Effects of Impurity Gases on Lamp Starting

Abstract

An arc discharge lamp starts usually through two steps, one is breakdown of gap and the other is glow-to-arc transition following the breakthrough.
If impurity gases exist, lamp starting voltage, namely, breakdown voltage and glow-to-arc transition voltage, increase. The impurity effect is represented as (E²-E₀²)/E₀², that means an increasing ratio of electron energy loss by impurity gas to that by dominant gas, is proportional to relative impurity gas density, where E and E₀ are the axial electric field strength of the discharge space containing and not containing impurity, respectively.
Using values of lamp starting voltage, it is able to determine relative density of impurity gas and also decreasing constant of impurity gas density in the discharge lamp operating with impurity getter action. For example, density of the impurity gas, N₂ in a 400 watt metal halide lamp decreases as n=n₀ exp (-t/3), where t is time (hours).