Dimming properties of medium and small wattage ceramic metal halide lamps

Ceramic metal halide lamps with rated powers of 70 W and 150 W are dimmed by reducing their operating power. Under dimmed conditions, photometric, colorimetric and electric parameters, as well as the temperature distribution of the outer wall of the arc tube have been measured. As the lamp power is reduced, luminous efficacy and colour rendering index decrease while correlated colour temperature rises. For each lamp, at 70% of its rated power, the luminous efficacy is greater than 80 lm/W, colour rendering index is above 74 and correlated colour temperature increases by less than 800 K. Dimming applications of these lamps are validated by these parameters. Furthermore, the complicated variation of lamp parameters as a function of operating power is explained by the variation of metal halide vapour pressure with wall temperature.