发光二极管(LED)已经成为头盔显示器液晶像源主流背光源。LED 发散角较大，大量的光线无法得到有效利用而造成能量的浪费，故需要对LED 进行二次光学设计以有效提高光效。背光包括由若干个LED 组成的阵列。对单个LED 设计了双曲面透镜，形成了8 mm 直径的均匀圆形光斑。对单个透镜进行切割，以形成矩形光斑。但由于切割面的全反射，矩形面积内存在亮斑。对透镜进行了优化设计，获得了亮度均匀的矩形光斑。4个矩形透镜拼接成透镜阵列。为消除杂散光斑，对透镜再次进行了优化设计。根据设计结果加工了透镜阵列并对背光进行了实际测试。测试结果表明：和传统背光结构相比，采用两层扩散膜和透镜阵列后，背光亮度提高了96.4%，非均匀性由23.8%略提高到23.1%，半亮度视角由39°降低到23°。满足头盔显示器液晶像源背光的要求。

Light emitting diode (LED) has become a mainstream backlight technology for liquid crystal display (LCD) imaging engine of helmet-mounted display.With large emitting angle, a lot of lights from LED can’t be used effectively, causing power waste. The backlight consists of LED array. The secondary optical design can help to improve the luminous efficiency. A double-freeform-surface lens is proposed, which forms a circle spot with a 8mm diameter. The lens is cut to form a rectangle spot. However, the hot spot appears, caused by the total reflection. The lens is then optimized, forming a rectangle spot with high uniformity. 4 lens are combined to become a lens array. In order to eliminate stray lights, the optimization on lens is performed again. The prototype of lens array is developed and the backlight unit with lens array is measured. The measurement results show that compared with the common backlight, the luminance of the backlight with 2 diffusers and lens array increases by 96.4%, the nonuniformity of 23.1% is slightly improved from 23.8%，the viewing angle of half-luminance decreases from 39° to 23°. The designed lens array satisfied the requirements of the LED backlight in LCD imaging engine of helmet-mounted display.