1. What's the difference between the CRI and CCT ratings?
“The table in sizes.com/units/CRI.htm says incandescents are rated at 100 CRI. Do they therefore most resemble natural daylight?”
Yes and no. The light of incandescents is like daylight in that it is free of peaks and valleys (high CRI rating), but it is unlike daylight in that it is much “warmer” (its CCT rating is very different from that of daylight). The easiest way to see what is happening is to examine graphs showing spectra.
The spectra of an incandescent bulb or sunlight is (mostly) a smooth continuous curve. One way to make light is to heat something. Everything above absolute zero is continuously emitting radiation. Even you, dear reader, are continuously emitting radiation by virtue of your temperature, but you are so cool your radiation is so far down in the infrared our eyes can't see it (a snake's tongue can). The difference between sunlight and incandescent light is mostly due to the difference in the temperature of the matter emitting the light, in the case of the light bulb, a filament at about 2800°K (depends on the wattage of the bulb), in the case of sunlight, the outer layer of the sun at about 6000°K. It's this difference which the CCT rating tries to capture. The hotter the emitting object, the more light emitted and the further the peak shifts toward the violet end of the spectrum (so higher temperature means “cooler” light).
The light from a fluorescent lamp is not much related to temperature. It comes from phosphors, chemicals that give off visible light when they are hit by ultraviolet light, in a fluorescent lamp supplied by excited mercury atoms. The light the phosphor gives off is characteristic of the substance; different substances, different wavelengths of light emitted. A peak in the spectrum of a fluorescent bulb is the color of the light (actually, the wavelength) emitted by one of the phosphors coated on the tube.
The light from the fluorescent looks white because human color perception is a near-miraculous process. Like a digital camera, we are continually setting a “white-balance.” Otherwise, at sunset everything would look orange and at noon, blue. There are an infinite number of different combinations of light of differing wavelengths which will look white to a human observer. The people who design fluorescent lamps chose a mixture of phosphors that produces an impression of white light.
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Last revised: 16 August 2009.