A stationary compressible three-dimensional (3D) model of photothermal processes inside an arc tube of a high-intensity discharge lamp is developed on the basis of the finite element method. It takes plasma, electrodes, and the tube wall into account and enables simulation of acoustic phenomena. The temperature profile of the discharge arc is used as a marker for the emission of visible light. Complementary, experimental investigations are conducted at different modulation frequencies. A photodetector array is used to record 2D information about the light intensity distribution. The shape and length of the discharge arc are determined and compared to numerical results.