Rear projection screen and method of manufacturing the same

Abstract: A screen is made having the same characteristics as of a known lenticular lens by having a number of organic mate­ rials provided in scatter arrangement within a substrate of a screen and shaped into bar-like configuration by stretch­ ing for the purpose of anisotropic diffusion of light.

Description: The present invention relates to a high quality rear projection screen for use with a transmitting type (rear-view) projection television. It is essential for a rear projection screen employed as the viewing screen of a rear-projection television to have a wider viewing angle and provide a projected image of high resolution and brightness. Such a rear projection screen has a substrate containing a mixture of light diffusing medium for rendering the projected light visible. The rear projection screen also incorporates a lenticular lens(es) provided on the surface thereof for anisotropic diffusion of the projected light while enhancing the luminance of an image throughout the viewing angle. Additionally, a Fresnel lens which serves as a field lens to the flux of projected light, is used in combination with the projection screen for minimizing decline in the luminance at the edge region of the projection screen. For enhancement of the resolution of the rear projection screen, it is practical to have the substrate section of the same, which contains the light diffusing medium for rendering the projected light visible, reduced in the thickness and allow the lenticular lens provided on the surface to reduce in the lens pitch. A method of thinning the substrate section containing light diffusing medium is disclosed in Japanese Patent Laid-open Publication 63-273850(1988) in which a thin film of substrate is formed from a mixture of light diffusing medium and resin and attached to the surface of a rear projection screen. However, it is still difficult in practice to have a lenticular lens of narrower lens pitch adapted for anisotropic diffusion of the projected light to provide high resolution. For example, it becomes remarkably intricate to fabricate forming molds of providing a lenticular lens on the surface of the rear projection screen. Also, the useable life of such complex forming molds is hardly extended in the relation to forming shots, which causes the overall cost to increase. A known transmitting type projection television is generally arranged in which an image developed on the phosphor screen of a cathode ray tube is projected through a projection lens onto a large-size rear projection screen. It is also known that liquid crystal panels are used as light valves to constitute a projector in a transmitting type projection television. More particularly, when patterns of image information are developed on the liquid crystal panels by means of variation in the transmittance according to image signals, the light incident on the back of the liquid crystal panels is modulated to produce a visual image on the rear projection screen through projection by a projection lens. In this method, each of the liquid crystal panels comprises a plurality of very small light valve elements provided in a matrix arrangement and thus, an image projected through the projection lens onto the rear projection screen is constituted by a corresponding matrix form of pixels. The interaction between the pixel matrix and the lenticular lens of the rear projection screen will cause Moire effects, thus lowering picture quality. It is also necessary for visual effect that the pitch of the lenticular lens of the rear projection screen is considerably narrowed so as to ensure no decline in the picture quality. Similarly as described previously, it Becomes troublesome to fabricate forming molds of providing such a narrow-pitch lenticular lens on the surface of the rear projection screen. Also, the life of the forming molds cannot be kept long as the number of forming shots increases. As a result, the production cost will rise. According it is preferred that a rear projection screen for a transmitting type projection television should allow no Moire effect to result from the optical interaction between a Fresnel lens and a lenticular lens or from the interaction between the pitch of a matrix pattern of pixels formed on the rear projection screen and the pitch of a lenticular lens of the rear projection screen while ensuring high resolution. To accomplish the above, an arrangement is made in which a substrate of a transparent resin material constituting a rear projection screen contains a plurality of elongate members of another resin material different in refractive index from and not soluble in the transparent resin material, the elongate resin members being dispersed and orientated in the same direction. In practice, the projected light to the rear projection screen is thus distributed in anisotropic relationship by the elongate resin members having a different index of refraction from that of the transparent resin material and arranged in the same direction within the substrate of transparent resin material constituting the rear projection screen. At the same time, the projected light is diffused by the elongate resin members of very small diameter. As the result, a picture of high resolution can be obtained. Since the elongate resin members are respectively bound at random intervals with the transparent resin material or in the substrate of the rear projection screen, the rear projection screen with a Fresnel lens produces no visual annoyance of Moire effect. Similarly, in a transmitting type projec- tion television employing liquid crystal panels as light valves, no Moire pattern will appear on the rear projection screen. A similar arrangement is shown in US-A-4,566,756 which aims to provide a rear projection screen which optimises brightness, colour and contrast characteristics. The screen according to US-A-4,566,756 includes filamentary particles in the screen substrate which extend in one direction in the plane of the screen. The particles have a different refractive index from the substrate, but are not described as being a resin material. The present invention provides a method of manufacturing a rear projection screen, said screen being a planar sheet made of a first transparent resin material in which a plurality of elongate members made of a second transparent material are dispersed, said first and second transparent materials having different refraction indexes and being not soluble in each other and said elongate members being