Abstract: An apparatus is described for efficiently transforming light from concentrated sources into uniform diffse light. The apparatus comprises (1) a light guide resonator which comprises (A) a light guide and (B) mirrors for reflecting light back into the light guide; (2) means for injecting light from a light source into the light guide resonator; and (3) a light-diffusing element placed within the field distribution of the light in the resonator, the whole apparatus being designed to set up a regime of optical resonance. The light guide resonator is uniformly filled with light; within the resonator the uniformly lit light-diffusing element acts as an efficient light source of uniform brightness.