Most LED illumination applications require generation of complex light patterns for which the secondary optics with two free-form surfaces needs to be used. We present a novel optimization method for computing such type of optical elements. An analytical solution for the generation of the initial surfaces is proposed. To accelerate the optimization process, a specific surface representation is used, that eliminates the need to run a time-expensive raytracing procedure. As an example, an optical element generating uniformly illuminated rectangular area with size of 60° by 40° is computed. Lighting efficacy for the extended Lambertian source 1x1 mm is 88.5% and nonuniformity is less than 8.5%.