Theoretical Analysis of the Behavior of Mercury plus Thallium Iodide Arc Lamps with Variable Mercury Loadings

A theoretical analysis of the behavior of 400-watt Hg + TlI arc lamps with variable mercury loadings is performed. It is quantitatively shown that, as the Hg loading is doubled, the following effects may be predicted: (a) the effective arc temperature decreases from 5100 K to 4800 K, (b) the effective radius increases by ~10%, (c) the escape factor for the imprisoned Tl 5350 A line increases ~40%, (d) the power radiated in the Tl 5350 A line increases ~10%, and (e) the power radiated in the Hg visible line decreases ~15%. These results are in agreement with the experimental observations of Larson. These effects result in an increase in the lumen-per-watt ratings, which is of technological significance. Self-absorption is an important factor in the radiation of the Tl 5350 A line, and it is shown that the experimental observations are reasonably consistent with the assumption of local thermodynamic equilibrium.