English: 160W mercury-vapor "black light" ultraviolet bulb. It creates ultraviolet light with an electric arc through mercury vapor in a small quartz tube inside. The outer bulb has a dark blue-purple filtering material (Wood's glass is one type) which blocks most of the visible light, passing only the ultraviolet light. It emits long-wave (UVA) ultraviolet radiation, from the 253.7 nanometer mercury spectral line. These lamps are used mostly for lighting effects in theaters and stages.