Optimization design of single freeform lens based illumination system for CF-LCoS handheld pico-projectors

Abstract An optimization method for the design of an illumination system with a single freeform lens for CF-LCoS handheld pico-projectors is proposed in this paper. Based on the Snell's law and the energy conservation law, the front and back freeform surfaces of the initial lens are calculated numerically by solving the corresponding first-order partial differential equations. Being different from other designs, the solution method assigns the light deflection angle of the two freeform surfaces, by which small-angle incidence on LCoS is guaranteed, and the freeform lens can also be reasonably controlled to avoid unmanageable surfaces in injection-molding process. To improve the spatial uniformity, the optimal surfaces are searched by using simplex algorithm for an extended LED system. The design example shows that a single 2 mm × 2 mm LED chip powered illumination system, which employs a CF-LCOS panel with a diagonal of 0.59-in and an aspect ratio of 4:3, satisfies the needs of the optical engine for handheld pico-projector, which has a transmission efficiency about 55% and an ANSI 9-point uniformity over 95%, with the whole optical engine volume 60 mm × 28 mm × 16 mm. The design method could not only achieve collimating and uniform illumination, but also show the great prospect in application on other specific light conditions.