Inside the arc tube of a metal halide lamp, a mixture of mercury, an inert gas, and various metal halides is sealed. When the lamp is turned on, the mercury evaporates, creating a vapor pressure that can reach several atmospheres. At the same time, the metal halides also vaporize, diffusing into the high-temperature arc column where they are ionized and excited, producing characteristic spectral lines. As these metal atoms move back toward the cooler regions near the tube walls, they react with halogen atoms to reform halide molecules. This continuous cycle ensures a steady supply of metal vapor to the arc, maintaining efficient light output.
The partial pressure of the metal vapor at the center of the arc is typically around 1330 to 13300 Pa, matching the halide vapor pressure at the tube wall. The average excitation energy of the metals used is generally about 4 eV, while mercury requires a higher excitation potential of 7.8 eV. Because of this, the visible spectrum from the metal halides often dominates over that of mercury, making the lamp's output primarily a metal-based spectrum. By varying the types of metal halides used, the color rendering properties of the lamp can be significantly improved, with average color rendering indexes (Ra) ranging from 70 to 95.
Although only about 23% of the total emission from mercury in the lamp falls within the visible spectrum, the overall radiation from the metal halides exceeds 50% in the visible range. This leads to high luminous efficiency, often reaching 120 lumens per watt or more. However, the high temperatures inside the lamp can cause chemical reactions between the metal halides, electrodes, quartz glass, and other components. Additionally, metal halides are hygroscopic, meaning even small amounts of moisture can lead to abnormal discharge or blackening of the lamp envelope.
To prevent unwanted reactions, the electrode materials often include compounds like ruthenium oxide. Some metals, such as sodium, may migrate within the arc tube, leading to excess halogen and causing issues like arc shrinkage or increased starting and operating voltages. Metal halide lamps usually require a dedicated starting method, such as a bimetal starter or a high-voltage leakage transformer, and many modern systems use electronic triggers. A current-limiting ballast is also essential to control the initial surge of current during ignition, which is typically higher than that of a standard high-pressure mercury lamp of similar power.
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