Ceramic metal halide lamps for general illumination utilize translucent polycrystalline alumina (PCA) discharge vessels. PCA is translucent, not transparent, due to birefringence of the hexagonal alumina grains. Because of the lack of transparency, a PCA discharge vessel is generally not suitable for focused-beam, short-arc lamps such as projection lamps and automotive headlights. For focused-beam lamps, a transparent ceramic like sapphire is required.
Aluminum oxynitride (AlON) is a transparent ceramic material with in-line transmittance values as high as that of sapphire. AlON has a cubic spinel structure and a composition that may be generally represented by the empirical formula Al(64+x)/3O32−xNx where 2.75≦x≦5. The mechanical strength and thermal expansion of AlON are close to those of PCA, so that AlON should be able to survive the stresses in high-intensity discharge (HID) lamps. In fact, several sources have identified AlON as a material suitable for HID lamps, for example, Japanese Patent No. 09-92206 and U.S. Pat. Nos. 5,924,904 and 5,231,062.