More than 105 million high-intensity discharge (HID) lamps, ranging from 20W to 2,000W in size, are now in use in a broad range of interior and exterior applications in the United States. HID lighting systems currently consume about 12% of all lighting loads in the commercial sector, 31% in the industrial sector, and 87% in all outdoor stationary applications, which averages out to 17% of all electric energy consumed by all lighting systems in the United States.

This base represents a significant opportunity for energy savings, peak demand reduction, and greater flexibility in multi-use spaces through dimming. Dimming can be employed in HID lighting systems to save energy and enable the space to adapt to different uses, ambient conditions, and time of day. A number of technologies can be deployed to enable HID dimming and capture the same benefits that are used for dimming fluorescent and incandescent lamps.

Saving energy. HID lamp dimming can reduce electric utility operating costs through an occupancy-based strategy, daylight harvesting, and scheduled peak demand reduction. Dimming can be achieved manually (switch) or automatically (input from control device). Automatic dimming can be set to respond to a preset schedule or variable ambient conditions, such as occupancy and available daylight.

Occupancy-based. Dimming can be used in spaces that are intermittently occupied but where the lamps need to continue operating for security and safety reasons, such as a warehouse or parking lot.

In outdoor applications like parking lots, dimming has the added bonus of reducing spill light that may affect adjacent properties.

Daylight harvesting. Dimming can be used to adjust light levels based on available daylight via input from a photocell.

Peak demand reduction. Dimming can be scheduled using a time-programmable controller during times of peak loading to shave the facility's peak demand and thereby reduce utility demand charges.

HID lighting systems are fixed-output systems, but some spaces like school gymnasiums or supermarkets may require different light levels because they're used for multiple purposes. Dimming makes both the lighting system and the space more flexible. In the case of a school gym, the lamps can be dimmed to provide suitable lighting for sports, assembly, social events, maintenance, and other purposes. In the case of a supermarket, the lamps can be dimmed during maintenance and stocking operations. Many spaces can be dimmed after hours to provide lighting for safety and security.

Dimming technologies. HID lamps can be dimmed using step-level or continuous-dimming systems.

Step-level dimming. Step-level dimming typically entails using a constant-wattage autotransformer (CWA) magnetic ballast with one or two additional capacitors added to the circuit. Relay switching of the capacitors results in additional system impedance, which reduces the lamp current and the wattage. The capacitor circuit configuration may be parallel (Fig. 1) or series (Fig. 2) connected. That selection is a compatibility issue that will depend on the type of control system you're working with. Any type of HID lamp can be dimmed using this method.

Step-level dimming can be activated based on input from manual switches, scheduling devices, occupancy sensors, and photocells.

When the lamps need to return to full light output, they're brought from their reduced light output to about 80% of light output, followed by a brief warm-up time between 80% and 100% of light output.

The result of the capacitive-switching method is power reduction in stepped increments of two (two-level or bi-level dimming) or three (three-level or tri-level dimming) — usually at 100% and one or two steps between 100% and 50% of rated power.

Bi-level dimming is ideal for saving energy and providing lighting for safety and security during times when the space isn't occupied. Depending on the lamp type and wattage, the low level in a bi-level dimming system may be 15% to 40% of light output and 30% to 60% of wattage. This can result in energy savings as high as 40% to 70% during dimming periods.

Tri-level dimming does the same but offers a greater degree of flexibility that addresses multiple uses of the space.

Step-level dimming is generally less expensive than continuous dimming and is often more cost-effective than HID dimming panels for applications with few light fixtures. It allows for individual fixture control, is suitable for retrofit, and is available in fixtures that have a dedicated occupancy sensor and dimming ballast for direct fixture replacement.

Continuous (line-voltage) dimming. Several technologies are available for smooth, continuous reduction of lamp wattage, including panel-level HID dimming and relatively new electronic HID ballasts.

Continuous dimming is ideally suited for applications where it's advantageous to adapt the lighting system to a wide range of light levels to meet various uses of the space, such as airports, lobbies, classrooms, industrial facilities, sporting arenas, gymnasiums, and auditoriums. Continuous dimming is also well suited for daylight harvesting because it allows the HID lamp output to be tuned to maintain a constant light level in the space.

Panel-level HID dimming is used by control systems installed at the electrical panel that reduces the power supplied to the circuit. These control systems accept inputs from occupancy sensors, photocells, and time-programmable systems.

