The arc tube is filled with mercury, an inert gas, and more than one metal halide. During operation, mercury vaporizes and the mercury vapor pressure in the arc tube reaches several atmospheres (a few megapascals); the halide also evaporates from the tube wall, diffuses into the high temperature arc column, and the metal atoms are excited by ionization, radiating the characteristic spectrum. line. As the metal ions diffuse back to the tube wall, they meet the halogen atoms in the cooler regions near the tube wall and recombine to form halide molecules. This cyclic process continually supplies metal vapor to the arc. The partial pressure of the metal vapor at the axial center of the arc is similar to the partial pressure of the halide vapor at the wall of the tube, and is generally 1330 to 13300 Pa. The average excitation potential of the metal is usually about 4 eV, and the excitation potential of mercury is 7.8 eV. The total radiant power of the metal spectrum can greatly exceed the radiant power of mercury. As a result, the line of output of a typical metal halide lamp is primarily a metal spectrum. Filling different kinds of metal halides can improve the color rendering of the lamp (average color rendering index Ra is 70-95).
Only 23% of the total emission of mercury halide in the metal halide lamp is in the visible region, while the total radiation of the metal halide arc is more than 50% in the visible region, and the luminous efficiency of the lamp can be as high as 120 lm/W or more. Metal halides and electrodes, quartz glass, and halides can cause chemical reactions at high temperatures. Metal halides are prone to deliquescence, and the inhalation of a very small amount of water can cause abnormal discharge and blacken the lamp. The electrode electron-emitting material is ruthenium oxide, ruthenium oxide, ruthenium oxide or the like to prevent the emission material from reacting with the halogen. Some metals (such as sodium) in the arc tube will migrate, resulting in an excess of halogen, resulting in a very negative halogen, causing arc shrinkage and starting voltage, operating voltage rise. The metal halide lamp can not be reliably activated by the action of the trigger electrode. Generally, a bimetal starter is used, or a leakage transformer having a sufficiently high starting voltage is used, and an electronic trigger is also used. The ignition of metal halide lamps also requires a current limiter (ie, a ballast) that operates at a higher current than a high-pressure mercury lamp of the same power.
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