefore illuminate the edges and the corners with less intensity of light and less directly than in the center. One method to deal with the brightness of the shore is to use a fresnel lens behind the diffuse or lenticular panel of the screen. The fresnel lens is a collimating lens that has a focal length equal to the axial distance between the collimator lens and the pupil of the projection lens. The purpose is to redirect the light beams diverging from the projectors such that the beams along the projection axis of each projection tube emerge from the screen, parallel to the axes. A fresnel lens is subdivided into ridges that are progressively more inclined towards the edges of the lens, having a slope substantially equal to the slope of a solid collimator lens, the specific angles of the flanges being chosen in such a way that the refraction at the interfaces of air / glass (or air / plastic) on the surface of the lens that deflects the beams in the required direction. In particular, the beams that diverge from the center axis of the screen bend inward toward the axis of the center to emerge parallel to the axis of the center. This requires a larger progressive refraction on the edges of the screen and no refraction at the center. It is known, in a conventional projection screen, to increase the focal length of the fresnel rims proceeding outward from the center of the image. Beams of light outside the axis on the edges of the screen curve beyond parallel to the axis of the center, and are directed somewhat inward towards the axis of the center. This makes the edges of the image look brighter, as long as the screen is visible along the axis of the center, but it is not useful for viewing from other positions. Another problem of brightness variation may occur in projection televisions where a fresnel is configured to direct light in the direction of a user's view from a point above the center of the screen, for example on a television. projection that has a relatively low cabinet. This is done by compensating the centerline of the fresnel upwards in relation to the center of the screen. Although, this can improve the relative brightness, especially in the corners, the upper part of the screen also appears generally brighter than the bottom of the screen. Despite the many years of impetuous development in the design of projection screens, the improvements at the most have increased. On the other hand, there has been no success in overcoming certain benchmarks. The angle of incidence defined by the geometric configuration of the image projectors, referred to herein as the ce angle, has generally been limited to the range of more than 0o and less or equal to approximately 10 ° or 11 °. The size of the image projectors and / or their optics, essentially makes the angles of a close to 0. In the range of the angles of a less than about 10 ° or 11 °, the best performance of the color change that is has achieved is approximately 5, as determined in accordance with equations (I) and (II). In the range of angles greater than 10 ° or 11 °, the best color change performance that has been achieved is not commercially acceptable. In fact, it is not known that they have commercialized projection television receivers that have angles greater than 10 ° or 11 °. The small angles of. they have a significant and undesirable consequence, namely, a very large cabinet depth is required to accommodate a projection television receiver. The large depth is a direct result of the need to accommodate optical paths that have small incidence angles (a). For a given size of the image projectors and optical elements, the angle of incidence can be reduced only by increasing the length of the optical path between the image projectors or their optic and the screen. Techniques for reducing the size of projection television cabinets generally rely on mirrors to fold long optical paths. The success of these efforts in the change of color is finally limited because there is a low limit for the range of possible angles of incidence. Polaroid Corporation sells a photopolymer designated as DMP-128®, which Polaroid Corporation can manufacture as a three-dimensional hologram, using patented processes. The holographic manufacturing process is described, in part, in U.S. Patent Number 5,576,853. Holographic photopolymers are usually useful for recording photographic images by dividing coherent light into a beam of illumination and a reference beam. The ray of illumination radiates to the subject. The ray reflected from the subject and the reference beam, which avoids the subject, irradiate the medium of the photopolymer, which contains a sensitive photographic composition of light that can be revealed. The light waves of the two beams interfere, that is, by constructive and destructive interference they produce an erect wave pattern of sinusoidal peaks which locally expose the photographic composition, and null that do not locally expose the composition. When the photographic medium is revealed, a corresponding interference pattern is recorded in the medium. By illuminating the medium with a coherent reference beam, the subject's image is reproduced and can be viewed over a range of apparent angles. The pattern of recorded interference of a hologram representing a typical photographic subject is complex, because light from all points illuminated in the interference of the subject with the interference beam at all points of the hologram. It would be possible by recording the image of a blank "subject" (effectively by interference of two interference beams), to make a blank hologram, where the interference pattern is more regular. In that case, the interference pattern would resemble a diffraction grating but the resolution of the diffraction grating would be very fine compared to the resolution of a projection screen having macro-sized lenticular elements formed to bend or refract the light in a particular direction from the rear projection tubes. A three-dimensional holographic screen for a projection television was proposed by Polaroid Corporation, as one of the many suggestions made during efforts to establish a market for the photopolymer DMP-128® holographic product. The proposal was based on the advantages that Polaroid Corporation expected in terms of superior brightness and resolution, lower manufacturing costs, lower weight, and resistance to abrasion to which two-piece screens would be subjected during shipment. Polaroid Corporation never proposed a particular holographic configuration for holographic elements in volume which could form a holographic projection television screen, and never even considered the problem of color change in projection television screens of any kind, holographic or otherwise. way. Especially, despite years of intense development to provide a projection television receiver that had a screen with color change of less than 5, even significantly less than 5, or that had a color change as low as 5 for IO angles even greater than 10 ° or 11 °, there has been no progress in solving the problem of color change more than with the increasing changes in the shapes and positions of the lenticular elements and diffusers in conventional projection screens. On the other hand, despite the suggestions that three-dimensional holograms could be useful for projection screens, although for reasons that have nothing to do with the color change, there has been no effort to provide projection televisions with three-dimensional holographic screens. A felt need for a long time has been unsatisfied, a projection television receiver that has a significantly improved color change performance, in the performance of the color change, which can also be integrated into a significantly smaller cabinet.

