Traditionally, HID lamps were dimmed using stepped dimming typically implemented with magnetic ballasts that reduced lamp current and therefore lamp wattage to a preset increment between full output and 50% of full output. The ballast is usually a constant-wattage autotransformer (CWA) ballast with one or two additional capacitors added to the circuit, depending on whether the ballast using bilevel or trilevel dimming (see below). Relay switching of the capacitors results in additional impedance, which reduces the lamp current and the wattage. The capacitor circuit configuration may be parallel or series.
Typically, there would be one step, causing step-dimming systems to be sometimes called “two level” or “bilevel” dimming systems. However, some systems, sometimes referred to as trilevel dimming systems, can operate at three fixed levels.
Step dimming is ideal for saving energy while providing a minimum amount of illumination for safety and security during hours of non-occupancy. For example, an occupancy sensor responds to the absence of people by signaling a step-dimming ballast to reduce lamp power to 50%, saving energy. Trilevel systems provide this benefit but with greater flexibility. Ideal applications include spaces that may be unoccupied for long periods of time but still need to be lighted, such as parking lots, warehouses, supermarkets and malls.
Some projects require an even higher degree of flexibility, while making the control effect as transparent as possible to avoid intrusion with a sudden change in the lighting state. In these applications, continuous dimming may be preferable. Ideal applications include airports, lobbies, classrooms, industrial facilities, sporting arenas, gyms and auditoriums. Continuous dimming is also well suited to daylight harvesting by enabling HID lamp output to be tuned to maintain a constant light level in the space, but without irritating occupants.
Line-voltage dimming: Continuous dimming may be implemented using line-voltage dimming devices that change the primary voltage to the ballast or electronically modify input voltage and current waveforms. The devices include:
• variable-step transformers that reduce voltage to the load, and which typically operate with CWA magnetic ballasts;
• variable-reactor devices that keep voltage constant but reduce current; and
• devices that modify the waveform (so-called “wave choppers”) to reduce the RMS voltage to the load to reduce rated power.
This method is sometimes called panel-level HID lamp dimming because these devices typically install at the electrical panel. For more information about line-voltage dimming, consult the product manufacturer.
Electronic HID ballasts: Alternately, continuous dimming may be achieved using electronic low- or high-frequency switching circuits or through combinations of electronic and mechanical devices to modify lamp power. Typically, continuous dimming is implemented using dimmable electronic HID ballasts. These ballasts will be the focus of the rest of this whitepaper.
Dimmable eHID ballasts
Dimmable electronic HID ballasts offer continuous dimming across a given range and may also offer other benefits such as higher efficiency (up to 94%), compact size, lighter weight, and better lumen maintenance and color stability than magnetic ballasts.
Availability of dimmable electronic HID ballasts recently expanded to cover wattages as low as 100W and as high as 750W, operate both metal halide and high-pressure sodium lamps, and provide ability to join 0-10VDC, DALI-based and proprietary control networks.
• Empower digital ballast (0-10VDC 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 175-575W metal halide and 100-600W high-pressure sodium lamps using 0-10VDC or proprietary digital interface);
• Philips Advance Dynavision (0-10VDC dimming of 320-400W pulse-start lamps with automatic 15-minute warm up), Dynadimmer (outdoor lighting control) and CosmoPolis programmable digital ballast (DALI dimming for ceramic metal halide lamps);
• Sylvania Quicktronic QTO (0-10VDC continuous dimming or standalone step dimming of 100-200W metal halide or high-pressure sodium lamps in outdoor applications); and
• Universal Lighting Technologies 210W ballast (0-10VDC dimming of Philips 210W C183 metal halide lamp, with automatic 15-minute time delay).
High-pressure sodium lamp dimming
According to Guidelines for The Application of Dimming to High-Intensity Discharge (HID) Lamps (LSD 14-2010) published by the National Electrical Manufacturers Association (NEMA), most high-pressure sodium lamps are approved for dimming in any orientation, although dimming performance may vary depending on whether the ballast is magnetic or electronic; consult the lamp manufacturer.
