Transparent heat conserving coating for metal halide arc tubes

摘要

A metal halide arc tube is provided having a light-transmitting envelope and electrodes for generating an arc discharge within a sealed chamber of the envelope. The envelope includes a transparent film at a cold region of the sealed chamber where metal halides migrate and condense. The transparent film transmits visible radiation and absorbs ultraviolet radiation emitted by the arc to impose an additional heat load on the cold region so that a high metal halide vapor pressure can be maintained. The temperature of a hot region of the envelope can be maintained at a lower level by reducing power input or increasing chamber surface area. Preferably, the transparent film reflects infrared radiation emitted by the light source back to the arc to increase efficiency and further increase the temperature of the cold region due to decreased thermal emittance. The transparent film is preferably a dichroic coating having alternating layers of high and low index of refraction refractory materials.