High Pressure Mercury Lamps This lamp consists of a quartz arc tube sealed within an outer glass jacket or bulb. The inner arc tube is made of quartz to withstand the high temperatures, resulting when the lamp builds up to normal wattage. Two main electronemissive electrodes are located at opposite ends of the tube; these are made of coiled tungsten wire. Near the upper main electrode is a third, or starting, electrode in series with a ballasting resistor and connected to the lower main-electrode lead wire.

The arc tube in the mercury lamp contains a small amount of pure argon gas that is vaporized. When voltage is applied, an electric field is set up between the starting electrode and the adjacent main electrode. This ionizing potential causes current to flow, and, as the main arc strikes, the heat generated gradually vaporizes the mercury. When the arc tube is filled with mercury vapor, it creates a low-resistance path for current to flow between the main electrodes. When this takes place, the starting electrode and its high-resistance path become automatically inactive.

Once the discharge begins, the enclosed arc becomes a light source with one electrode acting as a cathode and the other as an anode. The electrodes will exchange functions as the ac supply changes polarity.

The quantity of mercury in the arc tube is carefully measured to maintain quite an exact vapor pressure under design conditions of operation. This pressure differs with wattage sizes, depending on arc-tube dimensions, voltage-current relationships, and various other design factors.

Efficient operation requires the maintenance of a high temperature of the arc tube. For this reason, the arc tube is enclosed in an outer bulb made of heat-resistant glass that makes the arc tube less subject to surrounding temperature or cooling by air circulation. About half an atmosphere of nitrogen is introduced into the space between the arc tube and the outer bulb. The operating pressure for most mercury lamps is in the range of two 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.

Figure 6-15.- Mercury-lamp ballast circuits. Ballasts are classified into three major categories depending on the basic circuit involved: nonregulating, lead-type regulating, and lag-type regulating. Each type has different operating characteristics.