The present invention relates to a projection screen used for a rear projection type television set.
Conventional projection screens used for rear projection type television sets have been composed of a circular Fresnel lens sheet and a lenticular lens sheet. The circular Fresnel lens sheet is formed by arranging a great number of circular unit lenses on a plane in such a way that the apex lines of the lenses form concentric circles. This lens sheet has a function of converging the projected light toward the viewer side. The lenticular lens sheet is formed by arranging a great number of linear unit lenses on a plane in such a way that the longitudinal lines of the lenses form parallel lines. This lens sheet has a function of diffusing the projected light to form an image.
Further, in the projector optical system of the conventional rear projection type television sets, the projector tube has been so far mainly arranged on the central axis of the screen rear surface. Recently, however, another projector arranging method has been developed to make the television set compact. In this case, the projector tube is arranged in a limited space offset from the central axis of the screen surface, for instance over or under the screen.
The present invention relates to an improvement of the so-called table top type projection screen, which is suitable for use with the rear projection type television set. In this table top projection screen for a rear projection television set, since the screen lower portion can be fairly shortened as compared with that of the ordinary rear projection television set, it is possible to give the viewers such an impression as if they were seeing a direct viewing CRT television set. In this screen, a linear Fresnel lens sheet is further arranged near the rear surface (on the projector tube side) of the circular Fresnel lens sheet for correction of the optical axis of the projection screen. That is, in order to correct the offset position of the projector tube, the optical axis is corrected to some extent, before the light is allowed to be incident upon the circular Fresnel lens sheet. Here, the reason why the linear Fresnel lens sheet is used is as follows: since the linear Fresnel lens sheet is formed by arranging a great number of prism-shaped unit lenses on a plane in such a way that the longitudinal lines thereof are arranged in parallel to each other, it is possible to change the directional relationship between the incident light and the outgoing light.
The above-mentioned technique of adding the linear Fresnel lens sheet has been proposed by Japanese Published Unexamined (Kokai) Patent Application No. 59-15925 (issued on Jan. 27, in 1984). In this prior art, a linear Fresnel lens sheet is arranged on the projection side of the circular Fresnel lens sheet (the screen). In other words, the sheets are arranged in the order of the circular Fresnel lens sheet and the linear Fresnel lens sheet when seen from the viewer side. Further, the light is allowed to be incident upon the projection screen at a predetermined inclination angle with respect to the screen (without allowing the light to be incident upon the screen in parallel to the optical axis thereof), by designing the inclination angle of the lens surfaces arranged on the linear Fresnel lens sheet at an appropriate angle, in order to reduce the depth of the projector system, that is, the thickness of the projection type television set.
In the above-mentioned prior art technique, the linear Fresnel lens sheet is additionally provided, so that it has become possible to change the angle of the optical axis of the incident light with respect to the normal line to the screen surface, but there arises another problem in that the picture brightness is inevitably lowered.