We have studied the thermal emission of a cavity quantum electrodynamic system in the ultrastrong-coupling regime, where the atom-cavity coupling rate becomes comparable with the cavity resonance frequency. In this regime, the standard descriptions of photodetection and dissipation fail. By following an approach that was recently put forward by Ridolfo et al (2012 Phys. Rev. Lett. 109 193602), we were able to calculate the emission of systems with arbitrary strength of light-matter interaction, by expressing the electric-field operator in the cavity-emitter dressed basis. In this paper, we present thermal photoluminescence spectra, calculated for given temperatures and for different couplings, in particular, for available circuit quantum electrodynamics parameters. © 2013 The Royal Swedish Academy of Sciences.