Metal halide lamp having open tungsten coil electrodes

In a metal halide lamp of the sodium-scandium-thorium iodide type utilizing tungsten electrodes without emission mix, lumen depreciation during life results primarily from blackening of the arc tube wall by electrode sputtering during the glow-to-arc transition phase of lamp start-up. The glow-to-arc transition is speeded up by a lower glow-to-arc transition voltage which is achieved by using electrodes comprising an open tungsten wire coil on a tungsten shank, the coil comprising two layers of a composite wire made by open-winding a 2 mil overwind on a 4 mil core and then close-winding two layers of the composite wire on the shank. This decreases sputtering at starting and improves lamp maintenance.