COMPENDIUM A projection television receiver according to the configurations of the invention taught herein, provides such a significant improvement in the performance of the color change, measured in orders of magnitude, that a color change of 2 or less is achieved. with projection television receivers that have angles of incidence a in a range of less than 10 ° or 11 °. On the other hand, the performance of the color change is so significant that commercially acceptable projection television receivers having angles of incidence of up to about 30 ° can be provided in much smaller cabinets. The performance of the color change of these wide-angle receivers is at least as good as that of conventional receivers from small angles, for example having a color change of 5, and can be expected to approach or they even reach values as low as approximately 2, as in small-angle receivers. These results are achieved by completely abandoning the extruded lens screen technology. In contrast, a projection television receiver according to a configuration of the invention has a screen formed by a three-dimensional hologram formed on a substrate, for example, ® a polyethylene film, such as Mylar. This three-dimensional holographic screen was originally developed for its expected advantages in terms of higher brightness and resolution, and lower manufacturing cost, lower weight and resistance to abrasion to which two-piece screens are subjected, for example during their shipment. The discovery of the color change performance of the three-dimensional holographic screen was presented when the test to determine if the optical properties of the three-dimensional screen would be at least as good as a conventional screen. The performance of the color change of the three-dimensional holographic screen, as measured by equations (I) and (II), was so unexpectedly low that it was surprising. The barriers that limited the improvements of the prior art to the steps in increments had been eliminated altogether. Moreover, smaller cabinets with projection geometry characterized by larger incidence angles can now be developed. A projection television having the unexpected properties associated with the three-dimensional holographic screens, and in accordance with the configurations of the invention taught herein, comprises: at least three projectors images for respective images of different colors; a projection screen formed by a three-dimensional hologram arranged on a substrate, the screen receiving images from the projectors on a first side and exposing the images on a second side, with controlled light scattering of all the exposed images; one of the projectors having a first optical path in an orientation substantially orthogonal to the screen and having at least two of the projectors having respective optical paths converging towards the first optical path in a non-orthogonal orientation defining angles of incidence and representing the Three-dimensional hologram A three-dimensional arrangement of lenticular elements that have an effective configuration for reducing the color change in the exposed images, the screen having a color change less than or equal to approximately 5 for all angles of incidence in a range greater than 0o and less than or equal to approximately 30 °, as determined by the maximum value obtained from at least one of the following expressions: red (0) = 20 * log1 (blue i?) green (0) C (?) = 20 «log10 () blue (0) where 0 is any angle within a range of horizontal view angles, C (0) is the color change in the angle? , red (0), is the red luminance level at angle 0, blue (0) is the blue luminance level at the angle 0, and green (0) is the green luminance level at the angle. It can be expected that the color change of the screen is less than 5, for example, less than or equal to about 4, 3 or even 2. In terms of the known barrier at an angle of incidence of about 10 ° or 11 °, the color change of the screen is less than or equal to approximately 2 for all incident angles in first subrange of incident angles greater than 0o and less than or equal to approximately 10 °, and, the screen color change is lower or equal to about 5 for all angles of incidence in a second subrange of angles of incidence greater than about 10 ° and less than or equal to about 30 °. The screen further comprises a light-transmitting reinforcing member, for example, of an acrylic material in a layer having a thickness in the range of about 2 to 4 millimeters. The substrate comprises a highly durable transparent water repellent film, such as a polyethylene terephthalate resin film. The substrate can be a film having a thickness in the range of about 25.4 to 254 microns. It has been found that a thickness of about 177.8 microns provides adequate support for the three-dimensional hologram. The thickness of the film is not related to performance. The three-dimensional hologram has a thickness in the range of no more than about 20 microns. The projection television can also comprise one or more mirrors between the projectors of the image and the screen. In accordance with an aspect of the invention, a projection screen is specifically configured to improve its brightness and uniformity over a wide range of angles of incidence of the projection beams. This is carried out using a holographic screen as described, which has a substantially higher gain proceeding towards the edges, backed by one or more linear panels of fresnel that contain flanges that vary progressively in focal length from the center to the edges. The fresnel panels on the screen can also have modestly different focal lengths between the bottom and top of the screen to optimize the brightness and uniformity of the screen from the perspective of the viewer located outside the central axis of the screen.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic representation of a projection television according to the configurations of the invention taught herein. Figure 2 is a simplified diagram of the geometry of the projection television useful for explaining the configurations of the invention. Figure 3 is a side elevation of a reinforced projection screen according to the configurations of the invention. Figure 4 is a schematic representation of an alternative mode of a projection screen with three linear lines and a hologram. Figure 5 is a schematic representation of the effects of variations in holographic gain and fresnel focal length across the length and width of the screen, along with the compensation of the gain centers and / or the variation of the focal length. Figure 6 is a graphical representation of the effect of variations in the focal length of the linear fresnel lens in the apparent image.

