TRANSMISSION TYPE PROJECTION SCREEN

Abstract: A screen (1) formed with a plurality of lenticular lenses on both the surfaces is provided. There are formed on the incident surface a plurality of sets of incident side main lenticular lenses (L₁) and incident side auxiliary lenticular lenses (L₂, L₃), each set being arranged on both the sides of the main lenticular lens. Further, there are formed on the emission surface a plurality of emission side main lenticular lenses (L₄) and a plurality of pairs of emission side auxiliary lenticular lenses (L₅, L₆), each pair being arranged on both the sides of the main lenticular lens. The greater part of an incident light is incident from the incident side main lenticular lens (L₁) and is emitted from the emission side main lenticular lens (L₄) at a predetermined diffusion angle. The remaining incident light is incident from the incident side auxiliary lenticular lenses (L₂, L₃) and is emitted from the emission side auxiliary lenticular lenses (L₅, L₆) at a different angle. Thus, the combination of main and auxiliary lenticular lenses makes it possible to widen an angle of view and to suppress color shift, thus providing an improved contrast.

Description: BACKGROUND OF THE INVENTION The present invention relates to a transmission type projection screen having an incident surface and an emission surface, wherein a plurality of incident side main lenses are formed on said incident surface and a plurality of emission side main lenses are formed on said emission surface, each of said incident side main lenses being a cylindrical convex lens whose cross section on a plane perpendicular to its longitudinal axis contains a first curvature of circle, ellipse, parabola or other curves, and each of said emission side main lenses being a cylindrical convex lens whose cross section on a plane perpendicular to its longitudinal axis contains a second curvature of circle, ellipse, parabola or other curves. Transmission type projection screens, in which an image is projected from the rear side and the projected image is observed from the front side, have been known in the art. Transmission type projection screens of various configurations have been already proposed. Particularly, for the transmission type screens having been recently used for color television projectors, it is required to have not only a function essential to the screen to diffuse a light toward the side of an observer, but also a function to correct "color shift" produced resulting from the fact that light rays indicating colors of red (R), green (G) and blue (B) are projected from different positions. In addition, reduction of the external reflection is required for the screen which is used for the color television projectors, because the luminance of the color television projectors is not so high that, in a well-lighted room, the indoor or outdoor light is reflected on the screen surface so as to lower the contrast of the projected image, thus to damage visual recognizability. For the article to satisfy the above-requirement, there is known a lenticular sheet with black stripes comprising a large number of lenticules arranged in a vertical direction on both the surfaces of the sheet, and light absorption layers provided at portions from which light is not emitted of light emission surfaces which will be simply referred to as "emission surfaces" hereinafter. However, the drawback with the above-mentioned lenticular sheet is that an angle of view or visual field in a horizontal direction is limited to a range of 30 degrees in each of clockwise and counterclockwise directions with the optical axis being as the center, i.e., a narrow range of 60 degrees in total. On the other hand, in order to widen the horizontal viewing angle, there is also known a transmission type screen having a portion where a part of the incident light is totally reflected. However, since an element for correcting color shift is not included in the screen of this type, color shift becomes large. In addition, it is required for reducing the reflection of an external light to provide light absorption layers on the surface from which light is not emitted of the emission surface. Since, in the case of this type of screen, the surface from which light is not emitted is the one on which a part of incident light is totally reflected, it is necessary to provide light absorption layers on the total reflection surface. To realize this, it is required to first provide reflection layers on a surface which is to serve as a total reflection surface and to second provide light absorption layers thereon. In addition, since the surface from which light is not emitted generally exists on a concave portion, it is very difficult to provide respective layers on such a concave portion. EP-A-0 077 117 discloses a rear projection screen comprising cylindrical convex lenses on both the incident (projector-facing) side and the emission (viewer-facing) side, the lenses having the same radius of curvature in order to reduce colour shifting, and further comprising black painted projected portions on the emission side. SUMMARY OF THE INVENTION The present invention has been made for solving the above mentioned problems and its object is to provide a transmission type projection screen having a broader angle of view in a horizontal direction. Another object of the present invention is to provide a transmission type projection screen having less "color shift" produced resulting from the fact that light rays indicating three primary colors of R, G and B are reflected from different positions, respectively, in the