A lamp comprises a light source in the form of a light emitting resonator (1), a magnetron (2) and a stub tuner (3). A reflector (4) is fitted at the junction of the light source and the stub tuner, for directing the light in a generally collimated beam (5). The light emitting resonator comprises an enclosure (11) formed of inner and outer envelopes (12,13) of quartz. These are circular cylindrical tubes (14,15) with respective end plates (16,17). A tungsten wire mesh (18), of a mesh size to exhibit a ground plane to microwaves within the resonator, is sandwiched between the tubes and the endplates respectively. Each envelope, comprised of its tube and end plates is hermetic. An earth connection (18') extends from the mesh to the outside of the envelope. The length axially of the enclosure between the wire mesh sandwiched between the end plates is Lambda/2 for the operating microwave frequency. At one end of the enclosure, a molybdenum drive connection 19 extends to a tungsten disc (20). This is arranged transverse the axis A of the enclosure at 1/16 Lambda from the mesh at its end of the enclosure. The enclosure is filled with excitable plasma material, such as a dose of metal halide in a rare earth gas. The disc acts as antenna and is driven by the magnetron (2), via the matching circuit (3).