DESCRIPTION OF PREFERRED MODALITIES Figure 1 shows a projection television receiver. An array 12 of projection cathode beam tubes 14, 16 and 18 provide red, green and blue images, respectively. The cathode beam tubes are provided with respective lenses 15, 17 and 19. The projected images are reflected by a mirror 20 on a projection screen 22. Additional mirrors may also be used, depending on the particular geometry of the optical paths. The green cathode beam tube 16 projects the green image along an optical path 32, which in this example is oriented substantially orthogonal to the screen 22. In other words, the center line of the optical path is at right angles to the screen. The red and blue cathode beam tubes have optical paths respectively 34 and 36, which converge towards the first optical path 32 in a non-orthogonal orientation defining angles of incidence a. The angles of incidence present the problem of color change. The screen 22 comprises a three-dimensional hologram 26 arranged on a subtracted 24. The hologram 26 is an impression of a master hologram that substantially forms a diffraction pattern that handles the distribution of light energy from the three projectors 14, 16, 18, and it can be made variable across the width and / or height of the screen. In a preferred configuration, the hologram is a "central only" hologram that tends to reorient incident light. The screen receives images from the projectors on a first side of the entrance surface 28, and exposes the images on a second side of the exit surface 30 with controlled light scattering of all the exposed images. The substrate is preferably a highly durable, transparent, water repellent film, such as a polyethylene terephthalate resin film. One of these films is obtained in E.I. du Pont de Nemours & Co. under the Mylar® trademark. The substrate of the film has a thickness in the range of about 25.4 to 254 microns, equivalent to about 0.001 to 0.01 inches or about 25.4 to 254 microns. A film having a thickness of about 177.8 microns has been found to provide adequate support for the three-dimensional hologram disposed thereon. The thickness of the film does not affect the performance of the screen in general or the performance of the color change in particular, and films of different thicknesses can be used. The three-dimensional hologram 26 has a thickness of no more than about 20 microns. Three-dimensional holographic screens are available in at least two sources. Polaroid Corporation uses a proprietary wet chemical process to form a three-dimensional hologram in its DMP-128 photopolymer material. The process includes the formation of a holographic diffraction pattern in the photopolymer material, whose pattern can include variations in the gain of the screen through the range of horizontal and / or vertical view angles. A master hologram can be prepared by exposing the holographic photopor medium to coherent light including a reference beam and a ray reflected from a flat pattern having light-to-dark variations corresponding to the desired variation in gain. A preferred embodiment of the three-dimensional holographic screens used in the projection television receivers described and claimed herein was manufactured by the wet chemical process of Polaroid Corporation, in accordance with the following performance specifications: Average viewing angle horizontal: 38 ° ± 3 °. Vertical average viewing angle: 10 ° ± 1 °, Screen gain: >; 8, Change color: < 3, where the horizontal and vertical viewing angles are measured conventionally, the gain of the screen is the quotient of the intensity of the light directed from the source towards the rear of the viewing surface, and of the intensity of the light from the front part of the viewing surface towards the viewer, measured orthogonal to the screen, and the color change is measured as described above. The extraordinary performance of color change of the three-dimensional holographic screen was, as explained in the Compendium, totally unexpected. Figure 2 is a simplified simplified television projection diagram, omitting the mirror and lenses, to explain the performance of the color change. The optical axes 34 and 36 of the red and blue cathode beam tubes 14 and 18 are aligned symmetrically at angles of incidence a with respect to the optical axis 32 of the green cathode beam tube 16. The minimum depth D of a cabinet determined by the distance between the screen 22 and the edges of the back of the cathode beam tubes. It will be appreciated that if the angle a were to be reduced, the cathode tubes should be placed closer together and / or be further apart from the screen to provide tolerance for the tubes. At a sufficiently small angle, interference can not be avoided. This undesirably increases the minimum depth D of a cabinet. Conversely, as the angle becomes larger, the cathode beam tubes can be moved closer to the screen 22, reducing the minimum depth D of a cabinet. On the viewing side of screen 22, two horizontal mid-view angles are designated as -ß and + ß. Together they define a total horizontal view angle of 2ß. The average viewing angles can typically be in the range of + 40 ° to + 60 °. Within each average angle there is a plurality of specific angles f, in which it can measure and determine the color change according to the equations (I) and (II) explained above. In terms of the known barrier at an angle of incidence of approximately 10 ° or 11 °, the color change of the three-dimensional holographic screen is less than or equal to approximately 2 for all angles of incidence in a first subrange of higher incidence angles that 0o and less than or equal to approximately 10 °; and the color change of the screen is less than or equal to about 5 for all incident angles in a second subrange of incident angles greater than about 10 ° and less than or equal to about 30 °. It is expected that a color change less than or equal to about 2 can also be achieved, as in the first subrange, in the second subrange of larger incident angles. Referring to Figure 3, the substrate 24 comprises