to four times the atmospheric pressure. Lamps can operate in any position; however, light output is reduced when burned in positions other than vertical. Mercury lamps  for  lighting  applications  range  in  wattage  from  40 to 1,000 watts. The 175- and 400-watt types are the most popular. Mercury lamps are used in streetlighting, security lighting, and outdoor area lighting. In new installations today, mercury lamps are being replaced with  more  efficient  metal  halide  or  high-pressure sodium systems. Metal Halide Lamps The halide lamps are similar to mercury lamps in construction because the lamp consists of a quartz arc tube mounted within an outer glass bulb; however, in addition to mercury, the arc tubes contain halide salts, usually sodium and scandium iodide. During lamp operation, the heat from the arc discharge evaporates the iodide along with the mercury. The result is an increase in efficiency approximately 50 percent higher than  that  of  a  mercury  lamp  of  the  same  wattage together with excellent color quality from the arc. The amount of iodide vaporized determines lamp efficiency and color and is temperature-dependent. Metal halide arc tubes have carefully controlled seal shapes  to  maintain  temperature  consistency  between lamps. In addition, one or both ends of the arc tube are coated  to  maintain  the  desired  arc-tube  temperature. There is some color variation between individual metal halide lamps owing to differences in the characteristics of each lamp. Metal halide lamps use a starting electrode at one end of the arc tube that operates in the same manner as the starling electrode in a mercury lamp. A bimetal shorting switch is placed between the starting electrode and  the  adjacent  main  electrode.  This  switch  closes during  lamp  operation  and  prevents  a  small  voltage from developing between the two electrodes, that in the presence of the halides could cause arc-tube seal failure. High Pressure Sodium Lamps The   high-pressure   sodium   lamp,   commonly referred to as HPS, has the highest light-producing efficiency of any commercial source of white light. Like  most  other  high-intensity-discharge  lamps,  high- pressure sodium lamps consist of an arc tube enclosed within an outer glass bulb. The arc operates in a sodium vapor at a temperature and pressure that provide a warm color with light in all portions of the visible spectrum at a high efficiency. Owing to the chemical activity of hot sodium, quartz cannot be used as the arc-tube material; instead, high-pressure sodium arc tubes are made of an alumina ceramic (polycrystalline alumina oxide) that can withstand the corrosive effects of hot sodium vapor. There are coated-tungsten electrodes sealed at each end of the arc tube. The sodium is placed in the arc tube in the form of a sodium-mercury amalgam that is chemically inactive. The arc tube is filled with xenon gas to aid in starting. High-pressure sodium lamps are available in sizes from 35 to 1,000 watts. They can be operated in any burning position and have the best lumen-maintenance characteristic of the three types of HID lamps.   Except for the 35-watt lamp, most high-pressure sodium lamps have rated lives of more than 24,000 hours.  The 35-watt lamp has a rated life of 16,000 hours. The 50-, 70-, and 150-watt sizes are available in both a mogul-base and a medium-base design. Fluorescent  Lighting Fluorescent lamps of high-pressure, hard glass are used to some extent for floodlighting where a low-level, highly diffused light is desired. This would include club parking  lots,  outside  shopping  areas,  parks,  or  grass areas. This bulb is much the same in operation as the mercury-vapor  lamp  with  the  exception  that  the fluorescent tube has an inside coating of material, called phosphor, that gives off light when bombarded by electrons. In this case, the visible light is a secondary effect of current flow through the lamp. Just like the HID lamps, the fluorescent lamp requires a ballast for operation. The color produced by the light depends on the type of phosphor material used. High-Intensity-Discharge  Lamp  Ballasts All   HID   lamps   have   a   negative-resistance characteristic. As a result, unless a current-limiting device is used, the lamp current will increase until the lamp  is  destroyed.  Ballasts  for  HID  lamps  provide  three basic functions: to control lamp current to the proper value, to provide sufficient voltage to start the lamp, and to match the lamp voltage to the line voltage. Ballasts are designed to provide proper electrical characteristics to the lamp over the range of primary voltage stated for each  ballast  design.  Typical  ballasts  are  shown  in  figure 6-15. 6-14