case of projecting a color pictorial image using a color television projector. A further object of the present invention is to provide a transmission type projection screen having a structure easy to provide light absorption layers in order to reduce the reflection of an external light, thus making it possible to provide an image of high contrast. To achieve the above mentioned objects, the present invention provides a transmission type projection screen as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a horizontal cross section showing that projection is conducted from a color television projector to a projection screen according to the present invention; Fig. 2 is a horizontal cross section partially showing an embodiment of a projection screen according to the present invention; Fig. 3 is a diagrammatical view showing optical paths in a partially enlarged portion of the screen shown in Fig. 2; Fig. 4 is a horizontal cross sectional view partially showing that an absorption layers are provided on non-light emission portions of the screen shown in Fig. 2; Fig. 5 is a horizontal cross section partially showing a screen specially formed for providing absorption layers as a modified embodiment of the invention; Fig. 6 is a vertical cross section showing one manner of use of a projection screen according to the present invention combined with a Fresnel lens; Fig. 7 is a vertical cross section showing another manner of use of a projection screen according to the present invention combined with another Fresnel lens; and Fig. 8 is a graph showing the comparison between an angle of view of a projection screen according to the present invention and an angle of view according to a conventional screen. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a schematic view showing an example of use of a transmission type projection screen of the invention. As seen from this figure, in the vicinity of a transmission type projection screen 1 of the invention, a Fresnel convex lens sheet 2 for converging an incident light to a suitable angular range is arranged. The horizontal cross sections of both the projection screen 1 and the lens sheet 2 are illustrated in this figure, respectively. As in the example shown, it is ordinary that the projection screen 1 is combined with the Fresnel convex lens sheet 2, but the Fresnel convex lens sheet 2 may be omitted. Blocks labeled R, G and B represent three projection CRTs for projecting color information indicative of Red (R), green (G) and blue (B) of the color television projector, respectively. Lines drawn from the CRTs to the center of the lens represent rays of incident light, respectively. When a light vertically incident from the CRT for projecting the information indicative of green is assumed as reference, incident angles of incident rays of light indicative of red and blue deviate from the incident angle of the incident ray of light indicative of green by an angle indicated by ε. This angular deviation of the incident light is the cause that deviation of distribution based on color of a light emitted, i.e., color shift is produced. The angle ε is, e.g., approximately 10 degrees. Fig. 2 shows an enlarged cross section of the screen 1 wherein a portion encompassed by the solid line is a unit portion. A large number of unit portions are arranged in upper and lower directions as indicated by double-dotted lines in this figure, thereby forming a screen. The left surface of the screen 1 forms an incident surface S₁ and the right surface thereof forms an emission surface S₂. The incident surface S₁ is formed of two kinds of lenses. Namely, the incident surface S₁ comprises an incident side main lens L₁ occupying the greater part of the incident surface, and incident side auxiliary lenses L₂ and L₃ occupying the remaining portions of the incident surface. The lenses L₂ and L₃ are so arranged on both the sides of the lens L₁ that they are in contact with each other. The dimensions of the main lens L₁ and the auxiliary Lenses L₂ and L₃, hold a relationship such that the ratio of sum of the width of the lens L₂ (which is defined as one measured in a direction of the pitch P and the same definition will be applied hereinafter) and the width of the lens L₃ to the width of the lens L₁ is equal to 1/9 to 4/6. Accordingly, the lens L₁ has 90 down to 60% of the incident light in its charge, thus allowing it to be incident, and the lenses L₂ and L₃ have the remaining 10 to 40% of the incident light in their charges, thus allowing it to be incident. The emission surface S₂ is also formed of two kinds of lens portions. Namely, the emission surface S₂ comprises an emission side main lens L₄ in the center thereof and auxiliary lenses L₅ and L₆ arranged on the both sides of the main lens L₄, respectively. M₁ and M₂ denote portions from which light is not emitted. In this embodiment, these portions are formed planar. The dimensional relationship between the main lens L₄ and the auxiliary lenses L₅ and L₆ corresponds to the dimensional relationship between the incident side main lens L₁ and the auxiliary lenses L₂ and L₃. Since a light refracted by the lens L₁ is focused on the lens surface of the lens L₄, even if the width of the lens L₄ is smaller than that of the lens L₁, there is no inconvenience. When a screen for a television of 40 inches or more is assumed, the pitch P is 0.3 to 1.5 mm and the thickness t is 0.3 to 1.5 mm in the screen thus configured, although they change in dependence upon a dimension of the entirety of the screen or a distance between the screen and an observer. Fig. 3 is a schematic diagram showing the