Metal halide lamps or metal halide arc lamps consist of a high performance arc discharge tube housed in an elliptical reflector that focuses the output into a microscopic optical train. The arc tube is filled with metal halide salts. A starter or ballast is usually used as the voltage source for these lamps.
Some of the notable characteristics of metal halide lamps are brighter images, intense light than other arc lamps, uniform irradiance and longer life span. These lamps are available in low, mid and high wattage ranges. This article will elaborate on the working, construction and applications of metal halide lamps.
Working and Spectral Output
Metal halide arc lamps are ignited by a high voltage trigger current applied to the electrodes. Argon gas which is used as a starter gas gets ionized and initiates the burning and ionization of the metal halide compound. Once the halide salts are heated to the operating temperatures, they ionize and move to higher energy levels. This results in the emission of spectral lines.
The choice of metal halide salt influences the color of the spectral output. The spectral output of these lamps lies in the UV region.
Metal halide lamps are made of a ceramic or a quartz envelope that houses the metal halide. Argon is used as a starter gas. The electrodes are made of tungsten alloys. Metal halide lamps are mostly used in microscopic applications; hence require a reflective assembly in order to focus the output light into the microscopic assembly.
Some of the main application areas of metal halide lamps are listed below:
- Outdoor lighting, road way lights, security lights, pedestrian walkways
- Digital Imaging
- Indoor ceiling lights.