a transparent film, such as Mylar®, as described above. The photopolymer material from which the three-dimensional hologram 26 is formed is supported on the film layer 24. A suitable photopolymer material is DMP-128. The screen 22 may further comprise a transmissive light-enhancing member 38, for example, of an acrylic material, such as polymethyl methacrylate (PMMA). Polycarbonate materials can also be used. The reinforcing member 38 is currently a layer having a thickness in the range of about 2 to 4 millimeters. The screen 22 and the reinforcing member are adhered to one another over the entire mutual boundary 40 of the holographic layer 26 and the reinforcing member 38. Adhesive, radiation, and / or thermal bonding techniques can be used. The surface 42 of the reinforcing layer can also be treated, for example by one or more of the following: inking, anti-gloss coatings, and anti-scratch coatings. Various surfaces of the screen and / or its constituent layers can be provided with other optical lenses or lenticular arrays to control aspects of the projection screen by relying on performance characteristics different from the performance of the color change. These aspects can be made complementary with the characteristics of the holographic screen. Figure 4 illustrates a first variation, where a stacked circular holographic element 26 is superimposed with linear fresnel elements. In this embodiment, a horizontally active fresnel 29 (lenticular vertically) and a vertically active fresnel 31 (horizontally lenticular) are provided. Stacking allows separate handling of horizontal and vertical collimation, and linear fresnels can be less expensive than circular ones. On the other hand, one or more linear fresneles provide one more degree of freedom, as illustrated in Figure 5. It is desired that the brightness of the exposure be as uniform as possible from all angles and at all points on the screen . Accordingly, a circular fresnel can be centered on the screen and a focal length equal to the distance between the exit pupil of the projection tubes and the screen can be provided. The fresnel directs the light from the projection tubes perpendicular to the surface of the screen without taking into account the angle at which the light arrives. That is shown in a rough way by the fine lens equation: 1 f where s is the distance from the exit pupil to the screen; s' is the distance from the screen to the apparent "image"; and f is the focal length of the fresnel. If s = f, then s' tends to infinity, and an image apparently at infinity implies that the beams of light that come out of the screen are parallel. It is known to provide a continuous variation in the focal length from the center of the screen to the edges as a means to improve the brightness of the edges of the screen compared to the center, effectively directing the light from the edges of the screen more towards inside the central axis of the screen. In the fine-lens equation, for example, if one places a variable d representing the difference in the increase in focal length between two points in a fresnel (for example, from the center to the edge) and substitutes f + d for f, the following solution shows the effect on f, the distance to the apparent image: 1 + - f + d . { - t¿ + f d) s' = - This function is illustrated in Figure 6, and shows that the apparent image becomes closer as the focal length decreases. In the case of a circular fresnel, the correction amount can be optimized in all directions to the outside from the center of the screen. However, the aspect ratio of a screen is generally wider in the horizontal direction (4: 3 or 16: 9), so that a greater correction is needed to optimize the edges of the horizontal screen. With a linear fresnel element and a horizontal fresnel element, all the power of the vertical and horizontal elements can be used to move the light inwards, towards the axis in the respective vertical and horizontal planes. The variation of the focal length in the vertical and horizontal directions from the screen to the edges can be done at different indexes. The result is a minor axis and improved corner lighting compared to a circular fresnel. Referring to Figure 5, a further degree of freedom with linear fresnels is that the vertical and horizontal directions can be independently centered. In general, it is advantageous if the appearance of the screen is symmetrical through the range of horizontal viewing angles. However, with respect to the vertical, it may be desirable to have more than one angle from above or from one angle from below, for example, in a projection screen mounted on a floor or ceiling, respectively. In order to accommodate the vertical vision compensation, a linear vertical fresnel can be compensated in the required direction, while the horizontal fresnel remains centered. One drawback is that the brightness of the screen is higher near the top of the screen than near the bottom. A conventional fresnel is symmetric around its center (whether or not the center is distributed on the screen). According to a further aspect of the invention, the vertical fresnel can vary in focal length at different rates proceeding upwards from the center rather than downwards. A modestly different focal length at the top and bottom of the screen balances the difference in brightness caused by vertical compensation of the center point of the fresnel, providing more uniform brightness. According to a further aspect of the invention, the differences in brightness at the center of the edge are compensated by the corresponding variations in the gain in holographic screen element 26. The following measurements were made to compare the brightness of the center and the edge of two holographic screens that do not have gain variation of 14.8 and 22.5 respectively, with a holographic screen that had a gain of 14.8 in the center and 22.5 in the edges. The points are identified using clock face numbering to distinguish the points of the shore as the major and minor axes, and the brightness measurements are in spark plugs.