effects of respective portions constituting the screen 1, particularly the effects of the lenses L₁ and L₄ and the lenses L₂, L₃, L₅ and L₆ wherein the screen 1 itself is formed in the same manner as that in Fig. 2. In Fig. 3, solid lines with an arrow denote rays of light incident to the screen. In this instance, all rays of light are incident in parallel to the optical axis 3 of the lens L₁. Rays of light between broken lines 11 and 12 are incident to the lens L₂, rays of light between broken lines 12 and 13 are incident to the lens L₁, and rays of light between broken lines 13 and 14 are incident to the lens L₃. Initially, rays of light between broken lines 12 and 13 are refracted by the lenses L₁ and L₄ and then are emitted to the outside. The emission range can be changed by the shapes of the lenses L₁ and L₄. These lenses L₁ and L₄ may be formed as a portion of circle, ellipse or parabola, or other curves. In the case shown in this figure, the lens L₁ is formed as a portion of ellipse in which the ratio of the long diameter to the short diameter is equal to 1/0.733 and the lens L₄ is formed as a portion of ellipse in which the ratio of the long diameter to the short diameter is equal to 1/0.655. Light is emitted in a range of ±49 degrees with respect to the normal vertical to the screen. Although light is emitted in a range of ±49 degrees, since quantity of light emitted is reduced according as an angle increases, the observable range of the light emitted is equal to approximately 30 degrees in practical use. In addition, rays of light incident to the ranges between broken lines 11 and 12 and between broken lines 13 and 14 are respectively refracted by the lenses L₂ and L₅ and the lenses L₃ and L₆ and then are emitted to the outside. Also in this instance, as the diffusion range of the light emission varies in dependence upon the shapes of the lenses L₂ and L₃ and the lenses L₅ and L₆, basically rays of light respectively incident to the lenses L₂ and L₃ are refracted in a manner that they reach the optical axis 3 to arrive at the lenses L₅ and L₆, thereafter to be further refracted by the lenses L₅ and L₆ so that they reach the optical axis 3. The lenses L₂, L₃, L₅ and L₆ performing such an effect are formed as a portion of circle, ellipse or parabola, or other curves. In the case shown in this figure, the lenses L₂ and L₃ are formed as a portion of ellipse in which the ratio of the long diameter to the short diameter is equal to 1/0.512 and the lenses L₅ and L₆ are formed as a portion of circle wherein light is emitted in a range of +32 to +63 degrees and in a range of -32 to -63 degrees. Thus, the screen of the invention can widen an angle of view as compared to the conventional screen in which the incident and emission surfaces are formed with one kind of lens groups, respectively. The horizontal diffusion characteristic of a light emitted in the present invention can be varied not only by the shape of each lens but also by the ratio of rays of light incident to the incident side main and auxiliary lenses, i.e., the ratio of the dimension of the main lens to that of the auxiliary lens. Accordingly, the present invention permits a diffusion range of approximately 30 to 50 degrees to be bright or a range of approximately 0 to 50 degrees, which is ordinarilly regarded as the most desirable range, to be kept at a uniform brightness, or whatever desired. In the embodiment shown in Fig. 3, the ratio of rays of light incident to the incident side main and auxiliary lenses is equal to 1/0.23. Since the respective diffusion ranges of the light emission are as above, light is generally emitted uniformly up to a range nearly equal to 0 to 45 degrees. In connection with the screen having lenses on both the sides thereof, it has been known that it is possible to lessen color shift by designing and arranging lenses on the incident and emission sides in particular condition. To speak theoretically, if the surface from which light is emitted is formed as a portion of circle or ellipse or the like, the degree of refraction toward the lens axis increases in proportion to the distance from the center of the lens. Accordingly, when rays of light having different incident angles are incident to the screen, the difference between the emission angles becomes small as compared to the difference between incident angles. Also in the present invention it is possible to lessen, in the same manner, color shift of a light emitted from the main lens. In addition, because of the principle as stated above, it is possible to reduce the color shift of a light emitted from the auxiliary lens by the same way. Accordingly, the screen of the invention can reduce the color shift of the entirety of rays of light emitted. A modified embodiment of a screen according to the present invention is shown in Fig. 4. This embodiment is characterized by the providing of light absorption layers 15 at non-light emission portions of the screen employed in the above-mentioned embodiment, thereby to make it possible to reduce reflection of unnecessary exterior light, thus permitting realization of a high contrast screen in which lowering of contrast is small even in a bright or well-lighted room. Such a light absorption layer may be provided by using the known printing, painting or transfer method, etc. For the light absorption layer itself, a composite material having light absorption property, e.g., a material in which a pigment of black, etc. and preferably a flatting agent are filled into a known paint or ink may be used. It is preferable that particularly portions at which the light