Patent ID: 7726826

Claim:
A direct-type backlight device comprising: a plurality of linear light sources that are arranged in parallel, a reflection plate reflecting light from the linear light sources, and a light diffusion plate for diffusing and emitting direct light from the linear light sources and reflected light from the reflected plate; the reflection plate, the linear light sources, and the light diffusion plate being arranged in this order; wherein: the light diffusion plate has the prismatic ridges ( 1 ) at least on the surface being on the opposite side to the light sources, each of the ridges having a sawtooth-shaped cross-section; the apex angle “y” (degree) of the prismatic ridge ( 1 ) of the light diffusion plate, the distance “a” (mm) between centers of the adjacent linear light sources, and the distance “b” (mm) from a center of the linear light sources to the surface of the light diffusion plate facing the light source satisfy the following relationship: 80×( b/a )+15 <y< 180×( b/a )+70; wherein the direct-type backlight device further comprises a prism sheet on a surface of the light diffusion plate, the surface being on the opposite side to the light sources, for adjusting the output direction of the light that has been diffused by and emitted from the light diffusion plate; the prismatic ridges ( 1 ) of the light diffusion plate has the apex angle “y” from 60° to 170° and the pitch from 20 μm to 700 μm; the prism sheet has a prismatic ridge ( 2 ) on the surface being on the opposite side to the light diffusion plate, the ridge ( 2 ) having a sawtooth-shaped cross-section; the prismatic ridge ( 2 ) of the prism sheet has the apex angle from 70° to 120° and the pitch from 20 μm to 700 μm, and wherein each of the longitudinal direction of the prismatic ridges ( 1 ) of the light diffusion plate and that of the prismatic ridges ( 2 ) of the prism sheet is substantially parallel to the longitudinal direction of the linear light sources.