Patent ID: 7887208

Claim:
A direct type back-light device comprising: a reflection plate, a plurality of linear light sources disposed in parallel to each other, and a light diffusion plate, disposed in this order, wherein the light diffusion plate comprises: a plurality of concave or convex structural units, in which each of the units has three or more sloped sides, on at least one main surface, a maximum height Rz of the main surface having the structural units being not more than 1000 μm, and a portion which lies immediately above a region between central axes of adjacent linear light sources in which the portion (1) has no less than two outlines of the sloped sides which have different gradients from each other in a vertical cross-section of the light diffusion plate, and (2) satisfies a relationship of 12.5−10×(bL/aL<Xn<85−25×(b L /aL), in which the aL(mm) is an average distance between the central axes of the adjacent linear light sources, the bL(mm) is a minimum distance between the central axis of the linear light source and the main surface facing the linear light source in the light diffusion plate, and the Xn(deg.) is a gradient of the nth outline in the vertical cross-section of the light diffusion plate in which the n is a subindex for representing each outline, in which a cutting-plane line of the vertical cross section of the light diffusion plate has a direction perpendicular to a central axis direction of the linear light source.