Patent ID: 6870681

Claim:
A partially transparent sheet for directional light transmission and image viewing comprising: a body of at least partially transparent solid material with a predetermined thickness substantially less than its other dimensions, and having a front surface and a back surface; a multiplicity of convex lenses with a predetermined index of refraction covering a substantial area on said body front surface with each of said lenses having a least transverse dimension of less than two millimeters; a multiplicity of concave lenses with a predetermined index of refraction of said body back surface, each being generally in register with a respective one of said convex lenses, the transverse dimension of said concave lenses being less than three-fourths of the center-to-center pitch thereof, surfaces between said concave lenses being at least in part in the form of a convex arcuate surface, the effective radii of curvature of said concave lenses being selected with regard to the effective radii of curvature of said convex lenses, said index of refraction, and said body thickness to cause parallel rays converged by said convex lenses to be returned approximately to parallelism upon passing through said concave lenses; whereby said sheet, when viewed in a direction nearly parallel with the paths from lenses on the front surface to corresponding lenses on the back surface, appears transparent and transmits a substantially undistorted image, while said sheet appears non-transparent to images when viewed in a substantially different direction.