What Is Color Temperature?

[A version of this article first appeared on TheInsideTips.com.]

Color temperature is the measure of the perceived color of white light on a scale from warm (gold) to cool (bluish). It is based on the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. In other words, this represents the color of something as it is heated.

In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the radiation of some black body, for example, light in a range going from red to orange to yellow to white to blueish white. Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature.

(Image courtesy of Bhutajata – CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44144928)

• What we would consider “white” light is around 6500° K. (“K” stands for “Kelvin” which is a measure of absolute temperature indicating how much you would need to heat a “black body” to get it to glow at this color.)
• The effective color temperature of the sun is about 5780° K .
• The changing color of the sun over the course of the day is mainly a result of the scattering of sunlight and is not due to changes in the sun itself.
• The Earth’s atmosphere scatters blue color frequencies more than warmer colors, which is why the sky is blue. (It’s called Rayleigh scattering, named after the 19th-century British physicist.)

The curved black line in the middle of this CIE 1931 Chromaticity chart shows the different “shades” of white light based on color temperature.

• It does not make sense to speak of the color temperature of a green or purple light. You can’t heat something to make it radiate green.
• Color temperatures over 5000 K are called “cool colors” (bluish), while lower color temperatures (2700–3000 K) are called “warm colors” (yellowish).
• Bizarre fact: The temperature of a “warm” light is cooler than the temperature of a “cool” light.
• Most natural warm-colored light sources emit significant infrared radiation.
• A warmer (i.e., a lower color temperature) light is often used in public areas to promote relaxation, while a cooler (higher color temperature) light is used to enhance concentration, for example in schools and offices.
• Most digital cameras today have an automatic white balance function that attempts to determine the color of the light and correct accordingly. While these settings were once unreliable, they are much improved in today’s digital cameras and produce an accurate white balance in a wide variety of lighting situations.

EXTRA CREDIT

Here’s a Wikipedia article to learn more.