Metal Halide Lamps with Ceramic Envelopes: A Breakthrough in Color Control
Most types of metal halide lamps have discharge tube fillings based on sodium and scandium halides or sodium and rare-earth halides. For decades, these halides have been contained within envelopes of fused quartz, henceforth called quartz. These halides provide spectral energy distributions that offer lamp designers a broad choice of color temperatures, color renderings, and luminous efficacies. However, these properties depend on the vapor pressure of the halides and hence on the operating temperature of the discharge tube. The higher the temperature, the better the color rendering, the smaller the color spread lamp-to-lamp, and the higher the efficacy. However, higher operating temperatures can cause severe color shift and reduce lamp lifetimes. The challenge to lamp designers is to improve the color control by minimizing the color spread and color shift, while maintaining lamp lifetimes that 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.