High-Intensity Discharge Lamps

Metal halide. The metal halide lamp (Figure 2) operates by passing an electric current through vaporized mercury, which is similar to fluorescent lamp operation. The addition of iodides of metals to the arc tube enables the lamps to offer very high efficacy and high-quality light. Those characteristics make metal halide lamps a very effective choice for energy-efficient lighting, but there are some drawbacks:

Some of these problems have been overcome by metal halide systems using what is known as pulse-start technology. In these systems, a brief, high-power pulse is used to ignite the lamps. Although pulse-start lamps can only be started with pulse-start ballasts,it is possible to use standard lamps with pulse-start ballasts. Pulse-start lamps are available in 50- to 450-W models.

Figure 2: Metal halide lamp

Metal halide lamps offer high efficacies and good color rendering.

The benefits offered by the pulse-start systems include superior efficacy; more-rapid, reliable starting, even in cold weather; less electrode damage with each start (which extends lamp life); better lumen maintenance; and faster restrike times (5 to 7 minutes, compared with 10 to 15 minutes for standard metal halide systems).

New ceramic metal halide lamps, which use a polycrystalline alumina material for the arc tube instead of quartz, offer improved performance in color rendering, color uniformity, and color stability over the life of the lamp. All ceramic metal halide lamps use the pulse-start method. Those running on electronic ballasts can match or exceed the performance of fluorescent systems in high-bay applications where color quality, fixture aesthetics, and uniform uplight are important, and where facilities are subject to high and low temperature extremes.

High-pressure sodium. Sodium lamps (Figure 3) vary more widely than other HID lamps in their efficacy and color quality. For example, the light from some sodium sources is predominantly emitted in the longer wavelengths, between yellow and red. Light emitted in these wavelengths is less effective than light emitted at shorter wavelengths because the human eye is less sensitive to photons in that region of the spectrum.

Figure 3: High-pressure sodium lamp

High-pressure sodium lamps can be operated in any orientation and do not require an enclosure, except when there is a need to prevent moisture from accumulating on the lamp.

Sodium lamps use no phosphors, and their low mercury content creates very little UV output, although some high-pressure sodium lamps are coated to reduce glare and to widen light distribution.

Color rendering index (CRI) is a scale for describing the effect of a light source on the color appearance of objects being illuminated. A value of 100 is the maximum possible CRI. There are three basic grades of high-pressure sodium lamps: Low-grade lamps with a CRI of about 21, which are typically used in outdoor lighting; general-purpose indoor lamps with a CRI of about 60; and “white” lamps with a CRI of 80 or more. Unfortunately, the units with higher CRIs have lower efficacies (Figure 4). The “white” high-pressure sodium lamps have an efficacy lower than that of ceramic metal halide lamps but greater than that of halogen units. They have, for the most part, been superseded by ceramic metal halide lamps, which boast not only higher efficacy but also better color and longer life.

Figure 4: Color quality and efficacy for sodium lamps

Sodium lamps are available in four distinct color-quality ranges. It is important to note, however, that sodium lamps with higher color quality have lower efficacy than those of the same power rating with lower color quality.

Ballasts. HID lamps are available with one of three types of magnetic ballasts or with electronic ballasts. Magnetic ballasts are less expensive, but they are bulky and less efficient than electronic versions. The most commonly used type today is the constant-wattage autotransformer, but electronic ballasts—which improve lamp life, reduce lumen depreciation, improve color stability, and are small in size—are beginning to penetrate the market (see the guide on metal halide ballasts).

Choose the right type of lamp for the application. Different HID lamps are appropriate for different applications. Table 1 provides a visual guide for selecting the right kind of HID lamp for your application.

Table 1: High-intensity discharge light sources and the best applications

Excellent color properties and a wide range of sizes make the metal-halide lamp the most versatile type of high-intensity discharge light source.

Consider safety. Metal halide lamps operate at high internal pressures and temperatures—and if the lamps are not used properly, a rupture can eject hot particles. To help users minimize fire risk, the National Electrical Manufacturers Association (NEMA) updated its best practices guide for metal halide lighting systems, presenting information on selection, operation, and maintenance: Best Practices for Metal Halide Lighting Systems, Plus Questions and Answers About Lamp Ruptures in Metal Halide Lighting Systems.

Choose the right lamp for the intended position. The output of some types of metal halide lamps is sensitive to lamp position. Manufacturers classify their products into three categories: BU and BD lamps are intended to be operated in the base up or base down (vertical) position, HOR (or H) lamps are designed to operate in the horizontal position, and U lamps are designed for universal operation and can be installed in any position. However, note that U lamps operated in positions other than vertical suffer from light output reductions of up to 25 percent, lower efficiency, and more lumen depreciation. High-pressure sodium lamps may be operated in any position, but operation in an orientation other than base up or base down can reduce efficacy and may shorten lamp life.

Make sure the lamps and ballasts are compatible. The American National Standards Institute (ANSI) is a nonprofit organization that develops voluntary industry standards for various products. With few exceptions, HID lamps and ballasts should have matching ANSI designations. Luminaire manufacturers normally label their fixtures with the ANSI designation of the ballast so that relamping personnel can be sure to install the correct lamp.

