Radiation spectrum of systems with condensed light

Experimental observation of Bose-Einstein condensation (BEC) of photons inside a microcavity induced an extensive study of the phenomenon. Beyond the purely theoretical interest, this phenomenon is believed to be used to create a novel source of light. The shape of radiation spectrum is therefore the main characteristic of the system with light BEC as an optical device. However, until now, there were no detailed calculations of this property. In the present paper we derive analytically the shape of radiation spectrum and show that our results are in excellent agreement with existing experimental measurements.