Improved Color Stability in Highly Loaded Metal Halide Lamps

摘要

The use of low wattage, high intensity discharge lamps for automotive forward lighting is well established. In most cases, the lamps perform well over the expected life of about 2500 hours. However, because of the small volume and high power loading, the interior of the lamp vessel is quite hot and physiochemical reactions that affect the virgin lamp performance are accelerated. Understanding the dynamics of these processes and their interrelationships permits tuning of the light sources to limit the color shift of the light output as the lamp ages. While a zero color shift is not realizable, it is possible to manage the (x,y) chromaticity swing to minimal and slowly changing values. Of course, the lamp arc gap and operating voltage must be the maintained at current specifications for optical considerations, and the lamp must function properly on existing electronic control gear. Improved color stability over life is achieved through a combination of changes to the lamp construction: reduction in size of the lamp plasma chamber (interior volume of the pre-form), change of the arc tube sleeving material, and a small modification to the fill. Intuitively, one would expect the reduced volume lamp to overheat (since wattage is maintained at 35W and power loading is thus increased), have erratic color control and reduced life. However, this does not occur if the interrelationships between the physiochemical processes are exploited in a unique and beneficial way.