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 high vapor pressure—typically several atmospheres or even a few megapascals. The metal halides also vaporize, diffusing into the hot central arc column where they are ionized and excited, emitting light at specific wavelengths. As these metal atoms cool and 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 the lamp's performance.
The partial pressure of metal vapor at the center of the arc is usually around 1330 to 13300 Pa, matching the pressure of the halide vapor at the tube wall. The excitation potential of the metal components is typically around 4 eV, while mercury has a higher excitation energy of about 7.8 eV. Because of this, the light emitted by the lamp is dominated by the metal spectrum, which can produce much higher luminous output than mercury alone. By using different types of metal halides, 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 halide arc exceeds 50% in the visible range. This results in a high luminous efficiency, often reaching 120 lumens per watt or more. However, the high-temperature environment inside the lamp can cause chemical reactions between the metal halides, electrodes, quartz glass, and halogens. Additionally, metal halides are hygroscopic and can absorb moisture, leading to abnormal discharges or darkening of the lamp if exposed to even small amounts of water.
To prevent unwanted reactions, the electrode material is often made of ruthenium oxide or similar compounds. Some metals, like sodium, may migrate within the arc tube, causing an imbalance in the halogen levels. This can lead to negative halogen effects, resulting in arc shrinkage, increased starting voltage, and higher operating voltage. Unlike some other lamps, metal halide lamps cannot be reliably started using just a trigger electrode. Instead, a bimetal starter, a high-voltage leakage transformer, or an electronic igniter is typically used. Also, a current-limiting ballast is necessary to manage the higher current demands compared to high-pressure mercury lamps of similar power ratings.
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