Concentrators based on geometrical optics increase the irradiance by increasing the projected solid angle, but conserve the radiance of radiation. The general principle for increasing the radiance, and thereby concentrating even diffuse radiation, resembles a light trap. Light, which enters the trap through a selective filter, is shifted in photon energy, for example, by a Stokes luminescent process. It is subsequently trapped because it is reflected by the filter. Concentration is limited, in the ideal case, by the reverse (anti-Stokes) process, which reaches equilibrium when incoming and concentrated radiation reach equal chemical potential. The laser is discussed as an example for a concentration not limited by thermodynamics. The limits imposed by quantum mechanics are derived. Real systems, with various losses, are discussed.