An electrodeless lamp includes a bulbous lamp envelope enclosing an inert gas and a vaporizable metal fill, the lamp envelope having a reentrant cavity; an electromagnetic coupler positioned within the reentrant cavity; and a cold spot structure configured for low temperature, low duty cycle operation and for room temperature, 100% duty cycle operation. In some embodiments, the cold spot structure includes a dimple in the lamp envelope, the dimple having a thinned sidewall. In further embodiments, a shield is positioned near the dimple to control cold spot temperature. In additional embodiments, the cold spot structure includes a heat sink attached to the exhaust tube of the lamp envelope and thermally isolated from the lamp base.