all substantially aligned in a direction parallel to the plane of said planar sheet, said method comprising the steps of: mixing particles of said second material with a fluid amount of said first material and forming the mixture obtained therewith into a planar shape in order to form a first sheet; said method being characterized in that: said second transparent material is a second resin material and said particles of said second material are beads of such second resin material; and by the step of stretching said first sheet formed of said mixture in one direction in order that said resin beads are shaped into an elongate form to constitute said elongate members dispersed in said first transparent material. As set forth above, the rear projection screen manufactured according to the present invention ensures the reproduction of a high resolution image and thereby, makes it possible to develop an advanced transmitting type projection television for reproduction of a higher quality picture, promoting the application to industrial uses. Fig.1 is a perspective view of a rear projection screen according to the present invention; Fig.2 is a diagram showing the characteristics of gain relative to a viewing angle on the rear projection screen; Figs.3 and 4 are schematic views showing the process of manufacturing at rear projection screen according to the present invention; Fig.5 is a cross sectional view describing the shape of elongate resin members 3 arranged in separate rows within a transparent resin material of the rear projection screen; Fig.6 is a schematic cross sectional view showing the arrangement of a transmitting type CRT projection television employing a rear projection screen of the present invention; Fig.7 is a schematic view showing a projector for a transmitting type projection television in which liquid crystal panels are used as light valves; and Fig.8 is a schematic cross sectional view showing the arrangement of a transmitting type projection television employing a rear projection screen of the present invention and the projector including liquid crystal panels used as light valves. A preferred embodiment of the present invention will be described in detail referring to the accompanying drawings. Fig.1 is a perspective view of a rear projection screen according to the present invention. The rear projection screen 1 of the present invention consists of a substrate of transparent resin material 2 which contains a plurality of elongate resin members 3 having a different index of refraction from that of the transparent resin material 2 and respectively arranged therein to direct in the same lengthwise direction. Accordingly, light projected onto the rear projection screen 1 is diffused by the elongate resin members 3, as also shown in Fig.1, from the Z-axis in the X-axis direction rather than the Y-axis direction. This is expressed in Fig.2 as the relation between gain and viewing angle on the rear projection screen of the present invention in which the curve 4 represents gain characteristics when the viewing angle is extended from the Z-axis in the X-axis direction and the curve 5 in the Y-axis direction. As apparent from Fig.2, the rear projection screen according to the present invention can provide the same effect as the anisotropic distribution of light by a known lenticular lens. Also, its functional arrangement for anisotropic distribution of light can easily be made compact in size, thereby making it possible to produce a high resolution rear projection screen. Figs.3 and 4 illustrate a method of manufacturing such an improved rear projection screen according to the present invention in which a resin mixture 6 produced by mixing a transparent resin material of transparent thermoplastic property with beads of another resin material different in refractive index from and not soluble in the transparent resin material, is infed from a hopper 7 to an extruder machine 8 and after molten by heat, transferred under pressure with a screw 9 to a dice 10. The resin mixture 6 is then flattened into a planar shape by the dice 10 and arranged to a predetermined width and thickness between a plurality of forming rolls 11 thermally controlled by a thermal regulator (not shown) in order to form a sheet 12. In the sheet 12 of resin mixture, the resin beads which are different in the refractive index from and not soluble in the transparent resin remain in the same spherical shape as before the process. Then, the sheet 12 is gripped with a plurality of clips 13 and stretched out by tension in the directions represented by the arrow 19. As the resin mixture is further shaped into a stretched sheet 14, the resin beads different in the refractive index from and not soluble in the transparent resin material turn into elongate members. The stretched sheet 14 is processed by a masking procedure to prevent surface injury and keep from impurities such as dirt and finally, cut to a given size to form a desired rear projection screen. It would be understood that the transparent resin material and the resin material of beads do not solve in each other in a heat molten condition so that each of the resin beads mixed with the transparent resin material which has a different index of refraction from that of the resin beads, can be formed into the elongate shape by stretching out a mixture sheet of the transparent resin material and the resin beads in one direction. It is also required that the refractive index is different between both the resin materials. From the result of an experiment by the inventor, it was found that the application of light remained efficient when such a difference of refractive index was, at the most, not more than 0.15 at the d line of 587.6 nm wavelength. More particularly, if the difference in the refractive index is greater than 0.15, reflection or total reflection of light occurs on the boundary surface between the two resin materials and thus, the transmittance of light or the efficiency of use of light will be decreased lowering the brightness of a reproduced image. To satisfy the foregoing requirements, the transparent resin material may be acrylic resin, polycarbonate resin, polyester resin, polyvinyle chloride resin, etc. The acrylic resin is most preferred in the form of a polymer consisting mainly of alkyl methacrylate such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, or the like. The resin bead is preferably of transparent silicone resin or allyl resin such as allyl methacrylane. Particularly, when the transparent resin material is acrylic