Lens Array Design With Double Freeform Surface For LED Backlight In LCD Imaging Engine

Liquid crystal display(LCD)imaging engine products tend to be small volume,light weight,high brightness,and low power consumption.The existing LCD imaging engine products have some problems such as low light efficiency and low luminance,which are unable to meet the special requirements of the head-up display.Light emitting diode(LED)has become a mainstream backlight source for LCD imaging engine.When used in imgaing engine,the power,luminance,and volume have to be comprehensively studied.This paper proposes a simple calculation method based on the secondary optical design thoery to design the double freeform surface lens,which can collimate light emitted from a LED.Data of the double freeform surface can be obtained by solving the equation based on the law of conservation of energy and Snell law.The lens is then cut to be rectangular.90 lenses are combined to form a lens array.The cutting way is then optimiazed according to the simulation results.To solve the uniformity problem caused by the extended source,the optimization design method is put forward based on the theory of edge-ray.The feedback optimization method is proposed based on the simulation results to change the energy distribution on the surface of the target to improve the uniformity.Multi-round optimizations help satisfy the requirements on luminance and uniformity.The prototype of the lens array is manufactured and the backlight unit with the lens array is measured.To solve the problem of nonuniformity,the reflective appertue is designed and processed.The measurement results show that the reflective appertue effectively improves the uniformity of ghost image.Compared with the common backlight,the luminance of the backlight with two diffuser films and the lens array increases by 179.7%,the nonuniformity of 12.46% deceases by18.32%,the viewing angle of half-luminance decreases from 32° to 18°.The designed lens array satisfies the requirements of the LED backlight in LCD imaging engine of head-up display.The freeform surface lens array designed by the method proposed in the paper can help LED backlight module achieve high luminance and good uniformity,which could also be used for other optical systems with the limited volume and required high light efficiency.