It has been reported that high-intensity discharge (HID) lamps, such as high-pressure mercury-vapor lamps, can constitute a safety hazard if the outer envelope is broken and the lamp continues to operate, because of the generation of short wavelength ultraviolet radiations which are passed by the quartz arc tube. Lamps which are protected by a fuse are generally known in the art and a projection lamp which is fused in order to prevent dangerous arcing currents is described in U.S. Pat. No. 2,859,381, dated Nov. 4, 1958.
High-intensity discharge lamps have also been operated with a tungsten filament included between the arc tube and the outer envelope for purposes of ballasting the discharge and also generating some visible light. While the purpose of the ballasting and light-generating filament is not that of a link or safety switch, such a filament will normally oxidize and fail, particularly under lamp start-up conditions, if the outer envelope is broken, since this permits the filament to come in contact with air. Of course, such a supplemental ballasting filament consumes an appreciable portion of the total power consumed by the lamp, which decreases the lamp operating efficiency.