ABSTRACT
Central Development Lighting, Philips Lighting, P.O. Box 80020, 5600 JM Eindhoven, The Netherlands 2Predevelopment Center Lighting Electronics, Philips Lighting, P.O. Box 80020, 5600 JM, Eindhoven, The Netherlands Contact: riceijser@philips.com
INTRODUCTION White HPS lamps require some form of electronic control to keep the colour of the lamps between acceptable limits. A control algorithm used in practice is based on the existence of a correlation between the colour temperature of the lamp and the lamp voltage [1,2]. This correlation is based on the fact that both the spectral distribution and the electrical characteristics are directly determined by the Na and Hg vapour pressures. This, however, holds only in case of a well-defined relation between these vapour pressures, which is only the case for a fixed amalgam composition. It fails in a situation where, for example, the amalgam composition changes as a result of Na corrosion or diffusion as the lamp ages. The impact of Na corrosion processes - even at the high Na vapour pressures in these types of lamps - can be suppressed sufficiently to obtain adequate lamp life [3]. However, as the lamp ages, ultimately a point will be reached where the Na loss becomes significant. The consequence of this is that a colour control approach based on sensing of the steady state electrical lamp characteristics will fail. Fig. I illustrates the situation: for relatively new lamps (fixed amalgam composition with Na mole fraction xr.h,=0.74) there exists a unique relation between colour temperature Tc and lamp voltage Via (data are for 50 Hz operation). However, for old lamps (with severe loss of Na) we see a large drop in Tc as compared to normal lamps with the same lamp voltage, resulting in an unacceptable yellowish appearance of the lamp. Also, the correlation between Tc and Vh, is lost.
