Optical design of an ultrashort throw ratio projector with two freeform mirrors

An approach to design an ultrashort throw ratio projection system by combining an off-the-shelf refractive lens and two off-axis freeform mirrors is presented in this paper. The freeform profiles of the mirrors are firstly calculated by partially coupling more than three fields. The specifications of the conventional refractive lens are taken into account, and the pupil matching principle is applied to ensure good performance between the sub-systems. To step further, the calculated two freeform profiles are fitted into odd polynomials to evaluate the image quality and then re-fitted into XY polynomials for optimization. An exemplary ultrashort throw ratio projection lens with a large 80-inch diagonal image at 400mm throw distance is designed and evaluated..