Light therapy in the visible spectral region, for which radiation sources emitting UV radiation are provided, uses fluorescent films that age during operation such that the intensity of the light irradiated by the fluorescent films gradually decreases. In order to solve this problem, the invention relates to a radiation converter located in a treatment device and comprising a fluorescent layer (2) for emitting the visible light. Said radiation converter comprises a housing (1) that can be penetrated by radiation emitted by the radiation source (11). The housing (1) comprises a front wall (3) consisting of a UV-permeable material and a rear wall (4) consisting of a UV-impermeable material, on the side of the housing (1) opposing the front wall (3). A liquid chamber (5) is formed between the front wall (3) and the rear wall (4), and the fluorescent layer (2) is arranged between the front wall (3) and the rear wall (4).