Choosing the Right Light

Nobody wants to waste energy or money to light their home or workspace. It seems logical to simply repopulate every fixture with the most efficient lamps available. But that may be a poor solution because it ignores a fundamental principle of lighting design: Make only as much of the right kind of light as you need, and shine it only where it is needed.

New energy-efficiency standards for electric lights, based on “lumens per watt,” have been imposed in the United States and elsewhere. Lamps that don’t meet these standards are being phased out and replaced by more efficient (and more expensive) products. Unfortunately, standards based solely on lumens per watt don’t tell the whole story—they ignore that first fundamental rule of lighting.

Beyond Lumens Per Watt

I’ve been shopping for lighting based on lumens per watt since I first moved off the grid in 1991. With my first tiny solar electricity and battery system, it was critical to use as little energy as possible. That need remains, even now that I’m lighting a bigger space with a bigger PV system. I’ve had many lighting failures. Besides a shorter-than-expected lamp life and poor light quality, many of the lamps used more energy than claimed for the amount of light produced. Running two “efficient” lights to replace a single inefficient one doesn’t make sense. So I asked acclaimed lighting engineer Howard Brandston, an outspoken critic of government lighting bans based on lumens per watt, what I had been doing wrong all these years.

“All lumens are not created equal,” Brandston says. “Those metrics are simply agreements amongst scientists, and have very little to do with the human perception of light. Eyes are extremely sensitive—your visual system, all the way from eye to brain, works at 85% effectiveness in only 7.5 foot-candles of illumination. That is not much light. Mozart wrote his music under the light of only two candles, and the average person can read The New York Times...in only two foot-candles.” To put this into perspective, full sunlight has an illuminance of about 10,000 foot-candles; an overcast day, about 1,000 foot-candles.

Both biology and psychology are at least as important as physics. The lumens per watt standard was developed in 1924 to compensate for the difference in sensitivity of the human eye to various frequencies of light. But it doesn’t compensate for how the eye shifts its color sensitivity in different brightness conditions, or for the fact that most energy-efficient lamps emit light in a very narrow spectrum compared to the broad spectrum of sunlight. These effects can make a room seem brighter or dimmer, and also result in lighting that seems inadequate or is unpleasant in quality.

The graph (See ITA) shows the spectral distribution of light from different sources. Both sunlight and incandescent light have an even distribution of frequencies. Fluorescent (FL) and compact fluorescent (CFL) lighting have large spikes at certain frequencies, with light-emitting diodes (LEDs) somewhere in between. This is why the light from FL and CFL lamps can seem harsh and fatiguing, and also why Brandston says that incandescents are “the gold standard of lighting.”

Both the lighting industry and the U.S. Department of Energy (DOE) are aware of the limitations of lumens per watt and are developing new metrics. I asked Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute, why the situation is so confusing for consumers, and what is being done to improve it.

“We need to do a better job at recasting the problem as a benefits-versus-cost decision,” Rea says. “There is a lot of effort in the industry right now focused on ‘bundling’ the numerous benefits of different lighting choices into an easy-to-understand format. For example, if a lightbulb has good color quality, no detectable flicker, and is dimmable, it could be labeled as a Class A light source.”

Rea and other industry experts are working with the DOE and its Energy Star program to make things easier, but it will take time. “It’s still the wild, wild West out there with residential lighting,” says Rea, “and consumers still have to choose by trial and error.”