Spotlight on Dimming

Energy codes, green building projects, user interest in flexibility and other market forces are driving demand for lighting solutions, including high-intensity discharge (HID) lighting systems, such as high-pressure sodium and metal halide.

HID lamps may be dimmed using either step or continuous dimming. Step dimming, typically using a magnetic ballast, involves changing the lighting state from full lamp power to a preset increment between full and off. An occupancy sensor, for example, detects that an area is unoccupied and signals the step dimming ballast to reduce lamp power to a preset level, thereby saving energy.

For applications requiring more flexibility as well as greater “transparency” of the control effect in occupied spaces, continuous dimming may be suitable and may be achieved using panel-level line-voltage dimming devices or dimmable electronic HID ballasts. These ballasts may offer additional benefits, such as efficiency up to 94 percent, a smaller size and lighter weight, and better lumen maintenance and color stability.

Both types of ballasts generally offer a dimming range from 100 down to 50-plus percent of rated lamp power. In its “Guidelines for the Application of Dimming to High-Intensity Discharge Lamps” (LSD 14 2010, available free at, the National Electrical Manufacturers Association (NEMA) recommends not dimming HID lamps below 50–70 percent, depending on the specific type. Dimming below the minimum can degrade lamp life—potentially voiding lamp warranties—while affecting efficacy, color and lumen maintenance.

NEMA further suggests that the lamps be operated at full light output for at least 15 minutes before dimming. If a voltage interruption extinguishes the lamp, the timer should be reset only after the lamp has restarted; for more information, consult the manufacturer.

Continuous dimming capability increasingly is becoming available for electronic HID ballasts. Dimmable electronic HID ballasts are available for wattages from 100–750W; for both metal halide and high-pressure sodium lamps; and with the ability to join 0–10V DC, DALI-based and proprietary control networks.

Examples include the Empower digital ballast (0–10V DC dimming of 157–750W metal halide and high-pressure sodium lamps); GE UltraMax eHID ballast (250–400W quartz or ceramic metal halide); Metrolight SmartHID Plus (dims 450–575W metal halide and 100–600W high-pressure sodium lamps using proprietary digital interface); Philips Advance Dynavision (0–10V DC dimming of 320–400W pulse-start lamps with automatic 15-minute warm-up) and CosmoPolis programmable digital ballast (DALI dimming for ceramic metal halide lamps); Sylvania Quicktronic QTO (0–10V DC continuous dimming or stand-alone step dimming of 100–200W metal halide or high-pressure sodium lamps in outdoor applications); and Universal Lighting Technologies 210W ballast (0–10V DC dimming of Philips 210W C183 metal halide lamp, with automatic 15-minute time delay).

Most high-pressure sodium lamps are approved for dimming in any orientation, according to NEMA, although performance may vary depending on whether the ballast is magnetic or electronic. NEMA also recommends that these lamps not be dimmed below 50 percent of the rated lamp wattage.

Note that dimming some high--pressure sodium lamps (and retrofit metal halide lamps designed to operate on high-pressure sodium ballasts) may result in a shift in color temperature, color rendering and efficacy. Also note that high-pressure sodium lamps that age about 15,000 hours may “drop out” when dimmed rapidly, which users may mistake for a failed lamp. NEMA recommends reducing the dimming rate to about 1.5 minutes from full output to the low end of the dimming range, while maintaining sufficient voltage to sustain the arc.

Most metal halide lamps are approved for dimming except self-extinguishing T-type lamps or high-pressure sodium retrofit lamps designed to operate on metal halide ballasts.

Probe-start quartz lamps should be operated in a base-up position when controlled by dimming systems; NEMA recommends that these lamps not be dimmed below 50 percent. Many pulse-start quartz and ceramic metal halide lamps may be dimmed in any operating position, with some exceptions, notably 150W and larger ceramic metal halide lamps. NEMA recommends that these lamps not be dimmed below 50–70 percent of lamp power, depending on the lamp type (see LSD 14 2010). For specialty quartz and ceramic metal halide lamps that use external ignitors for starting and do not use probes and switches, consult the manufacturer about guidelines and viability for dimming.

Note that metal halide lamps typically experience some degree of color shift during dimming; clear quartz lamps, for example, may exhibit a significant increase in color temperature, with the shift extending to as high as 5,000–6,000K compared to the typical 3,000–4,000K. The shift in a phosphor-coated lamp may be much less pronounced.

Lighting is increasingly becoming controllable. With advances in HID lamp dimming coupled with smart application, HID systems can participate in dimming strategies that support visual needs and/or save energy.

DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at

About the Author

Craig DiLouie

Lighting Columnist
Craig DiLouie, L.C., is a journalist and educator specializing in the lighting industry. Learn more at and .​

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