BackPage Primer from Environmental Building News
March 1, 2008
Shedding Light on Light Quality
It’s no wonder that confusion abounds regarding light quality. We talk about
color temperature in kelvins (a temperature scale), but higher temperatures are “cooler”—how’s that? A more important property,
color rendering index, tells us how accurately a light source shows off colors, but what does that mean?
Color temperature, measured in kelvins or K, describes the color of a light source. Some light sources, such as sodium lamps, are quite orange, while others, such as cool-white fluorescent lamps, are bluish-white. Color temperature is measured in kelvin (one kelvin is equal to one degree Celsius, but the scales start at different points: 0 K is absolute zero, or –273°C).
The color-temperature scale references the color changes that happen as a theoretical “black-body” radiator is heated. When the steel gets hot enough (about 1,500 K), it begins to glow a deep orange-red. As it gets hotter, the incandescing becomes yellow, then white, and finally—over about 6,000 K—distinctly bluish. Confusion arises because yellowish, lower-color-temperature light feels warmer to humans, while bluish, higher-color-temperature light feels cooler.
Cooler light (higher color temperature) is generally preferred for tasks that require a high degree of visual acuity because it produces higher contrast than warmer light (lower color temperature). However, warmer white is generally preferred for living spaces because it is more flattering to skin tones, clothing, and artwork. People generally prefer warm light in dimly lit spaces and cool, bluish light for brightly lit spaces, perhaps because humans evolved with warm, dim light from fires at night and bright, bluish daylight during the day.
Color rendering index (CRI) is a measure of how effectively colors are rendered (or represented) by a particular light source. Every light source has a distinctive spectrum (the distribution of colored light emitted), and if that spectrum is deficient in certain wavelengths (colors), the color of an object (the light reflecting off that object) will appear altered. The CRI scale measures a light source’s similarity to natural daylight or a 100-watt incandescent light bulb (both of which have a CRI of 100) in rendering color.
The lower the CRI, the less natural objects appear. Twenty years ago, most fluorescent lamps had CRIs as low as 50—giving skin color a ghost-like hue. Low-pressure sodium lamps have the lowest CRIs of common light sources because they emit light in only a very narrow spectral band. The next time you’re at a tollbooth with low-pressure sodium lights (which give off deep-orange light), notice how you can barely distinguish a blue car from a red car—the CRI is extremely low—close to zero. If the CRI is higher than about 80, most people (but not all) consider the light quality to be generally acceptable. Fluorescent lamps with the highest CRIs, those with trichromatic phosphors, achieve CRIs of 85 to the mid-90s, and one LED light now on the market has a CRI of 92 (see
EBN
Vol. 16, No. 11).
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More comments
"The CRI scale measures a light source’s similarity to natural daylight or a 100-watt incandescent light bulb (both of which have a CRI of 100) in rendering color."
Those are 2 different scales of measurement, daylight and 100 watt incandescent? Or daylight and 100 watt incandescent are almost identical in all lighting qualitites?