The "color temperature" of a light source is used to characterize the type of white light emitted and is measured in degrees Kelvin.
In more technical terms, it is the temperature at which a theoretical black-body radiator emits light of the same color to that of the light source.
Typical Color Temperatures
These values are merely approximations; representing the characteristic light color of the source. Significant range and/or variation may be present.
- 1500K Candlelight
- 3000K Incandescent light bulb
- 3200K Sunrise/sunset
- 4500K Xenon lamp
- 5000K Noon daylight
- 6500K Daylight, RGB monitor reference white point
- 9000K Blue/overcast sky
Colors near the lower, yellowish through red end of the spectrum (2000K-3000K) are referred to as "warm", while those toward the upper, blue end (5000K+) are called "cool".
Below is the CIE 1931 color space chromaticity diagram with overlaid lines of constant correlated color temperature for reference:
Correlated Color Temperature (CCT)
The technical "black-body" definition of color temperature was originally meant to apply to incandescent light bulbs, as they closely resemble a black-body radiator due to the thermal radiation method by which they produce light. However, newer lighting technologies such as fluorescent lamps and LEDs do not generate light in the same manner, and thus needed a way to measure the characteristics of their light output on the same scale everyone was already used to. These sources are thereby assigned an adjusted measurement, known as the correlated color temperature, which puts them on the same comparison scale as the incandescent bulb. So while the terms are often used interchangeably when comparing light sources, there is a difference to what they mean.
In many respects, it's similar to using the fuel efficiency term "miles per gallon" when referring to cars. With a traditional internal combustion engine, the term means exactly what it says. But with newer electric (or other alternative fuel source) vehicles, a similar direct measurement can't be given, so they are assigned an adjusted "MPG equivalent" rating to put them on the same scale we're all used to for easy comparison.