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Guest Post by Jim Brodrick: LED Source Efficiency Versus Luminaire Efficiency

by Jim Brodrick, lighting program manager, U.S. Department of Energy, Building Technologies Program

As you’re probably aware, there’s been quite a debate lately in some quarters about how best to measure energy efficiency for lighting, particularly in residential fixtures. At the heart of the debate are two terms I’m sure you’re familiar with by now: source efficacy and luminaire efficacy.

Source efficacy measures the lumens per watt of the light source, without taking the fixture into account while luminaire efficacy measures the lumens per watt of the entire luminaire, taken as a unit. Both are useful measurements in the right context, but it seems to have been blurred lately.

The goal of a luminaire is to provide light that’s useful to the task at hand—whether it involves illuminating the surroundings, producing a visual signal, or creating a pleasant ambiance. Any other light it happens to produce is wasted energy.

As we all know, no light fixture is 100-percent efficient. A certain amount of waste is inevitable, because not all of the light that’s produced shines in the desired direction. As a result, some of it must be reflected, trapping a sizable portion, or it is absorbed by the various optical components.

But not all light fixtures are equally efficient—or inefficient. Some waste significantly more energy than others. If the goal is to conserve as much energy as possible, source efficacy alone is not an adequate measurement. That’s because even a highly efficient light source will waste a considerable amount of energy if it’s housed in an inefficient fixture—or one poorly designed for that type of source. It’s like the proverbial cow that yields plenty of milk but has an unfortunate habit of kicking the pail over.

Why, then, is source efficacy still around for measuring residential lighting fixture efficiency? One reason is that residential light fixtures haven’t been subjected to photometric testing to calculate luminaire efficacy, so there is a general lack of awareness about the extent of the light losses within the fixture.

But that’s changing, as the importance of energy efficiency continues to hit home on multiple fronts, making people more energy-conscious and increasing the number of products tested and rated for energy efficiency. DOE’s CALiPER program has conducted photometric testing of residential fluorescent fixtures to determine their luminaire efficacy, and the results were eye-opening. In the products tested, a great deal of the light produced by the source was trapped and therefore wasted—more than 50 percent in some products. That leaves plenty of room for improvement.

Another reason why there’s been more emphasis on source efficacy than on light loss has to do with market forces such as what happened with the introduction of compact fluorescent lamps (CFLs) in the late 1980s and 1990s. Although CFLs were up to four times more energy-efficient than incandescent lamps, consumers were reluctant to switch over to them, making manufacturers reluctant to design CFL-specific fixtures.

To remedy that, energy efficiency programs took a jumpstart-the-industry approach that only considered the efficiency of the light source, not the fixture. That made it fairly easy for fixture manufacturers to qualify their products for consumer rebates and other incentives. Though it made sense at the time, the approach set an unfortunate precedent by ignoring the issue of light loss. The result was that, in general, manufacturers didn’t redesign fixtures to maximize energy efficiency but instead tended to retrofit existing designs to accept CFLs.

Today, conditions are much different. With energy efficiency now a national priority, manufacturers recognize it as a powerful selling point and are eager to incorporate it into their designs. What’s more, the U.S. government has raised the bar with regard to residential lighting. The Energy Independence and Security Act of 2007 calls for minimum efficiency standards for general lighting service bulbs to take effect in January 2012. Advanced incandescent lamps will likely meet the new standards, but only CFLs or LEDs will likely meet the next tier scheduled for 2020.

This means that any energy efficiency program that focuses only on source efficacy will soon become obsolete, as more residential fixtures start using light sources with efficiencies that are equivalent to those of CFLs. The old way of measuring lighting performance just doesn’t work anymore, for the simple reason that a chain is only as strong as its weakest link. Sponsors of energy efficiency programs will have to look at the whole chain, and to focus on new kinds of lighting that can achieve greater energy savings.

That’s why the arrival of LEDs on the general illumination market is so timely. Unlike incandescent and fluorescent lamps, which emit light in all directions, LEDs are directional light sources. This means that the right kind of fixture design will minimize or even eliminate the reflective surfaces, lenses, and diffusers traditionally used to guide light and thus drastically reduce light loss.

But if—as happened with CFLs—LEDs are merely popped into fixtures designed for incandescent lamps, much of the energy efficiency potential of those LEDs will go untapped. This makes it especially important to base lighting programs on luminaire efficacy rather than on source efficacy, so that manufacturers have the incentive to make fixtures that take full advantage of LEDs.

Yes, photometric testing for luminaire efficacy is a new paradigm, particularly for the residential lighting industry. But the reality is we can no longer afford to ignore the issue of light loss in residential fixtures. It’s time to begin photometric testing of these fixtures, and to make the results readily available. Buyers and builders can then make well-informed choices between lighting fixtures that are truly efficient and those that just contain efficient light sources.

  • AMI April 28, 2009, 2:31 AM

    Can we send the text of Jim Brodrick to some of our customer for a complementary explanation on the particularity of LED light ? Permission with mention of source and author.

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