A lucent crucible of a Lucent Waveguide Microwave Plasma Light Source (LWMPLS) comprising a Light Emitting Resonator (LER) in form of a crucible (1) of fused quartz which has a central void (2) having microwave excitable material (3) within it. In one example, the void is 4 mm in diameter and has a length (L) of 21 mm. The LWMPLS is operated at a power (P) of 280 W and thus with a plasma loading P/L of 133 w/cm and a wall loading of 106 w/cm2. The lamp is thus operated with a high efficiency—in terms of lumens per watt—while having a reasonable lifetime.