A lamp has microwave resonant body (11) of transparent quartz. The body has a central bore (16), having a sealed plasma enclosing bulb (17) inserted in it. The bulb is of quartz also and has an external diameter which is a close fit in the bore. The bulb itself is of drawn quartz tube (18) and as such has a smooth internal bore (19). End caps (20) are fused to the tube and encapsulate a charge of a material excitable to form a light emitting plasma in the bulb when microwaves are fed into the body via an antenna (7) in a bore (21) in the body. The body is sized to establish resonance within the Faraday cage in the body (11), bulb (17) and fill containing void (22) within the bulb. There is negligible gap between the bulb and body, whereby they can be regarded as one for resonance purposes. The bulb is fixed in the body by welds.