Electrode temperatures in low power metal halide lamps

Summary form only given, as follows. The temperature of the tip of the electrode in a low-power metal halide lamp is a critical design parameter. Temperatures above 2800 K reduce lumen output of the lamp by evaporation or chemical attack with subsequent transport of tungsten to the wall. Temperatures below 2200 K cause a constriction of the cathode arc spot with subsequent electrode erosion. The temperature depends on the energy balance between the power delivered to the tip by the plasma and the power conducted and radiated away. It depends on the diameter of the electrode, on the current through the lamp, on the mercury pressure, and, to a lesser extent, on any noble-gas present and the other dimensions of the lamp. Scaling rules and models have been used to predict. These have been compared with physical measurements made with a near infrared pyrometer.