Evaporation Studies of the Sintered Aluminum Oxide Discharge Tubes Used in High Pressure Sodium (HPS) Lamps

Abstract

In order to achieve the highest possible luminous efficacy, HPS lamps are generally designed to operate at the highest possible wall loading, and thus wall temperature, for the sintered aluminum oxide discharge tube. At wall temperatures above 1150°C-1200°C, excessive alumina sublimation rates can produce dark deposits on the outer envelope that can cause significant lumen losses within an unacceptably short time period. In order to determine whether higher wall temperatures could be realized for the HPS discharge tube using a fill gas rather than a vacuum in the outer envelope, evaporation studies were carried out on HPS discharge tubes mounted in evacuated and gas-filled outer envelopes.