resin, beads of denatured acryl resin with a high degree of refraction, polystyrene resin, or polycarbonate resin will be most desirable. With the complementary use of other light diffusing medium, e.g. organic or inorganic ultrafine particles of SiO₂, Al₂O₃, CaCO₃, or Al(OH)₃, which is added to and commingled with the transparent resin material for enhancement of the diffusion of light, the flexibility in designing an improved rear projection screen will be increased. Fig.5 illustrates the external shape of the bar-like resin member 3, the plurality of which are dispersed within the transparent resin substrate of the rear projection screen according to the present invention. The ratio of a lengthwise extent a and a widthwise extent b of the elongate resin member 3 is determined according to the characteristics of light distribution of a desired rear projection screen. In an experiment, the result was acceptable when the ratio (a:b) is greater than 3:1. Fig.6 is a cross sectional view of a transmitting type projection television 18 provided with the rear projection screen 1 of the present invention, in which an optical image developed on a CRT 17 is projected through a projection lens 16 and a reflector mirror 15 onto the transmission projection screen 1. In the rear projection screen of the present invention, the substrate of transparent resin material contains the plurality of elongate resin members having a different index of refraction from that of the transparent resin substrate and dispersed in the same orientation for the control of a viewing angle and thus, can allow the same functional elements as of a known lenticular lens to be arranged with much higher precision. Fig.7 is a schematic view showing an arrangement of a projector 24 for a transmitting type projection television employing liquid crystal panels as light valves, in which a liquid crystal panel 22 which produces an image corresponding to variations in the transmittance of light responsive to signals of image information, is illuminated from the back by an optical illumination system comprising a lamp 20 and a light condenser lens 21. The light passed across the liquid crystal panel 22 is thrown through a projection lens 23 to form an enlarged image on the projection screen as a focus plane. Fig.8 is a cross sectional view showing a schematic arrangement of a transmitting type projection television equipped with the rear projection screen 1 of the present invention and the foregoing projector 24 employing liquid crystal panels as light valves. The projected light from the projector 24 is reflected on a reflector mirror 15 and thrown to yield an optical image on the transmission projection screen 1. In the rear projection screen 1 of the present invention, the substrate of transparent resin material contains a plurality of elongate resin members which are different in the refractive index from the transparent resin substrate, arranged in the same direction, and specifically, spaced at random intervals from each other for the control of a viewing angle and thus, can allow no Moire effect to appear resulting from the interaction between the elongate members dispersed in the rear projection screen and the pattern of pixels on the rear projection screen corresponding to the matrix arrangement on the liquid crystal panel, ensuring the reproduction of a picture without decline in the picture quality.

Claims: A method of manufacturing a rear projection screen, said screen being a planar sheet made of a first transparent resin material in which a plurality of elongate members made of a second transparent material are dispersed, said first and second transparent materials having different refraction indexes and being not soluble in each other and said elongate members being all substantially aligned in a direction parallel to the plane of said planar sheet, said method comprising the steps of: mixing particles of said second material with a fluid amount of said first material and forming the mixture obtained therewith into a planar shape in order to form a first sheet; said method being characterized in that: said second transparent material is a second resin material and said particles of said second material are beads of such second resin material; and by the step of stretching said first sheet formed of said mixture in one direction in order that said resin beads are shaped into an elongate form to constitute said elongate members dispersed in said first transparent material. A method of manufacturing a rear projection screen according to claim 1, wherein said first resin material comprises an acrylic resin. A method of manufacturing a rear projection screen according to claim 1, wherein said first resin material comprises a polycarbonate resin. A method of manufacturing a rear projection screen according to claim 1, wherein said first resin material comprises a polyester resin. A method of manufacturing a rear projection screen according to claim 1, wherein said first resin material comprises a polyvinyl chloride resin. A method of manufacturing a rear projection screen according to claim 1, wherein each of said resin beads comprises a transparent silicone resin. A method of manufacturing a rear projection screen according to claim 1, wherein each of said resin beads comprises an aryl resin. A method of manufacturing a rear projection screen according to claim 2, wherein each of said resin beads comprises a denatured acrylic resin having a high refractive index. A method of manufacturing a rear projection screen according to claim 2, wherein each of said resin beads comprises a polystyrene resin. A method of manufacturing a rear projection screen according to claim 2, wherein each of said resin beads comprises a polycarbonate resin. A method of manufacturing a rear projection screen according to claim 1, wherein the difference of refractive index relative to the d line of 587.6nm wave length between said first resin material and said second resin material is at most 0.15. A method of manufacturing a rear projection screen according to claim 1, wherein said first sheet is stretched so far that a lengthwise extent "a" and a widthwise extent "b" of each of said elongate resin members result to be expressed as: a/b≧3. A method of manufacturing a rear projection screen according to claim 1, wherein said first resin material comprises a transparent thermoplastic resin.

Inventors: YAMAMOTO YOSHIHARU, YAMAMOTO, YOSHIHARU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G03B21
IPC codes: H04N5, H04N9, G03B21, B29D11