Design of refractive optical elements with two free-form surfaces for generation of prescribed illuminance distribution

New optimization method for design of refractive optical elements with two free-form surfaces is presented. Algorithm of design consists of two parts: generation of initial approximation and the further optimization of outer surface parameters from condition of producing prescribed illuminance distribution. As an example the optical element generating uniformly illuminated rectangular region with angular sizes $60^\circ \times 40^\circ$ with extended $1 \times 1$ mm Lambertian light source is computed. The light efficacy of optical element is about $88,5\%$ and the relative root-mean-square error of generated illuminance distribution is less than $8,5\%$.