Example 1. Non-Variable Holographic Gain of 14.8 POINT W% of Ctr Means ctr 115.9 100.00 Axis Major 3 22.1 27.70 25.19 9 26.3 22.69 Axis Minor 6 38.1 32.87 39.52 12 53.5 46.16 Corner 2 11.7 10.09 7.74 4 8.3 7.16 8 6.4 5.52 12 9.5 8.20 Example 2. Non-Variable Holographic Gain 22.5 POINT W% of Ctr. Means ctr. 172. .7 100.00 Ex e Major 3 55. .8 32.31 28.84 9 43, .8 25.36 Axis Minor 6 63, .3 36.65 41.40 12 79. .9 46.15 Corner 2 18. .6 10.77 8.31 4 13. .3 7.70 8 10. .8 6.25 12 14. .7 8.51 Example 3 Variable Holographic Gain 14.8 in the Center and 22.5 in the POINT% of Ctr. Means ctr 115.9 100.00 Axis Major 3 55.8 48.14 42.97 9 43.8 37.79 Axis Minor 6 63.3 54.62 61.69 12 79.7 68.77 Corner 2 18.6 16.05 12.38 4 13.3 11.48 8 10.8 9.32 12 14.7 12.68 In the previous examples you can see that as a problem of proportions and considering the average brightness of the opposite edges, varying the gain of the holographic screen in this problem produces an improvement in the brightness on the edges of the major axis (3 and 9 o'clock) of 72 percent, measured as the improvement in the Brilliance of the bank as a proportion of central brilliance, an improvement in the edge of the minor axis (6 and 12 o'clock) of 55 percent and an improvement of the extreme corner of 50 percent. The center prior to the variations of the bank are useful individually and in combination, and can be incorporated, for example, as multi-layer screens.

