Relationship between efficacy, arc tube temperature and power dissipation in a high-pressure sodium lamp

Abstract

Operating wall temperature and electrode separation are important factors in the design of efficient high-pressure sodium lamps. The authors have measured and calculated how the arc efficacy varies with wall temperature and input power per unit length. In their experiments they controlled the wall temperature by passing gases with different thermal conductivities over the arc tube. Their experiments showed that the arc efficacy increases at rates of 3.75+or-0.15% per 100K rise in wall temperature and 3.5+or-0.3% per 1 Wmm-1 increase in electrical power input. The corresponding calculated average rates are 2.1% per 100K and 1.7% per 1 Wmm-1. The satisfactory agreement of their experiments with their calculations resolves a discrepancy between their computer model predictions and those of Waymouth and Wyner (1981). Their two-zone model predicted that the efficacy decreases as the electrical power input increases. Although they have found that the proportion of the total radiation emitted in the D-lines decreases as the power input increases, this is more than compensated by an increase in the total radiated power at the expense of thermal conduction.