Uniform light distribution by using microlenses with various focal lengths

The increasing demands for non-self-emissive portable display devices jointly stimulate the needs for additional front light units to provide extra illumination to compensate for the ambient light in dim environment. The advantages of light emitting diodes (LED), such as high efficiency and small size, make it an ideal source for the compact unit, but direct LED lighting is impractical. Secondary optics is commonly adopted in illumination occasions to ensure the output from the LED dies meet the overall specification. Considering an oblique incidence situation, light mapping redistribution is inevitable. In this paper, we propose a beam shaping method adopting microlens array with various focal length to achieve uniform light distribution. The design is based on edge ray principle, which considers each microlens individually. Microlenses with various focuses were made by photoresist thermal reflow process to verify the novel design.