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The phototoxicity of medical ultraviolet (UV) sources used in dermatology has long been recognized. Less obvious are potential hazards to the eye and skin from many other optical sources-both to the patient and to the health-care worker. To assess potential hazards, one must consider not only the optical and radiometric parameters of the optical source in question but also the geometrical exposure factors. This knowledge is required to accurately determine the irradiances (dose rates) to exposed tissues. Both photochemically and thermally induced damage are possible from intense light sources used in medicine and surgery; however, thermal injury is rare unless the light source is pulsed or nearly in contact with tissue. Generally, photochemical interaction mechanisms are most pronounced at short wavelengths (UV) where photon energies are greatest, and also will be most readily observed for lengthy exposure durations.