NEMA recommends that high-pressure sodium lamps not be dimmed below 50% of rated lamp power. Dimming below the recommended minimum can significantly degrade lamp life—potentially voiding lamp warranties—while also affecting efficacy, color and lumen maintenance. For retrofit metal halide lamps designed to operate on high-pressure sodium ballasts, and specialty high-pressure sodium lamps (e.g., internal ignitor, etc.), consult the lamp manufacturer about whether dimming is allowed and resulting parameters and limitations.
Besides dimming range, NEMA recommends that high-pressure sodium 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 Guidelines.
Note that dimming some high-pressure sodium lamps and retrofit metal halide lamps can result in changes to color temperature, color rendering and efficacy. NEMA further points out that high-pressure sodium lamps are susceptible to “drop out”—sudden extinguishment—if rapidly dimmed after the lamp operates for about 15,000 hours. Users may confuse this with the lamp having failed; if the user allows the lamp to cool, the lamp will relight and operate at its rated wattage for its rated life in an undimmed mode. To reduce the potential for this, NEMA recommends slowing the dimming rate to about 1.5 minutes from full output to maximum dimmed condition (while maintaining sufficient voltage to sustain the arc).
Metal halide lamp dimming
Dimming is possible with pulse- and probe-start quartz metal halide lamps, pulse-start ceramic metal halide lamps and many types of specialty lamps, such as lamps with internal ignitors or starting aids that operate on probe- and pulse-start metal halide ballasts. NEMA does not recommend dimming for self-extinguishing T-type metal halide lamps and retrofit high-pressure sodium lamps designed to operate on metal halide ballasts.
NEMA recommends that metal halide lamps not be dimmed below 50-70% of lamp power, depending on the lamp type, for the same reason as high-pressure sodium lamps—to dim below recommended levels may degrade lamp life, efficacy, color and lumen maintenance. Below is the recommended dimming range for each type of metal halide lamp, per NEMA:
• 100% to 50% of rated lamp power for probe-start quartz metal halide lamps;
• 100% to 60% of rated lamp power for 150W or smaller pulse-start quartz metal halide lamps;
• 100% to 50% of rated lamp power for >150W pulse-start quartz metal halide lamps;
• 100% to 70% of rated lamp power for <150W ceramic pulse-start metal halide lamps; and • 100% to 60% of rated lamp power for 150W or larger ceramic pulse-start metal halide lamps. For specialty metal halide lamps, consult with the lamp manufacturer about allowable dimming and wattage range. [caption id="attachment_3913" align="alignnone" width="404" caption="Correlated color temperature shift for quartz metal halide lamp across dimming range. Image courtesy of NEMA."][/caption]
As with high-pressure sodium lamps, NEMA recommends that metal halide 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 Guidelines.
Manufacturers limit the operating position of probe-start lamps to the base-up position if the lamp is to be dimmed. Most pulse-start quartz and ceramic metal halide lamps may be dimmed in any operating position, with some exceptions, including ceramic metal halide lamps 150W and larger; consult the lamp manufacturer for specific information about allowable dimming and operating position for the selected lamp type.
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 5000-6000K compared to the typical 3000-4000K. The color shift in a phosphor-coated lamp may be much less pronounced. For example, a 3700K coated lamp may increase to just 4000K during dimming.
Additionally, as with high-pressure sodium lamps, lamp efficacy and color rendering may decline with dimming.
Mercury vapor lamp dimming
Regulations created by the Energy Policy Act of 2005 eliminated the manufacture and import of mercury vapor lamp ballasts, although subsequent legislation—specifically, the Energy Independence and Security Act of 2007—allows specialty ballasts for which dimming may be feasible. Specialty applications, according to NEMA, include UV curing, microscopy, fiber optic lighting and other uses.
As with high-pressure sodium and metal halide lamps, NEMA recommends that mercury vapor 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 Guidelines.
NEMA offers no further guidance, stating only that questions about dimming specialty mercury vapor systems should be directed to the manufacturer.
New HID ballasts expand dimming options
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 save energy.