The Dim State of Affairs

By Craig DiLouie | Aug 15, 2011




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Dimming offers greater flexibility for lighting systems, providing users with more control over their lighting conditions to support visual needs and enabling energy management strategies that can reduce energy costs. Traditionally limited to high-end commercial building applications, such as conference rooms and private offices, dimming is now being implemented throughout buildings.

Fluorescent lamps require ballasts that apply the proper voltage to start the lamp and then regulate current after startup. Ballasts dim lamps by reducing current, which reduces lamp output and power.

The current state-of-the-art dimmable linear fluorescent lamp ballasts offer expanded control opportunities, and the future promises to provide energy savings through flexibility.

Introduction to dimming
Dimming may be implemented in steps or over a continuous range. Step dimming provides a limited choice of light levels, with one or more preset increments between off and full output. Typically, there is no fade between light levels; the change in lighting state is abrupt. Because the change occurs at the ballast, it is technically categorized as dimming.

Another type of step-dimming ballast, sometimes called a load-shedding ballast, offers only one preset or a range of levels—­typically from 100 percent to as low as 60 percent—but with smooth transitions. Step dimming typically presents a lower cost for the dimming ballast but is less flexible. It is best suited for applications where there is a desire to reduce lighting power without the visual effect of alternate lamps or lighting fixtures being switched on separate circuits. Step dimming is typically accomplished by switching two hot legs into the ballast, requiring multiple voltage lines to be run to the fixture or a line-voltage occupancy sensor incorporated into the fixture to control the high/low setting. This dimming strategy is best suited to applications such as public spaces and circulation areas, where the abrupt change in light level will not be irritating to occupants.

Continuous dimming enables light levels to be raised or lowered over a specified range, and the change in lighting state is smooth (ideally transparent or virtually unnoticeable), using an automatic energy-saving control strategy. It is ideally suited for applications where the goals are more precise response from the control system and limited irritation among room occupants.

The practicality of dimming
Dimming is often expressed as a fraction (percentage) of relative lamp output or lamp power. The dimming range of fluorescent continuously dimmable ballasts, for example, is typically expressed as full output (100 percent) to its lowest achievable level of light output (e.g., 1 percent). Full output, in turn, is based on the ballast factor for the given dimming ballast (e.g., 0.88, 1.0 or 1.18). The dimming range for linear fluorescent step-dimming ballasts, meanwhile, is expressed as a single preset level of lamp power or multiples thereof (e.g., 50 percent).

When applying dimming to the built environment, there are two relevant questions: What is the effect on vision? And, how fast and how deep can the lighting system dim before users notice and before they find the dimming intrusive?

When lamps are dimmed, the human eye may perceive a higher light level than is actually falling. The human eye overcompensates for lower light levels by allowing more light to enter the pupil. Dimming to 25 percent, for example, may appear to be about 50 percent of full output. According to the Illuminating Engineering Society, this effect is predictable using the square law, which defines the theoretical relationship between light level and perceived brightness as perceived light (%) = 100 × square root (measured light (%) ÷ 100).

Various organizations have studied the threshold for when users are likely to notice the lights being dimmed and concluded that a majority of users cannot detect a 15–20 percent reduction in light level. In another study, the National Research Council Canada determined users are less likely to notice and more likely to accept reductions in light level where above-­average daylight is available.

Selecting equipment
Demand for fluorescent dimming ballasts has increased dramatically in recent years, driven to a large extent by daylight harvesting in green building projects and interest among retailers in using their lighting in demand response. Fluorescent dimming ballast sales increased 35.5 percent in 2010, according to the National Electrical Manufacturers Association (NEMA), and represented 5.4 percent of the $788 million fluorescent ballast market, up from its traditional 1–2 percent.

Most dimmable ballasts are electronic programmed-start models with control of up to four linear T8 lamps (including energy-saving lamps), one or two linear and twin-tube T5 and T5HO lamps, and one or two four-pin compact fluorescent lamps (CFLs). T8 lamp types covered by dimmable ballasts include 2-foot, 17-watt (W); 3-foot, 25W; 4-foot, 32W (and 23W, 25W, 28W and 30W); and 5-foot, 40W. Note that some amalgam lamps are dimmable but do not respond instantly. Programmed-start dimming ballasts typically operate at a loss in efficacy (lumens per watt) of nearly 10 percent compared to fixed-output instant-start ballasts. Several recent introductions of dimming ballasts, however, operate at the same level of efficacy as instant-start systems; these ballasts incorporate filament cutout technology at the top end of the dimming range down to a 0.71 ballast factor, where constant filament heat is not required. For the most efficient 4-foot, T8 ballasts, look for the NEMA Premium mark on the label.

Analog dimmable T8 ballasts can dim to 3 percent, and T5HO ballasts can dim to 1 percent. Many factors, such as voltage to the ballast, ambient temperature, lamp life used/remaining and lamp seasoning, may affect actual dimming range in a field installation. Digital ballasts typically dim to 1 percent.

Traditionally, dimming range determined whether the ballast was categorized as an architectural dimming (100 to less than 1 percent) or energy management dimming ballast (100 to 20 percent). As dimming range for energy management ballasts continues to drop to 5–10 percent and below, the line between these ballast categories is blurring. Note, however, that efficacy deteriorates over the dimming range, meaning that energy savings decline below a 20 percent dim level.

