A compact light source has been developed. It consists of a high-pressure mercury lamp, a shutter, and water-cooled filters for selected wavelength regions. A mixing device permits simultaneous irradiation from two lamps of different wavelengths. The spectral distribution of the light for seven filter combinations and the construction of a power meter are also described. The application of the lamps to clinical investigations is illustrated by determination of minimal erythemal and blister doses, as well as pigmentation, for various groups of patients. The light intensities available are high enough to make pain threshold measurements possible in the UV and visible regions.