The control system may be one of three types: variable-step transformer, which typically operates with existing CWA ballasts and can reduce rated power down to 50%; variable-reactor, enabling reduction in rated power down to 30%; and waveform modification, which can reduce rated power down to 50%.

Electronic dimming ballasts for HID lamps (Fig. 3) are available in new fixtures and provide continuous dimming, typically from 100% to 50% light output for metal halide lamps, and 100% to 30% light output for high pressure sodium lamps to preserve lamp life. In addition to dimming, they're designed to operate at a higher efficacy, improved color control, less stroboscopic effect, and harmonic distortion under 20%.

Electronic HID ballasts are generally not cost-effective in a retrofit but can produce significant energy savings in new fixtures. They can operate with occupancy sensors, photocells, and time-programmable controllers. The control signal can be transmitted along the power circuit or via low-voltage wires.

Related performance issues. Important performance issues related to HID lamp dimming include efficacy, lamp life, color, compatibility, and flicker.

Efficacy. The ratio of reduction in wattage to reduction in light output isn't proportional during the operation of panel-level and step-dimming control systems. Light output will be reduced further than the wattage reduction (Table).

In general, light output reductions are about 1.2 to 1.5 times the power reduction for metal halide lighting systems (Fig. 4 below), and about 1.1 to 1.4 times the power reduction for high pressure sodium lighting systems.

Lamp life. When HID lamps are dimmed below 50% of rated power, they may experience degradation in service life, efficacy, color, and lumen output, or they may extinguish. In fact, dimming below 50% of rated power may reduce high pressure sodium and metal halide lamp life by 90%. As a result, dimming below 50% may void lamp warranties.

The National Electrical Manufacturers Association (NEMA) suggests a maximum recommended dimming level of 50% rated lamp wattage for both metal halide and high-pressure sodium lamps. NEMA further recommends that the lamps should be operated at full power for at least 15 minutes prior to dimming (unless the lamp is extinguished from a voltage interruption and the input voltage activates the timer, in which case 30 minutes is recommended before dimming.)

Color. HID lamps can experience a color shift during dimming and also a reduction in color-rendering ability. Metal halide lamps are most susceptible to changes in lamp color characteristics. Coated metal halide lamps experience a much smaller shift and a smaller reduction in CRI than clear lamps. High pressure sodium lamps can also be affected, typically experiencing a 50°K to 200°K reduction in color temperature when they're dimmed, causing them to appear more yellow, while CRI experiences a minimal change.

Compatibility. Some panel-level dimming systems aren't compatible with electronic ballasts. Self-extinguishing lamps aren't recommended for use with dimming systems. Some manufacturers recommend that metal halide lamps be operated base-up to preserve lamp life. Some panel-level dimming systems may introduce harmonic currents into the electrical system.

Flicker. Dimming HID lamps, particularly high pressure sodium lamps, can make flicker more visible.

Increasing emphasis on energy efficiency is going to make it necessary for lighting system designers to find ways to reduce load consumption wherever they can, meaning dimming options for HID lamps may become an attractive alternative.

DiLouie is communications director for the Lighting Controls Association and principal of ZING Communications, Inc., Calgary, Alberta, Canada.

Sidebar: Overcoming HID Re-Strike Time

Typically, occupancy sensors are used with fluorescent lamps, turning the lights on or off after a determined interval based on detected occupancy in the space. In applications where HID lamps are used, it often isn't practical to switch the lamps based on occupancy if the space must be made usable again quickly.

The reason is that high-pressure sodium lamps can take three to five minutes to warm up; they take less than a minute to hot re-strike but don't reach full light for three to four minutes. Metal halide lamps take two to 10 minutes to warm up and 12 to 20 minutes to hot re-strike, while pulse-start metal halide lamps take only one to two minutes. These delays are unacceptable in many applications. In addition, switching the lamps can also result in shorter-than-rated lamp life, since most lamp manufacturers rate HID lamp life at a minimum of 10 hours per start.

It's impractical to switch HID lamps based on occupancy in a number of applications, but dimming is a viable alternative. The lamps can be dimmed in response to a signal from an occupancy sensor (recommended for spaces where occupation isn't predictable) or timer or other time-programmable controller (for spaces where occupation is highly predictable), which can yield significant energy savings. When the space is occupied again, the lamps will be able to achieve full light output quickly.

Figures 1-3 courtesy of Advance Transformer Company