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EDN, May 26, 2011

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The challenge in the near future is to provide a lighting experience that matches consumers' expectations for how a light should work. Consumers don't necessarily want incandescent lights but rather lighting "experiences" that match their expectations—lights that come on instantly; work with cur- rently installed light switches, including TRIAC (triode-alternating-current)- based dimmers; deliver a warm- to bright-white light; cost-effectively save energy, and have lifetimes of more than 10,000 hours. As a recent EDN article notes, Avnet illumineer George Kelly believes that our preference for warm colors dates back to prehistoric times when firelight was the only option for light at night (Reference 2). Blue light is more prevalent during the day when the sun is high, whereas redder, warmer light is a signal that the day is winding down and it's time to relax. In other words, the goal of indoor lighting should be to as closely as possible match the black-body curve rather than simply to meet a color temperature or CRI (color-rendering index). According to Wikipedia, German physicist Gustav Kirchoff introduced the term "black body" in 1862 to describe an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radia- tion, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body's temperature. At Earth-ambient temperatures, this emission is in the infrared region of the electromagnetic spectrum and is not vis- ible. The object appears black because it neither reflects nor emits any visible light (Figure 1 and Reference 3). Another recent EDN article sug- gests that lighting can influence sleep (Reference 4). The circuit in the article uses one cyan LED and one royal-blue LED to vary the current between them to achieve 32 shades of blue. According to the article, "When coach cars of long-range trains comprised compart- ments for six to eight passengers, the passengers could choose either 'white' or deep-blue light. The blue light helped passengers sleep, even when they were not in full darkness." This brief description is a bit hazy, MAY 26, 2011 | EDN 49 AT A GLANCE ↘ The Energy Independence and Security Act of 2007 dictates phas- ing out incandescent light bulbs starting in 2012. ↘ To satisfy consumer demands, energy-efficient lights must also be instant-on, work with currently installed light switches, deliver a warm- to bright-white light, cost- effectively save energy, and have a lifetime of more than 10,000 hours. but it implies that European trains once offered a blue light as a soothing nighttime color that would aid in sleep. Although the approach of using light to influence sleep is correct, the color is wrong. We now know that it's just the opposite: Blue light suppresses the production of melatonin, a hormone that helps induce sleep and, hence, drowsiness. Blue light of approximately 460 to 480 nm suppresses melatonin, an effect that increases with increased light intensity and length of exposure. Until recent history, humans in temperate climates were exposed to few hours of blue daylight in the winter; their fires produced predominantly yellow light INTENSITY (ARBITRARY UNITS) 14 12 10 8 6 4 2 0 0 5000K CLASSICAL THEORY (5000K) (Reference 5). In addition, blue light also has a strong link to the setting of circadian rhythms, also necessary for healthful living (Reference 6). A link between lighting and insom- nia may also be possible. Seth Roberts, a psychology professor at the University of California—Berkeley, has explored the connection between lighting and insomnia, using himself as a guinea pig and referencing related research studies. He concludes that people who experi- ence bright sunlight early in the day and no fluorescent lights just before bedtime have better sleep patterns (Reference 7). Lighting technology has so far been developing along a drunkard's walk of innovation: We started out with the incandescent light bulb, which seem- ingly by chance uses a filament that mimics the yellow-red tones of white light and is a good stand-in for the burn- ing embers of a prehistoric community fire. We then moved to fluorescent light, which in some instances has a distinct blue bias in its color tempera- ture—one of the worst color choices for nighttime lighting if you're interested in sleeping shortly afterward. One of the newest lighting tech- 4000K 3000K 0.5 11.5 2 WAVELENGTH (MICRONS) Figure 1 Black-body radiation has a frequency distribution with a characteristic fre- quency of maximum radiative power that shifts to higher frequencies with increasing temperature. As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, appearing red, orange, yellow, white, and blue. When an object is visually white, it is emitting a substantial fraction as ultraviolet radiation (courtesy Wikipedia). 2.5 3

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