Dimmable ballasts operate according to a dimming method, which may be analog (step dimming, 0–10 volts direct current (V DC), phase control and wireless infrared) or digital (DALI, proprietary). Digital and 0–10V DC are 4-wire, low-voltage methods, while 2-wire phase control is a line-voltage method using the line for both power and communication; wireless infrared is a short-range wireless method. Digital ballasts can serve as the point of control, simplifying wiring. Selection of dimming method is typically based on the use of the space, desired range of dimming, wiring, lamp type, physical size and/or budget.

Examples of T8 analog continuous dimming ballasts include the GE UltraStart (0–10V DC) and UltraMax T8 0–10V load-shed variable instant-start dimming ballast (dims to 60 percent); Lutron Hi-lume, Hi-lume 3D and ECO-10 (3-wire phase control), and Tu-Wire (2-wire phase control); Philips Advance Mark 10 Powerline (2-wire phase control), Mark 7 (0–10V DC) and EssentiaLine (0–10V DC); Sylvania Quicktronic PowerSense (0–10V DC and 2-wire); and Universal Lighting Technologies BallaStar (0–10V DC), SuperDim (0–10V DC) and DemandFlex (line-voltage dimming to 50 percent). Recent introductions include the GE UltraStart T8 ballast designed to operate 28W, 30W and 32W T8 lamps in parallel at a relatively economical cost; Lutron EcoSystem reduced-wattage ballast, designed to operate 25W, 28W and 30W T8 lamps; Philips EssentiaLine, which has a reduced feature set with dimming to 20 percent, presenting a lower upfront cost; and Sylvania Quicktronic Power­Sense, a high-efficiency ballast designed to work on both low- (0–10V DC) or line-voltage (2-wire) controls.

Digital addressable T8 dimming ballasts (DALI or proprietary) are also readily available, such as Sector ballasts by Leviton (also 0–10V DC), the EcoSystem and H-Series proprietary-protocol ballasts by Lutron Electronics, ROVR DALI ballast by Philips Advance, Quicktronic Professional (0–10V DC) by Sylvania, and DaliPro DALI ballast by Universal Lighting Technologies.

Sylvania further offers several options for T5/T5HO lamp dimming, including the Quicktronic PowerSense (0–10V DC and 2-wire), Quicktronic Helios (0–10V DC), Quicktronic T5 DALI, Quicktronic T5HO DALI, and Quicktronic QHE DALI ballasts.

In addition to continuous dimming ballasts, power line step-dimming ballasts are growing in popularity as an economical alternative. Examples include the GE UltraMax T8 bilevel, step-dimming, instant-start ballast (dims to 60 percent of lamp power without fade transition); Philips Advance Optanium step-dimming ballast for T5 lamps (50 percent); Sylvania Quickstep bilevel T8 switching ballast (50 percent); and Universal Lighting Technologies BallaStar (50 percent for T5 lamps and 50 or 60/30 percent for T8 lamps).

Note that fluorescent dimming systems are more sensitive to installation error than fixed-output systems. The National Lighting Product Information Program was tasked with determining why fluorescent lamps were failing prematurely in a dimming system in a big box retail store. Of the lighting fixtures with at least one inoperative lamp, 44 percent were found to have lamps with open cathodes; 41 percent suffered ballast failures; 21 percent had fixture problems, such as wires not connected or connected improperly; and 15 percent had installation problems, such as incorrectly installed lamps.

Dimmable ballasts need a secure contact with the lamp to ensure sufficient cathode heating. Proper lamp installation and/or special sockets ensure a secure connection. New lamps should be operated at full output overnight (or about 12 hours)—or at least a few hours if overnight operation is not practical—before dimming, which allows any residual impurities from the manufacturing process to be eliminated (a process called seasoning). Shorter lamp life may result for T8 lamps exposed to extended periods of dimming below 35 percent of full light output if the ballasts do not offer sufficient filament heating. Look for ballasts designed to the NEMA LL 9-2011 standard to gain certainty of sufficient cathode heating from 100 to 10 percent. Manufacturers may offer the same lamp-life ratings and warranty on dimming ballasts that meet NEMA LL-9 as their longest life fixed-output programmed-start ballasts.

Finally, be sure to specify ballasts that are dimmable, lamps and ballasts that are compatible, and ballasts and controls that are compatible, particularly when using analog dimming ballasts. Energy-saving lamps, such as the 28W T8, can be dimmed reliably, but the ballast must be designed with the proper starting voltage and striation control to operate these lamps without system issues. The ballast manufacturer should provide the certification and warranty to operate these lamps without problems. Additionally, avoid mixing dimmable and nondimmable loads on dimmer-controlled circuits, linear lamps and CFLs in the same dimming control zone, and different loads (e.g., incandescent with electronic ballasts) on the same dimming control.

The future is dimmable
Energy codes are trending toward requirements that both indoor and outdoor lighting systems be controllable as a way to increase energy savings through greater flexibility. Dimming offers multiple degrees of flexibility by controlling lamp output and power at the ballast without needing to turn any lamp off unless the space is unoccupied. Continuous dimming offers the ultimate in flexibility, enabling a greater degree of precision, energy savings and transparency for automatic control systems. However, dimming control systems are more sophisticated than standard controls and require careful product selection, design and installation to avoid operating problems.

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

About The Author

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





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