An instrument for photodynamic therapy applies treatment light from a dye laser, white light, and ultraviolet fluorescence excitation light from an LED onto a lesion and surrounding areas in a time-multiplexed manner. The reflected white light is analyzed in a spectrometer to determine a correction for the dynamic optical spectral properties of the patient's tissue. Light emitted by fluorescence from the lesion and the surrounding areas is analyzed in another spectrometer, and the results are corrected in a computer, using the correction. An optical switch has been developed for the instrument, using a bistable solenoid and a sled.