Design procedures for ideal radiation concentrators are described which are applicable to finite sources and/ or restricted exit angles. Finite sources are relevant for second stage concentrators which collect and further concentrate radiation from a primary focusing element (mirror or lens) in a manner similar to the field optic element in a telescope. Restricting the exit angle is useful for improving the optical efficiency of solar collectors by eliminating grazing angles of incidence of the absorber. It also serves to extend the useful range of angular acceptance values available from solid dielectric concentrators that function by total internal reflection. Concentrators of this type can be used to construct highly efficient radiation traps (spectrally selective filters).
Ari Rabl and Roland Winston, "Ideal concentrators for finite sources and restricted exit angles," Appl. Opt. 15, 2880-2883 (1976)