Patent ID: 7128457

Claim:
A surface light-emitting device comprising: a body outlining a light guiding space in the shape of a rectangular parallelepiped having a specific length, width, and height, and a light source which is arranged outside the body to supply light to the light guiding space, wherein light supplied from the light source through an incident plane formed of at least one of the sides of the light guiding space can be emitted to the outside through an emission plane formed of the side which intersects the incident plane at right angles, wherein the body includes a light-transmitting member arranged to cover the emission plane, the light-transmitting member includes a light-transmitting plate having a back face facing the light guiding space and a surface opposing the back face, and a prism sheet arranged on the back face of the light-transmitting plate, the surface of the light-transmitting plate is a light-emitting surface, wherein the incident plane is one of the opposing sides of the light guiding space in the longitudinal direction, the light-transmitting plate includes a diffusion filter covering a specific area of the surface of the light-transmitting plate near the light source, the remaining area of the surface of the light-transmitting plate at a distance from the light source is not covered with the diffusion filter, and wherein the diffusion filter is formed of a laminate in which a plurality of diffuse transmission films is layered, and the number of diffuse transmission film layers is greatest in the area close to the light source and gradually decreases as the distance from the light source increases so that light transmittance is increased as the distance from the light source increases due to a decrease in diffusion of light.