A New Metal Halide Arc Lamp
The production of light using medium-pressure electric discharge devices (one to two atmospheres) is one of the older lighting techniques. The commercial development of the medium-pressure device in which mercury was the active ingredient commenced in the period between 1920 and 1940. This source has been widely accepted. Unfortunately, however, the color rendition was poor, due mainly to the lack of red light being emitted by the elemental mercury. This serious drawback has been partially corrected through the use of fluorescent phosphors coated upon the inner wall of the lamp's outer jacket.
Spectroscopists have used electrodeless electric discharge devices containing metals to study their emission lines. Most of the metals that have been studies have low vapor pressures at temperatures which are compatible with envelope materials having high transmission to visible radiation. To circumvent this difficulty, metal iodides were used. The vapor pressures of these iodides are significantly higher than for the corresponding metals. Quite unexpectedly it was discovered that certain of the basic concepts of the electrodeless devices could be utilized for practical lighting devices. Some work in this direction has been reported to this Society by Larson et al. and Martt et al. who discussed the use of additive in conventional mercury lamps. This presentation will describe a completely novel source in which mercury has taken on a different role.