Patent ID: 7972042

Claim:
A light guide comprising: a cylindrical emitting surface; an incident surface adjacent to and substantially facing away from the cylindrical emitting surface; and a connecting surface connecting the cylindrical surface and the incident surface; wherein the light guide defines a plurality of grooves in the incident surface, each groove extending substantially along a direction parallel to the cylindrical surface and being bounded by a first surface substantially facing to the connecting surface and a second surface substantially facing away from the connecting surface, the grooves satisfying the following formulas: −0.58α+59°<90°−θ 1 <−0.54α+65°; and 5°≦θ 2 <25°, where θ 1 is an acute included angle between each first surface and the connecting surface, θ 2 is an acute included angle between each second surface and the connecting surface, and α is an acute included angle between the emitting surface and a plane passing through the intersection between each first surface and a corresponding second surface and parallel to the connecting surface.