Avoid retrofits with low power factor and high THD ballasts. The use of low power factor ballasts may increase current draw beyond circuit capacity, causing fuses to blow or breakers to open. Total harmonic distortion (THD) is a measure of how much the current waveform differs from a pure sinusoidal wave. Ballasts with a high THD could lead to operational problems if used to upgrade multiple fixtures in one location (as in a convention hall with many incandescent sockets).

Consider metal halide lights for outdoor use. High-pressure sodium systems are primarily used for outdoor lighting for security, roadways, and parking lots. Often, low-CRI units are used in such applications, but the quality of their yellow light can be unsatisfactory. Modern outdoor lighting systems are increasingly using metal halide sources because of their superior light quality. In some applications, metal halide lights applied intelligently may be able to replace high-pressure sodium lighting and reduce installed wattage. Light-emitting diodes (LEDs) are also becoming an attractive choice for outdoor lighting (see our advice on purchasing LEDs).

Think about maintenance costs. HID lamps generally have long lifetimes (Figure 5), but, because they are commonly used in high-ceiling applications, maintenance costs can significantly affect system economics. Labor costs can range from $5 to more than $100 per lamp change, depending on the difficulty of replacement and the equipment required. In high-ceiling applications, it may make sense to choose an open, lenseless fixture, even though it would necessitate the use of a more expensive lamp. Lamps in open fixtures can be easily replaced with a lamp-clamping device on a pole, which will greatly reduce change-out time compared to using a ladder. There are also fixtures available with an internal winch that lowers the entire unit for easy maintenance. When maintenance costs are really high, induction lamps may be a better option.

Figure 5: Lifetimes of high-intensity discharge lamps

High-intensity discharge lamps have some of the longest lifetimes of any light source. For comparison’s sake, this figure also shows incandescent and fluorescent lamp lifetimes.

For applications where color is critical, use electronic ballasts and ceramic metal halide lamps. Electronic ballasts that control lamp current help maintain nearly constant color. With magnetic ballasts, HID lamps exhibit significant color shifting over their lifetime. Ceramic metal halide lamps provide better color quality than fluorescent or other HID options.

Mix fluorescent and HID lighting to help combat color shifting. Standard metal halide and, to a lesser extent, high-pressure sodium lamps change color over time, most noticeably after about 70 percent of the rated lamp life. Spot relamping makes this phenomenon stand out, because lamps of different ages may be grouped near each other so that their differences in color are highlighted. The use of more-color-stable light sources (such as fluorescent lamps) in the same area can reduce the problem. Combining HID indirect lighting with fluorescents can also help, as the HIDs can reflect light off of surfaces not in the immediate view of occupants. Some designers also position multiple lamps so that their light will be mixed before reaching the target surface.

Consider the starting and restarting delays inherent in HID lighting. Ask these questions when considering HID lamps:

If the answer to any of these questions is yes, several potential solutions are available. Instant-restrike lamps (such as fluorescents) can be used in the same space, or an instant-restrike stand-by light source can be added to each HID fixture (typically a small halogen lamp). Instant-restrike HID lamp ballasts are also available, but they exist only in large sizes (250 W or more) and are costly. Note that stand-by lighting usually requires a 120-volt (V) supply, whereas many HID fixtures run on 277-V power. If incandescent stand-by lighting (regular or halogen) is included, the new ballasts must have a tap that provides 120-V power; otherwise a separate 120-V circuit will be required. Many HID ballasts do not include this feature. Because fluorescent ballasts are available for 277-V circuits, the use of fluorescent standby lighting does not create any special power-supply problems.

Is UV a potential problem? The high levels of UV put out by metal halide lamps can damage sensitive fabrics, papers, and artifacts. Special filtering can help prevent damage, but with some globes and sconces, available glass does not filter much UV and the flexible filters required lose their capacity at the temperatures common to metal halide fixtures. In those cases, ceramic metal halide lamps are the best bet because they contain UV blocking systems and have much lower UV output than conventional quartz-based metal halide lamps.

Consider the newest fluorescent lamps and fixtures in medium- and high-bay applications. HID lighting has been recommended for a number of years as the ideal replacement for linear fluorescent lamps in medium- and high-bay applications. HID was preferred because of its greater efficacy and superior color quality. However, the newest linear fluorescents have color quality and efficacy that surpasses HID lamps, and improved fixtures are once again making them an attractive option for medium- and high-bay areas. See the HID Versus Fluorescent for High-Bay Lighting topic for more information.

HID technology continues to advance on a number of fronts. With the introduction of ceramic metal halide lamps, HID became an effective alternative for several niches in the high-bay market. Whether continued improvements can open up wider niches is an open question. Fluorescents still provide a number of advantages, including higher efficacy at most operating conditions, lower lumen depreciation rates, better dimming options, and faster start-up and restrike times. In addition, until recently, dimming was a problem for HID lights—bi-level systems were the only option available. However, new products that provide continuous dimming down to about 50 percent of light levels have recently been introduced. Look for these products to make a bigger impact in the future. And in the near future, electronic ballasts for both fluorescent and HID systems may be dimmable via wireless controllers.