The arc tube is filled with mercury, inert gas and more than one metal halide. When working, mercury evaporates, the mercury vapor pressure in the arc tube reaches several atmospheres (fraction), halide also evaporates from the tube wall, diffuses into the high temperature arc column, and the metal atoms are ionized to radiate the characteristic spectral line. When the metal ions diffuse to the pipe wall, the halogen atoms are met in the cooler area near the wall of the tube, and the halide molecule is regenerated. This cyclic process continuously provides metal vapor to the arc. The metal vapor partial pressure at the arc axis is similar to the partial pressure of the halide vapor at the pipe wall, generally 1330~13300pa. The average excitation potential of the metal is about 4eV, and the excitation potential of mercury is 7.8eV. The total radiated power of the metal spectrum can exceed the radiation power of mercury significantly. As a result, the spectral lines of typical metal halide lamps are mainly metal spectra. Filling different kinds of metal halide can improve the color rendering of the lamp (the average color rendering index RA is 70~95). Only 23% of total mercury arc radiation in the visible region, while the total radiation of metal halide arc is more than 50% in the visible area, the luminous efficiency of lamps can be as high as 120lm/w.
Metal halides and electrodes, quartz glass, and halides can cause chemical reactions at elevated temperatures. Metal Halide easily hygroscopic, a small amount of water inhalation can cause the discharge is not normal, so that the lamp black. The electrode electron emission material is made of dysprosium oxide, yttrium oxide, scandium oxide, etc. to prevent the reaction between the emitting material and the halogen. Some metal in the arc tube (such as sodium) will migrate, resulting in halogen excess, resulting in a very strong halogen negative, resulting in arc contraction and start-up voltage, operating voltage rise. Metal Halide lamp Only by the role of trigger electrodes can not be reliably activated, the general use of Bimetal starter, or the use of high enough to start the voltage of the magnetic flux leakage transformers, but also the use of electronic triggers. Metal Halide lamp ignition also requires a restrictor (ie, ballast), its operating current than the same power high-pressure mercury lamp larger.