JPS649183B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a multifocal Fresnel lens used for rear projection screens and the like.

In order to enable bright and uniform viewing of the screen, rear projection screens are generally provided with concentric Fresnel lenses that converge incident light from a light source onto a viewing position. However, such a concentric Fresnel lens converges light uniformly in the horizontal and vertical directions, which may cause problems when observing a screen or the like. FIGS. 1 and 2 are explanatory diagrams showing desirable transmission states of a screen equipped with such a Fresnel lens. That is, in the horizontal direction, the light from the light source L is expanded as much as possible by the Fresnel lens F of the screen S, as shown in Figure 1, and conversely, in the vertical direction, it is limited as shown in Figure 2. Ideally, it should be configured so that it spreads within a defined range.

From this point of view, a multifocal Fresnel lens with different focal lengths in the vertical and horizontal directions was developed in 1983.
This method is already known as Publication No. 144347 and Japanese Utility Model Application No. 124837/1983. However, the above invention does not disclose a specific method for manufacturing such a Fresnel lens. Such a multifocal Fresnel lens can be manufactured by cutting, but the cutting method is complicated, the productivity is low, and the cost is high.

In view of this situation, the present invention attempts to easily manufacture a multifocal Fresnel lens by skillfully utilizing the contraction phenomenon caused by heating of a stretched thermoplastic synthetic resin. That is, the gist of the present invention is to form a lens surface by cutting a Fresnel lens into a thermoplastic synthetic resin plate manufactured with different stretching ratios in the vertical and horizontal directions, and then to A method for manufacturing a multifocal Fresnel lens, characterized by heat-treating a synthetic resin plate.

The present invention will be explained below with reference to the drawings of the embodiments starting from FIG.

FIGS. 3A to 3D are explanatory diagrams showing the manufacturing process of the present invention, and the process will be explained in the following order. FIG. 3A shows a base plate 1A of a thermoplastic synthetic resin plate, and the material used is a thermoplastic synthetic resin such as acrylic resin, polycarbonate resin, or vinyl chloride resin. Figure 3B shows this original plate 1A in one axial direction 2
The stretched plate 1B is produced by heating and stretching, and is stretched only in the vertical direction without stretching in the horizontal direction. It is known that when such a stretched plate 1B is reheated, the stretched portion shrinks and returns to almost its original shape.
The stretched plate 1B of the present invention is not limited to such a uniaxially stretched plate 1B, but may be a biaxially stretched plate with different longitudinal and horizontal stretching ratios.

Figure 3C shows the stretched plate 1 produced as described above.
B shows a Fresnel lens plate 1C on which concentric Fresnel lens surfaces are formed. Formation of such a Fresnel lens surface is performed by cutting, but it can be performed in the same manner as when manufacturing a general Fresnel lens. FIG. 3D shows a multifocal Fresnel lens plate 1D obtained by heat-treating the Fresnel lens plate 1C described above. When heated again in this manner, the stretched portion of the Fresnel lens surface is shrunk as shown in FIG. In other words, if the stretching direction 2 of the uniaxially stretched sheet is the vertical direction H of the screen, the shape of the Fresnel lens does not change in the horizontal direction V, so the inclination of the lens is larger in the vertical direction H, and it has an elliptical shape with a finer pitch. This results in a Fresnel lens with a focal length shorter in the vertical direction, as shown in the figure. Of course, even when a biaxially stretched plate with different stretching ratios in the vertical and horizontal directions is used, it can be shrunk by heating again to form a multifocal Fresnel lens surface. In addition, since the present invention utilizes shrinkage due to reheating during stretching, it is preferable to consider the stretching magnification according to predetermined vertical and horizontal focal lengths. Shrinking the Fresnel lens surface once cut in this way reduces the pitch of the lens,
Although this is advantageous from the standpoint of moire prevention, the minimum pitch that can be cut in general cutting processing is limited to about 0.1 mm, and the present invention also has the advantage of being able to reduce the pitch of the Fresnel lens.

Since the present invention has the structure as described in detail above, it has the advantage that a multifocal Fresnel lens can be easily manufactured by skillfully utilizing the properties of thermoplastic synthetic resin.

[Brief explanation of drawings]

FIGS. 1 and 2 are explanatory diagrams showing the desired transmission state of a screen equipped with a Fresnel lens, FIGS. 3A to D are explanatory diagrams of the manufacturing process of the present invention, and FIG. 4 is an illustration of the state of shrinkage due to heat treatment. It is an explanatory diagram. 1A... Original plate, 1B... Stretched plate, 1C... Fresnel lens plate, 1D... Multifocal Fresnel lens plate.

Claims (1)

[Claims] 1 A Fresnel lens is cut to form a lens surface on a thermoplastic synthetic resin plate manufactured with different stretching ratios in the vertical and horizontal directions, and then this thermoplastic synthetic resin plate is heat-treated. A method for manufacturing a multifocal Fresnel lens.