absorption layer 15 is provided are formed as projections having a flat surface when forming the lens plate as shown in Fig. 5 because the formation of the light absorption layer is further facilitated, when any one of printing, painting or transfer method is employed. As just described above, the screen of the invention makes it possible to widen an angle of view, to desirably design the horizontal diffusion characteristic, to reduce color shift, and to easily realize high contrast. The screen of the invention may be caused to have optical diffusion property to diffuse a light emitted in a vertical direction (in upper and lower directions when viewed from an observer). For a method for allowing the screen to have the optical diffusion property, there is known, e.g., a method to mix an optical diffusion material into the material constituting the screen. Namely, there may be used a material obtained by mixing fine particles exhibiting the optical scattering property into the material in the form of a paste. For the particle for scattering a light, there are shown, e.g., filler or extender such as fine powdered silica or powdered alumina, glass powder, and resin powder, etc. It is preferable that the particle diameter is approximately 0.5 to 30 µm. Further, it is preferable to mix the fine particles so that the ratio of the fine particle to the material constituting the screen is 0.5 to 5 weight %. It is to be noted that optical refractive index of the fine particle is required to be different from that of the material constituting the screen in order to scatter a light. For another method for allowing the screen to have optical diffusion property, there is known a method in which the light emission surface is formed as an optical diffusion surface. For example, there are shown a method to stick a plastic film obtained by mixing optical diffusion material by means of the technique, e.g., heat-fusing etc. at the time of formation of the screen, and a method to allow the emission surface to be formed as a coarse or rough surface by means of sandblast, etc. For the material constituting the screen, any material may be used as long as the material is transparent and can be formed in the shape of a sheet. It is preferable that thermoplastic resin, e.g., acrylic resin such as polymethyl methacrylate, polyester resin, cellulose system resin, polyvinyl chloride resin, polystyrene resin, or polycarbonate resin, etc., or glass is used. When the thermoplastic resin is used, the fabrication can be easily conducted with high efficiency. A method to apply heat and pressure to the sheet of the thermoplastic resin using a metal mold of an inversed type having a predetermined shape (plate- or roll-shaped), a method to implement the first-mentioned method to the sheet of the thermoplastic sheet which has not yet cooled immediately after the sheet is fabricated, or a casting or other forming method using a metal mold is suitable in the case of constituting the screen with the sheet of thermoplastic resin. In dependance upon the dimensions of the metal mold and the screen, a method to carry out manufacturing after cutting work is implemented per each thermoplastic resin sheet or a method to continuously carry out manufacturing a continuous roll-or coil-shaped sheet may be selectively conducted. The screen of the invention itself functions as a transmission type projection screen. In addition, when the Fresnel convex lens sheet 2 is disposed on the incident side as shown in Fig. 1, it is possible to irradiate a projected light uniformly in every nook and corner of the screen, thus enabling the screen to be more effectively used. On the right surface in Fig. 1 of the Fresnel lens sheet 2, inclined surfaces 21 are concentrically formed. By the inclined surfaces 21, an incident light is subjected to refraction so that it reaches the entire surface of the screen 1. Figs. 6 and 7 are vertical cross sections showing another manner how the screen of the invention is used. Since these figures are drawn as the vertical cross sectional views, the lenses L₁ to L₆ formed on both the sides of the screen 1 extend in upper and lower directions in these figures. Fig. 6 shows an example in which a Fresnel convex lens sheet 2a is disposed on the side of the incident surface of the screen 1. The Fresnel convex lens sheet 2a is provided with inclined surfaces 21 on the right surface in this figure, and lenses on the left surface in this figure. The longitudinal direction of each of the lenses L₁ to L₆ formed on the both sides of the screen 1 is in correspondence with vertical directions, whereas the lenticular lens 22 is disposed so that the longitudinal direction is in correspondence with the horizontal direction, thus functioning to widen an angle of view in a vertical direction of an image formed on the screen. Fig. 7 shows an example in which a Fresnel convex lens sheet 2b is disposed on the incident side of the screen 1. The Fresnel convex lens sheet 2b is provided with inclined surfaces 21 on the right surface in this figure and inclined surfaces 23 on the left surface in this figure. The inclined surfaces 21 are concentric surfaces and the inclined surfaces 23 are elongated rectangular surfaces such that the horizontal direction is in correspondence with the longitudinal direction. When such a Fresnel convex lens sheet 2b is disposed, it is possible to uniformly form a projected image on the entire surface of the screen 1 in the case that a projector 30 is disposed below as shown. Since the projector 30 is ordinarily installed on a floor etc., such a configuration is very practical. The