Patent ID: 8016450

Claim:
An illuminating apparatus comprising: a plurality of point light sources in one plane, wherein the point light sources are light emitting diodes (LEDs); and two optical sheets overlapped in a region facing the plurality of point light sources, wherein the plurality of point light sources are arranged in a first direction and also arranged in a second direction orthogonal to the first direction, the optical sheet on a side of the point light sources has a plurality of first three-dimensional structures extending in a direction parallel to the first direction and arranged in a direction parallel to the second direction, the optical sheet on a side opposite to the point light sources has a plurality of second three-dimensional structures extending in a direction parallel to the second direction and arranged in a direction parallel to the first direction, and each of the second three-dimensional structures has a shape which generates a larger amount of return light from normal incident light as compared with the first three-dimensional structures, wherein the first and the second three-dimensional structures satisfy the following expressions of P 3 /H> 1.3 P 4 /H> 1.3 and 20 >Tt 1 −Tt 2>5 where P 3 is the pitch in the first direction of the point light sources, P 4 is the pitch in the second direction of the point light sources, H is the distance between the point light sources and the optical sheet on the side of the point light sources, Tt 1 [%] is the total luminous transmittance of the optical sheet on the point light sources side when light is incident normal to the optical sheet on the point light sources side from the point light sources side, and Tt 2 [%] is the total luminous transmittance of the optical sheet on the side opposite to the point light sources when light is incident normal to the optical sheet on the side opposite to the point light sources from the point light sources side.