Nondiffusive light guide plate, lens film and surface light source

Abstract

A surface light source is capable of emitting light in a narrow directional luminance distribution, at a low loss in quantity of light and in a uniform surface luminance distribution, and consists of a relatively small number of parts. The surface light source is provided with a nondiffusive light guide plate and a lens film. The surface light source assembly need not be provided with any diffusive film on the outer side of the light exit surface of the light guide plate, because the light guide plate is not provided with any diffusive reflecting layer of printed dots on the back surface thereof. The back surface is a concavely curved surface extended opposite to the light exit surface, having a section in a plane perpendicular to both the side light entrance surface and the nondiffusive light exit surface, delineated by a concave curve, such as an approximate parabolic curve, and curved so that the space between the light exit surface and the back surface, and the gradient thereof decrease with distance measured in the direction of travel of received light rays from the side light entrance surface. The lens film has a light entrance surface provided with a plurality of triangular prisms in a two-dimensional arrangement. The vertical angle of the triangular prisms is determined so that light rays leaving the light exit surface of the light guide plate in a direction oblique to and deviating from a normal to the light exit surface can be refracted so as to leave the light exit surface of the lens film in a direction parallel to a normal to the light exit surface of the lens film.