Compact Fluorescent Dimming: Buyer Beware
Due to popular demand, manufacturers of screw-in compact fluorescent lamps, or CFLs, began introducing dimmable versions of these energy-saving bulbs in the late 1990s. Dimmable CFLs 
currently represent only a very small share of the nation’s incandescent sockets, but may become more common in the future as the provisions of the Energy Independence and Security Act of 2007 take effect in 2012-2014, eliminating today’s 40-100W incandescent light bulbs.
Typical incandescent dimmers are electronic devices that regulate the amount of power reaching the bulb’s filament by switching the power on and off. The dimmer does this so quickly that the average eye does not see it happening but instead perceives a smooth continuous transition to a lower light level.
Most CFLs are not designed for dimming; dimming a non-dimmable CFL could damage the bulb and the dimmer. CFLs rated for dimming, or dimmable CFLs, exhibit a number of performance issues that should be understood prior to dimming. While the basic principle of operation is the same across all electronic dimmers, variations in individual dimmers, while immaterial to incandescent bulbs, can produce variations in performance of dimmable CFLs.
⌂ Most CFLs are not dimmable
⌂ Special screw-in dimmable CFLs dim only to about 20% with abrupt transition to OFF
⌂ CFLs do not become visually warmer in color as they dim
⌂ Dimmable CFLs may flicker
A great deal of research has been conducted regarding non-dimmable CFLs, but little related to dimmable CFLs. Below are findings summarized from “Spectral and Electrical Performance of Screw-Based Dimmable Compact Fluorescent Lamps,” by Xin Hu, MS and Kevin W. Houser, PhD, LC of the University of Nebraska-Lincoln, 2002, mixed with other observations. The researchers studied the performance of four screw-base dimmable CFLs from reputable manufacturers—23-29W, 1450-1750 lumens, maximum overall length (MOL) of 145-170 mm, with integral digital electronic ballasting, and properly “seasoned” prior to testing—and compared their performance against a 90W incandescent reference bulb.
Dimming range. Incandescent light bulbs dim smoothly from 100% to 0% with a smooth transition to the OFF state. Screw-in dimmable CFLs dim down to 8-20%, depending on the model—with the typical being 20%, due to the use of low-cost ballasting—and with an abrupt transition to the OFF state. Higher-performance electronic ballasts are available that provide continuous dimming down to <5% but are not cost- or size-effective with screw-in CFLs. Pin-based CFLs with a separate dimming ballast used in some fixtures can dim down to 1-20%, with 3-20% being more common.
Color appearance. All light sources have a color appearance, expressed in terms of color temperature, that sets the color tone of the space and can impact mood and how the space is perceived. Incandescent bulbs have a low color temperature, around 2800K, and are therefore considered “warm” (orangish-white) light sources. As the 90W incandescent light bulb was dimmed over its complete dimming range, it exhibited a shift in color of about 718K towards the warmer part of the color spectrum. In other words, it got even warmer (orangish, reddish, more similar to candlelight).
The four dimmable screw-base CFLs behaved differently, exhibiting a small color shift of 114K, some bulbs getting a little warmer, some getting a little cooler. Conventional wisdom in the lighting industry is that CFLs get cooler in color as they dim, appearing more bluish-white.
This presents a major potential drawback for dimmable CFLs. Research by John Flynn in the 1970s suggested that a warm white color reinforces impressions of pleasantness, relaxation and preference. A.A. Kruithof’s previous research suggests that people prefer cooler light sources at high light levels and warmer light sources at low light levels.
Size. The reference 90W incandescent light bulb fit its light fixture perfectly, as the fixture had been designed for this light bulb. The researchers, however, found that screw-in CFLs do not always fit incandescent fixtures due to a MOL up to 50 mm larger than incandescent. What’s worse, CFLs retrofitted into light fixtures originally designed for incandescents may not distribute light output as intended, reducing the fixture’s photometric efficiency.
Other issues. Not discussed by the researchers were other potential performance issues related to CFLs, including flicker during dimming and a waiting period while the bulb warms up before it can be dimmed.
Conclusions. Homeowners have a number of ways to save energy in the home while improving the quality of their lighting. Lighting controls are recommended as a primary method of saving energy by either allowing users to adjust light output according to taste (dimming) or automatically turning the lights off when a space is not being used (switching). Options include vacancy sensors, motion sensors, timer switches, preset scene control systems and whole-house lighting control systems, each ideally suited to enhancing lifestyle and/or saving energy in certain applications.
In some cases, homeowners may wish to maximize energy savings by replacing incandescent light bulbs with screw-in CFLs. The Home Lighting Control Alliance is educating consumers that not all CFLs are dimmable and that special versions are needed to be able to operate CFLs on dimmers. HLCA is also educating consumers about dimming issues commonly experienced with dimmable CFLs. Due to these issues, HLCA recommends incandescent or halogen bulbs with dimming, providing average energy savings of 20% due to the fact that dimming saves energy. Additional savings can be realized by matching dimming (20% energy savings) with high-efficiency screw-in halogen light bulbs (about 30% energy savings) in addition to longer lamp life realized. Meanwhle, motion sensors can be assigned to control outdoor lighting and vacancy sensors can save energy in bathroom and utility spaces.
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