Patent ID: 7661863

Claim:
A back-light unit used for a transmission type liquid crystal display device comprising a liquid crystal panel, said back-light unit comprising: a light source for providing light exposed to said liquid crystal panel, a planar light guide for changing a direction of propagation of said light to said liquid crystal panel, and a prism sheet disposed adjacently to said planar light guide, said prism sheet comprising: continuous prism structures extending throughout a surface of the prism sheet in close proximity, each prism structure comprising: a sheet base; a higher order surface comprising: a peak; a small planar surface extending from the peak to an end portion; a curved surface continuous to and starting from the end portion of the planar surface extending toward the sheet base; wherein the curved surface is larger than the planar surface; and a connection plane that connects the sheet base and the peak of the higher order surface, said connection plane disposed at a predetermined angle to the sheet base, said angle allowing reflection of the light at an intensity peak within an angle distribution of launched light; wherein a top of the higher order surface extending above the end portion of the planar surface comprises a near triangular prism such that the peak forms an apex of the near triangular prism; wherein the curved surface extends from the top to an end portion of an adjacent connection plane toward the sheet base; and wherein the prism structure comprises a reflective surface extending from the sheet base to the top for providing substantially total reflection of light launched with an intensity distribution of a predetermined angle range.