Metal halide lamps with ceramic envelopes: A breakthrough in color control

A few decades ago, researchers transformed the high-pressure mercury vapor lamp by adding metal halides into the are tube, thus improving luminous efficacy and color rendering. Metal halides were chosen over metals because they could be vaporized at the lower temperatures which quartz are tubes were capable of handling. Market acceptance of these metal halide lamps was strong, despite some well-recognized shortcomings, including low initial color uniformity, moderate color rendering, high color shift over life, and low lumen maintenance. The initial color uniformity of existing metal halide lamps can be improved substantially by increasing tile operating temperature of the are tribe and thus the vapor pressure of the metal halides. However, increasing this temperature causes rapid deterioration of the quartz are tube, resulting in color shift reduced lifetime. Using are tubes made of ceramic instead of quartz allows for higher operating temperatures without the threat of rapid deterioration, and results in excellent initial color uniformity, high color rendering, lo-cv color: shift over life, and excellent lumen maintenance. Polycrystalline alumina are tubes, used for decades in high-pressure sodium lamps, provide excellent results in metal halide lamps as well, once several technical obstacles are overcome. This paper describes the unique photometric and lifetime properties of these new lamps, as well as the special are tube design that eventually made these possible, The challenge to lamp designers is to improve the color control by minimizing the color spread and color shift, while maintaining lamp lifetimes thar customers require. This can hardly be realized by using quartz discharge tubes. To achieve a breakthrough in color control, a new envelope material is needed. The chemical resistance of ceramic envelopes allows increased discharge-tube operating temperatures. The design of the ceramic discharge chamber cakes advantage of this fact to optimize photometric propel-ties that are important to customers, like a good color rendering, an excellent color control, and a higher luminous flux, both initially and throughout lifetime. A special feedthrough design ensures chat the lamps achieve the customers' required lifetimes.