The Correlated Color Temperature scale for common light bulbs ranges from 2700 to 5500 Kelvin. Warm white is 2700K-3500K, and cool white is 3600K-5500K. (further extremes on the scale are not typically used for architectural lighting.)
Sunrise and sunset light and candle flame light are very warm, around 1850K.
This is often very nice light for photographs of people.
The color temperature is based on the electromagnetic radiation emitted from an ideal black-body. It varies depending on its surface temperature in kelvins. This serves as a standard to which light sources are compared. An incandescent light bulb’s light is thermal radiation and the bulb approximates an ideal black-body radiator, so its color temperature is essentially the temperature of the filament. Incandescent bulbs are typically in the 2700-3300 Kelvin range, but often when dimmed the filament gets cooler (lower surface temperature, perhaps 2000 Kelvin, means “warmer” light.)
Compact Fluorescent bulbs do not emit light by thermal radiation, so they get a “correlated color temperature,” comparing the color of light to the ideal black body scale. You can buy them in a range of color temperatures.
Beware! CFLs that are sold as “Daylight” bulbs are around 5000K and considered to mimic daylight at midday. “Daylight” might sound nice, but this is a cooler light and might not be desirable for many spaces. Most people prefer the warm light of the incandescent, so should use CFLs in the 2700-3300 Kelvin range.
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