A lamp with a quartz electrodeless bulb has a ceramic wave guide with a central void, in which the bulb is accommodated. The wave guide is rectangular. The central void is centered on a central longitudinal plane of the wave guide, normal to front and back faces of the wave guide and equally spaced from end faces. Parallel with the central void and also on the central plane are two further voids for respective antennae. The central void is open through the front face for egress of light, but the antenna voids are not open in this face. The latter is metalized to inhibit egress of microwave energy from the wave guide.