Abstract: A photodynamic therapy light source for treating skin conditions comprising a light source (10), a filter wheel (24), a light guide (60), a camera (70) and a control unit (40) for controlling the source. A substance, which is absorbed by particular cells, is applied to an area of the patient. Ultraviolet light from the light source illuminates the treated area causing the absorbed substance to fluoresce. The mount of fluorescence of the area is captured by the camera and used to guide the photodynamic treatment. The filter wheel (24) includes a number of filter elements (24?) for selectively enabling a particular bandwidth of illumination to be provided. The illumination from the light source (10) can be modulated so that a cyclic treatment of illumination is applied. The period in which illumination is applied can be further pulsed or modulated to provide a pulsed illumination to the patient.