Secondary lens with hybrid structure of freeform surface and microstructure for ultra-thin backlight unit.

Ultra-thin has become the development trend of the direct-lit backlight unit (BLU). Double freeform surface lenses are commonly used in direct-lit BLUs to reduce thickness. However, for an ultra-thin BLU with quite small optical distance (OD) and a large LED pitch distance, the curvature of the designed lens would be quite large, which would make the final optical performance heavily affected by fabrication errors. This paper proposes a lens with freeform surfaces and microstructures. The rays from LEDs are first collimated by the freeform surfaces and the collimated rays are then reflected by the microstructures to the bottom of the BLU, which can effectively enlarge the spot size and reduce the OD. The simulation results show that the uniformity can be improved from 41.3% of the conventional double freeform surface lens to 83% when OD is 3 mm. Such hybrid lenses can avoid the fabrication of freeform surfaces with large curvature and the advantages of easy design and easy fabrication.