Electrode temperature diagnostics and a two-dimensional electrode model have been developed to improve our understanding of electrode behaviour in ceramic metal halide lamps. Using transparent YAG arc tubes in dc and ac operation, anode and cathode characteristics, like the effective work function, anode fall and electrode input power, could be derived from the measured temperature profiles. It is found that Dy-iodide in the metal halide lamp filling has a strong so-called gas-phase emitter effect. In order to improve our understanding of the observed phenomena and to help design electrodes for future lamps, a rotational symmetric two-dimensional electrode model has been created. The model is completely phase resolved so that time-dependent effects can be studied (both ac and dc). Furthermore, it contains various options for calculating the power input distribution, including a complex cathode sheath model as well as a simple anode model. The model has been shown to predict spot/diffuse transitions (in Hg-lamps with heavy electrodes/low currents) very similar to the behaviour seen in real lamps.

A diagnostic technique is presented to determine the electrode work function in ac-operated metal halide lamps. The heart of the experimental set-up is a high-speed photodiode array detector, which is able to follow real-time variations of electrode tip temperature and near-electrode plasma emissions in ac-operated experimental YAG lamps, enabling discrimination between the anode and cathode effects. Electrode tip temperature ripples have been measured for 100 Hz square wave operation and simulated with an existing electrode model. By using the electrode work function as main fit parameter for the simulations it is found that the measured cooling effect of the electrode tip in a NaTlDy-iodide lamp is caused by a gas-phase emitter effect of Dy. It is concluded that Dy coverage of the electrode tip causes an effective work function reduction of 0.3 eV at 100 Hz square wave operation, considerably less than the 1.0 eV reduction measured earlier for dc operation.