Metal halide (MH) lamps were introduced about 1965 or 1966 after some years of development and research by leading high intensity discharge lamp manufacturers in the United States. The first commercially available was the 400-watt size and offered 80 to 90 lumens per watt and a 6,000-hour estimated life upon its debut. In subsequent years other wattages became available, making this "white" light source quite versatile for a number of commercial interior and exterior lighting applications.
These are made in several wattages, ranging from 32 to 1,000 watts. Upon comparing a typical 400-watt metal halide and mercury lamp of the same rating, differences will be noted in the arc tube mounting from the stem of a mercury lamp. The lead wire from the arc tube, remote from the stem, is spaced away to reduce photoelectric currents, which otherwise can lead to sodium loss by electrolysis through the arc tube wall into the outer jacket, with possible arc tube failure as a result. A thermal switch in the starting probe lead has a similar purpose; when it is closed it removes the potential difference between the adjacent electrodes while the series resistor minimizes energy loss. A "getter" is often included to absorb any gassy contaminants which otherwise would discolor the inside of the outer bulb during lamp life.
MH lamps are usually enclosed in an elliptical or bulged tubular envelope and are very similar in appearance to a mercury vapor lamp. Some MH lamps have an internal coating of phosphor which helps reduce glare and in some cases, improves color rendition. These phosphors are applied in the same manner as mercury lamps, although their diffuse coatings may reduce light output slightly in some lamps. Like mercury ones, the elliptical lamps are entirely coated down to just above the screw base. Bulged-tubular MH lamps were coated with phosphor only in their widened mid section with the bulb's squared end remaining uncoated and clear until 1975 or 1976, as was the case also for mercury lamps. After that time manufacturers coated the entire inside of the lamp, except for a small section just above the screw base. MH lamps contain the iodides of several chemical elements in addition to mercury within their quartz arc tubes while some specially phosphor coated lamps have provided some excellent color rendering combined with good color appearance. In recent years metal halide lamps have been developed with color temperatures around 2900 degrees Kelvin, which, in appearance, is very similar to that of incandescent light. In addition, newer lamps are on the market that reduce noticeable color shift during the lamps' life expectancies.
Typical MH lamp wattages currently are 32, 50, 70, 100, 150, 175, 250, 400 and 1,000. Expected life expectancies of some MH lamps are less than stated above; the 50-watt lamp at 5,000 hours and 6,000 hours for the 175 and 250-watt lamp when operated horizontally. The lower wattage (below 175 watts) lamps typically have medium bases while all others are of the mogul screw type. Some 100-watt MH lamps are available with a mogul screw base for luminaries designed for larger lamps, particularly roadway styles.
Metal Halide Luminaries
MH lamps have not been common practice for streetlighting applications, likely because high-pressure sodium is somewhat more efficient and HPS lamps last longer. However, most streetlighting luminarie manufacturers offer them in conventional "cobra head" styles, typically from 100 to 400 watts. These can be readily identified during the day by the NEMA-compliant color-coded lamp wattage label (red designating MH) affixed to the luminarie. With emerging MH lamp technology, the trend may change more toward MH as a standard streetlighting illuminant in future years, particularly since the white light source of MH is more pleasantly appealing to the eye. Improved whiteness and life expectancies equal to HPS lamps will stimulate more competition with HPS lamps. Presently, the majority of MH applications for outdoor lighting include parking lots, security lighting, and many other industrial and commercial applications, etc. MH also has become quite popular in lighting of large interiors, such as retail stores, warehouses, etc. Their white light source and superior color rendering along with excellent optical control of the lamp are making this light source that of choice for these applications. Because some commercial users have found that illuminating their premises is more beneficial with MH than HPS on account of the inherent MH crisp, white light and improved color rendering, MH retrofit lamps are available in the 250, 325, 400 and 950 watt sizes. These lamps have light outputs comparable to their standard MH counterparts of the same rating and similar life expectancies as well, in the order of 10,000 to 20,000 hours, depending on lamp type and burning position.
Like mercury lamps, an inherent characteristic among MH lamps is a slight upward bowing of the luminous arc when the lamp is operated horizontally. MH lamp efficacy is reduced somewhat when operated in this position opposed to base up or base down. To compensate for this loss, some MH lamps have been and are manufactured with curved arc tubes for horizontal operation in special position-oriented sockets which mate with the lamp base's protruding lugs, thus providing an upwards arc stream within the lamp's similarly configured arc tube. These lamps have proven effective and have been utilized mostly in commercial applications rather than for street lighting. Most of these curved arc tube lamps have been made in the 400-watt size and their special sockets are clearly marked for use with these lamps only and/or are yellow porcelain glazed for such identification.
MH lamps require a ballast, just as mercury lamps do, and are manufactured in differing electrical configurations, dependent upon lamp wattage and customer requirements. Some of the higher wattage MH ballasts will also operate a mercury lamp; however a MH lamp will not run on a mercury ballast. MH lamps 100 watts and below usually require a starting aid or igniter within the ballast circuit to initiate a pulse of several thousand volts to start the lamp. For this reason, like with HPS lamps, sockets must be utilized which can withstand such voltages, which take place for usually a few milliseconds to initiate the arc. Very brief power interruptions or even a dip in supply voltage can cause a MH lamp to extinguish. Several minutes are usually required for the lamp to cool and restart. Automatic auxiliary incandescent lighting is sometimes built into some MH luminaries to provide illumination (generally for public safety reasons) until the MH lamp attains full brilliancy and then the incandescent circuit automatically switches off. In such instances the incandescent source should not be too close to the MH lamp so that its heat will not prolong MH lamp re-striking and warm-up time.
(c) 1998 by Joe Maurath, Jr.
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