[en] We discuss the physics of a photoluminescence-based pulse-energy detector that will be used to characterize hard x-ray free-electron-laser pulses at the Linac Coherent Light Source. We tested the detector in a quasi-steady-state mode of operation at the conventional synchrotron light source Stanford positron electron assymetric ring 3 at the Stanford Synchrotron Radiation Laboratory. We also developed a Monte-Carlo model for the x-ray interaction with the nitrogen gas and the resulting ultraviolet signal and found good agreement with the experimental data. Remaining discrepancies can, to a large extent, be attributed to the varying luminescence behavior of different chamber wall materials