Optical design of single plano-convex freeform lens-based illumination system for color filter liquid crystal on silicon pico projectors

An efficient and practical method for the design of an illumination system with a single plano-convex freeform lens for color filter liquid crystal on silicon (CF-LCoS) pico projectors is proposed. Based on Snell's law and the energy conservation law, the initial illumination system is derived and the freeform lens is numerically calculated from a first-order partial differential equation, which could be considered a good optimization start for further optimization. For an actual extended LED chip, a local optimization algorithm could efficiently achieve the design goals based on the specifically designed initial system. The design example shows that the illumination system employed a CF-LCOS panel with a diagonal of 0.38-in and an aspect ratio of 16∶9 meets the needs of the optical engine for a pico projector, which has a transmission efficiency of 50% and an ANSI nine-point uniformity over 95%, with the whole optical engine volume 28.4×18.5×8  mm3. Moreover, analysis indicates that the design method could obtain outstanding performance with compactness and low cost.