Claims (15)

CLAIMS:

1. A projection television, which comprises: a plurality of image projectors (14, 16, 18) for respective images of different colors; and a projection screen (22) formed by overlapping a linear fresnel element (29, 31) on a hologram (26), this hologram (26) being arranged on a substrate (24) superimposed on at least one light transmitting panel (38) , the screen receiving images from the projectors (14, 16, 18) on a first side and exposing the images on a second side with a controlled light scattering of the exposed images, the screen (22) forming an interference arrangement with properties optics that vary horizontally and vertically through a field of view, varying these optical properties at least one of the holographic gain, collimation, and centering.

2. The projection television of claim 1, wherein the screen (22) comprises at least one hologram (26) and at least one fresnel element.

3. The projection screen of claim 2, wherein the hologram (26) and the fresnel element have optical properties that vary in at least one of the circular, vertical, and horizontal directions, between a central point and the edges of the screen.

4. The projection screen of claim 3, wherein the center point corresponds to a center of the screen.

5. The projection screen of the claim 3, where the center point moves vertically from a center of the screen.

6. The projection screen of claim 3, wherein the optical properties of the hologram (26) and the fresnel element vary separately between the center and the edges.

7. The projection screen of the claim 6, which comprises at least two fresnel elements (29, 31) stacked behind the hologram (26), and wherein at least one of these two fresnel elements (29, 31) comprises a linearly lenticulate panel.

8. The projection screen of the claim 7, wherein the at least two fresnel elements (29, 31) have variable optical properties through the vertical and horizontal lapses of the field of view, respectively.

9. The projection screen of the claim 8, wherein the at least two fresnel elements (29, 31) have a variable focal length through the vertical and horizontal lapses of the field of view, respectively. The projection screen of claim 9, wherein the fresnel elements (29, 31) center horizontally and move vertically. 11. The projection screen of claim 3, wherein the hologram (26) has an increasing gain that proceeds from the center point toward the edges. The projection screen of claim 11, wherein the gain varies between about 14.8 at the center point, and 22.5 at least at the edges at one of a vertical and one horizontal plane. 13. The projection screen of the claim 11, wherein the gain varies from the center point towards the edges in a pattern corresponding to an aspect ratio of the screen. 14. The projection screen of claim 11, wherein the center point corresponds to a center of the screen. 15. The projection screen of claim 11, wherein the center point moves vertically from a center of the screen. SUMMARY A projection screen (22) for a television has a three-dimensional hologram (26) on a film substrate (24). The screen (22) may have a plurality of stacked holograms and / or presnel lenses having variable optical properties in the vertical and horizontal directions. The image projectors (14, 16, 18) for different colors, direct images that converge in a projection screen (22) formed by at least one hologram (26) on a light transmitting panel, which collects the converging light from the projectors (14, 16, 18) on the back, and which exposes the images on a front with controlled light scattering to orient the images substantially forward. The holographic screen (22) forms an interference array, and has optical properties that vary horizontally and vertically through the field of view, at least one of the holographic gain, collimation, and centering. The collimation is provided by at least one Fresnel element, which can have a focal length that varies from a central point towards the edges, to improve the brightness on the shore, by directing light rays more inwards, towards an axis central. The center point can be moved from the center of the screen, especially vertically, and the speed of change in the focal length of the fresnel can be different proceeding in opposite directions or in different planes from the center point. Preferably, fresnel elements are linear lenticular fresnels.