Lighting: High-Intensity Discharge Lamps

High-intensity discharge (HID) lighting can offer better efficiency and longer life than fluorescent lighting, with color quality approaching that of incandescent lighting. Originally intended for outdoor and industrial applications, the use of HID lamps has spread to office and retail applications as their color-rendering characteristics have improved and smaller sizes have become available.

Mercury vapor lamps, metal halide lamps, and high-pressure sodium lamps all fall in the HID category. Major applications have included high-bay lighting for industrial environments, outdoor floodlighting, and roadway lighting, but they are now also being used in track lighting for offices and retail environments.

HID lamps are available in sizes ranging from 35 to 2,000 watts. Figure 1 shows that HID light sources boast the highest efficacies now available. (Efficacy is a measure of the performance of a lamp, calculated as light output divided by power input and expressed in lumens per watt.) Within the HID category, the highest-wattage lamps offer the highest efficacies.

What Are the Options?

Mercury vapor. Mercury vapor lamps (Figure 2), which are available in sizes ranging from 50 to 1,000 watts, offer low initial cost, long life, and color stability. However, they are approaching obsolescence, now that other HID light sources with better efficacy, more accurate color rendering, and improved lumen maintenance are becoming available at lower cost, especially in larger lamp sizes. In smaller sizes, mercury vapor lamps can be replaced with screw-in compact fluorescent lamps that do the same job at less than half the wattage, while providing better color.

Metal halide. Adding certain iodides of metals to the basic mercury vapor lamp produces the metal halide lamp (Figure 3), which offers very high efficacy and high-quality light. Those characteristics make metal halide lamps a very effective tool for energy efficient lighting, but there are some drawbacks:

• Start-up typically takes 3 to 5 minutes, and restarting after a shutdown or power interruption takes 10 to 20 minutes. Lamps over 400 watts are available with instant restrike capability, but they require special ballasts.

• Metal halide units produce high levels of UV radiation that must be shielded by glass in the lamp or fixture.

• The smallest metal halide sources produce 2,500 lumens from a small 32-watt package—nearly as much light as a 4-foot fluorescent. This high intensity limits the range of application for metal halide lamps and requires the use of sophisticated fixtures.

Some of these problems have been overcome by new 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-watt models.

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 to 10 to 20 minutes for standard metal halide systems).

High-pressure sodium. Sodium lamps (Figure 4) 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 lower wavelengths because the human eye is less sensitive to photons in that region of the spectrum.

On the positive side, sodium lamps use no phosphors. Also, their low mercury content creates very little ultraviolet output, although some high-pressure sodium (HPS) 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. HPS lamps are available in three basic grades: Low grade lamps with a CRI of about 21, which are typically used in outdoor lighting; general purpose indoor lamps with CRI of about 60; and "white" lamps with a CRI of 80 or more.

The units with higher CRIs have lower efficacies (Figure 5). The "white" HPS lamps have an efficacy lower than that of compact metal halide lamps, but greater than halogen units. Their warm, incandescent appearance and wide range of beam spreads make them a good alternative for retail display lighting applications.

Ballasts. HID lamps are available with three types of magnetic ballasts or with electronic ballasts. In order of increasing sophistication and cost, the magnetic ballast choices are:

• Reactor ballasts, which are small, cheap, and simple. They may cause lamps to flicker or shut off if voltage varies, and they offer a lower power factor—typically about 0.5. Power factor is a measure of the efficiency with which input power is converted to useful work. The ideal power factor is 1.0, and many utilities require power factors of at least 0.9 on products that they recommend.

• High-reactance autotransformers, which are similar to reactor ballasts, but are capable of providing a voltage boost when line voltage is insufficient to start a lamp.

• Constant wattage autotransformers, which are the most common type. They regulate lamp power better than the other magnetic ballasts, eliminating flicker and inadvertent shutoffs when line voltage varies.

Electronic ballasts are available for HID lamps rated up to 400 watts. They are more expensive than magnetic ballasts, but they are smaller and lighter in weight. They also provide more sophisticated control of lamp voltage and current, yielding better lumen maintenance and color control throughout the lifetime of the lamp. However, unlike electronic ballasts for fluorescent lamps, they do not offer greatly improved energy performance.

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How to Make the Best Choice

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

Use UL-approved fixtures. Metal halide lamps operate at very high pressures and may burst if subjected to abnormal thermal or electrical conditions. Underwriters Laboratory (UL) requires that fixtures for metal halide lamps include a means of containment to prevent injury from broken lamps. (Containment is not required if the manufacturer specifies that a given lamp can be used in open fixtures.)

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/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 UNI or U lamps are designed for universal operation and can be installed in any position. However, note that UNI lamps operated in positions other than vertical suffer from light output reductions of up to 20 percent, lower efficiency, and more lumen depreciation.

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 some exceptions, HID lamps and ballasts should have matching ANSI designations. Luminaire manufacturers normally label their fixtures with the ANSI designation of the ballast so re-lamping personnel can be sure to install the correct lamp.

Make sure that ballasts with igniter circuits also have circuitry that disconnects the igniter when the lamp fails. In the absence of a disconnect circuit, the ballast may not recognize that the lamp has failed and will continually try to restart the lamp, destroying the igniter and possibly the ballast as well.

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 HPS lighting with a reduction in installed wattage.

Think about maintenance costs. HID lamps generally have long lifetimes (Figure 6), 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, greatly reducing 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.

For applications where color is critical, use electronic ballasts. Electronic ballasts that control lamp current help maintain nearly constant color. With standard ballasts, HID lamps exhibit significant color shifting over their lifetime. Ceramic arc tube metal halide lamps also offer improved color stability.

Mix fluorescent and HID lighting to help combat color shifting. Standard metal halide and HPS lamps change color over time, most noticeably after about 70 percent of the rated lamp life. Spot relamping makes this phenomenon stand out, since lamps of different ages may be grouped near each other, highlighting their differences in color. The use of more color-stable light sources (such as fluorescents) 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 to HID lighting. Ask these questions when considering HID lamps:

• Will occupants need light as soon as they enter the room?

• Will the lights be turned off during the normal workday and will the delayed restrike time be acceptable?

• Will temporary darkness after a brownout or blackout cause serious problems?

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 lamps are also available, but they exist only in large sizes (250 watts or more). Note that stand-by lighting usually requires 120-volt supply, while many HID fixtures run on 277-volt power. If stand-by lighting is included, the new ballasts must have a tap that provides 120-volt power; otherwise a separate 120-volt circuit will be required.

Is UV a potential problem? The high levels of UV put out by metal halide and mercury vapor lamps can damage sensitive fabrics, papers, and artifacts. Special filtering can help avoid 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 and mercury vapor fixtures. In those cases, "white" HPS sources are the best bet.

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.

What's on the Horizon?

HID technology continues to advance on a number of fronts. As the price of color-corrected "white" high-pressure sodium sources drop, more fixture manufacturers will offer lower-cost equipment, creating greater demand for electronically ballasted versions. Also, look for HID lamps to replace incandescents in even more applications as instant-on, instant-restrike technology filters down to lower-wattage lamps. 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 30 percent light levels have recently been introduced. Look for these products to make a bigger impact in the future.

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Who are the Manufacturers?