There are two ways to enhance the backlight brightness, BEF which stands for Brightness Enhancement Film, and DBEF stands for Dual-Brightness Enhance Film. BEF is usually used.

            
                           TFT Structure
BEF is two-layered prism film, that collects the wide-angle divergent light emitted by the light guide and the diffuser, into a 70 degree range and send out. In this way, the brightness of upright direction is increased, but the viewing angle is reduced.

                           

                           BEF Brightening  Principle
            
                                                                                                                 DBEF Brightening Principle
DBEF is a thin film reflective polarizer. It reflects about 50 percent of light which is gulped by the traditional polarizer, to enhance the brightness. The enhancement is made in all directions and all-round. It can increase the brightness of LCD’s full viewing range. DBEF is more expensive. The brightness of LCD modules with DBEF will be increased by about 20%. But DBEF cannot be used with the brightness enhancement film on the LCD.