Effects of the polycrystalline alumina arc tube transparency on the efficacy of the high pressure sodium lamps.

Abstract

The effects of the polycrystalline alumina (PCA) arc tube transparency on the luminous efficacy of the High pressure sodium lamps were investigated. Four PCA sample tubes with 8-88 micron average grain size were used and total transmittance and in line transmittance were measured. In-line transmittance of the PCA tube was derived dy assuming the superposition of each grain’s optical system and it increases with the average grain size. Luminous efficacy increases as the in-line transmittance increases if total transmittance is constant. The relationship between luminous efficacy and inline transmittance is similar to a hyperbola. In case the lamp used higher in-line transmittance, the energy radiated from the arc plasma was less reflected at the tube wall and less energy was absorbed in the non-radiated region and this may bring about a higher luminous efficacy.