Patent ID: 7771108

Claim:
A planar lighting device comprising: a light guide plate including a light exit plane being rectangular, two light entrance planes respectively containing two opposite longer sides of the light exit plane and being disposed opposite each other, two inclined planes being symmetrical and being increasingly distanced from the light exit plane with an increasing distance from the two light entrance planes toward a center of the light exit plane, a curved portion joining the two inclined planes, and scattering particles for scattering light propagating inside the light guide plate and being contained within the light guide plate; two light sources disposed opposite the two light entrance planes of the light guide plate, respectively; securing means for integrating the two light sources and the light guide plate such that the two light sources are secured to the light guide plate with a distance between the two light sources and the two light entrance planes of the light guide plate kept constant; a housing for accommodating the two light sources and the two light guide plate integrated by the securing means; and a sliding mechanism provided between the housing and the securing means for allowing the securing means to slide with respect to the housing in a direction from the two light entrance planes of the light guide plate toward the two light sources as the light guide plate expands or contracts with the two light sources and the light guide plate integrated and a distance between the two light sources and the light guide plate kept constant; wherein a distance between the two light entrance planes is from 480 mm to 830 mm both inclusive, the scattering particles have a particle diameter of from 4.0 μm to 12.0 μm both inclusive and a particle density of from 0.008 wt % to 0.25 wt % both inclusive, a light use efficiency representing a ratio of light emitted through the light exit plane to light admitted through the two light entrance planes is 55% or higher, and wherein a middle-high ratio of brightness distribution at the light exit plane representing a ratio of brightness of light emitted through a central area of the light exit plane to brightness of light emitted through an area of the light exit plane near either light entrance plane is from 0% to 25% inclusive.