screen of the invention is configured above, thus providing peculiar effects and advantages stated below. (1) In the screen of the present invention, the greater part of the incident light is diffused by the main lenses formed on the incident and emission sides of the screen, and a portion of the remaining part thereof is incident to the incident side auxiliary lenses being arranged both the sides of the incident side main lenticular lens and then is refracted by the emission side auxiliary lenses being arranged both the sides of the emission side main lens, so that it is diffused in a range outside the diffusion range of the main lens. Accordingly, the screen of the invention can diffuse an incident light in a range broader than of the conventional screen provided with a single kind of lenticular lenses on the both side thereof. The comparison between an angle of view in the screen of the present invention and that of the conventional screen is shown in Fig. 8. The solid line in this figure is a graph showing the relative luminance in the screen provided with two kinds of main and auxiliary lenses shown in Fig. 3 as a function of an angle with respect to the center of the screen, and broken lines in this figure is a graph showing the relative luminance in the conventional screen, which is similar to the graph indicated by the solid line. In the graph indicated by the solid line, a shoulder S appears by the effect of the auxiliary lens. Thus, it is demonstrated that the angle of view is widened. (2) Since the ratio of rays of light incident to the main and auxiliary lenses on the side of incident and emission sides can be adjusted by the ratio of dimensions of both the lenses, a setting of the horizontal diffusion characteristic such that a specified portion of an angle of view is caused to be brightened can be desirably made. (3) By suitably determining the shapes of the four lenses comprising two sets of main and auxiliary lenses on the incident and emission sides, it is possible to diffuse rays of light incident at different incident angles toward substantially the same direction. Accordingly, the screen of the invention can reduce color shift as compared to the screen provided with a single kind of lenticular lenses on both the sides thereof. (4) The screen provided with the light absorption layers which is implemented as one manner of the present invention can provide a high contrast screen in which lowering of the contrast is small even in a well-lighted room.

Claims: A transmission type projection screen (1) having an incident surface and an emission surface, wherein a plurality of incident side main lenses (L₁) are formed on said incident surface and a plurality of emission side main lenses (L₄) are formed on said emission surface, each of said incident side main lenses being a cylindrical convex lens whose cross section on a plane perpendicular to its longitudinal axis contains a first curvature of circle, ellipse, parabola or other curves, and each of said emission side main lenses being a cylindrical convex lens whose cross section on a plane perpendicular to its longitudinal axis contains a second curvature of circle, ellipse, parabola or other curves, characterized in that there are further formed on said incident surface a plurality of cylindrical convex incident side auxiliary lenses (L₂, L₃) whose cross sections on a plane perpendicular to their longitudinal axes contain a third curvature of circle, ellipse, parabola or other curves, said third curvature being different from said first curvature, said incident side auxiliary lenses are arranged parallel to, configuous with and on both sides of each of said incident side main lenses; there are further formed on said emission surface a plurality of cylindrical convex emission side auxiliary lenses (L₅, L₆) whose cross sections on a plane perpendicular to their longitudinal axes contain a fourth curvature of circle, ellipse, parabola or other curves, said fourth curvature being different from said second curvature, said emission side auxiliary lenses are arranged parallel to, configuous with and on both sides of said emission side main lenses; wherein an incident surface area of each of said incident side main lenses (L₁) is greater than that of each of said incident side auxiliary lenses (L₂,L₃) so that a greater part of incident light is incident on said incident side main lenses; and wherein said incident side main lenses (L₁) and said emission side main lenses (L₄) have one to one correspondence relationship, and said incident side auxiliary lenses and said emission side auxiliary lenses have one to one correspondence relationship, respectively, so that light incident on a lens on said incident surface is emitted from said emission surface through a corresponding lens thereof. A transmission type projection screen as set forth in claim 1, wherein light absorption layers (15) are provided at portions (M₁, M₂) from which light is not emitted. A transmission type projection screen as set forth in claim 1, wherein projected portions are provided at portions (M₁, M₂) from which light is not emitted and light absorption layers (15) are further provided on said projected portions. A transmission type projection screen as set forth in claim 1, wherein the screen comprises optically diffusing material. A transmission type projection screen as set forth in claim 1, wherein said emission surface serves as an optical diffusion surface.

Inventors: SEKIGUCHI, HIROSHI
Applicants: DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1987
Main classification: G03B21
IPC codes: G03B21, G02B3