Color Temperature
Correlated Color Temperature (CCT)

(Term of photometry)

The color temperature of light refers to the temperature to which one would have to heat a "black body" source to produce light of similar spectral characteristics. Low color temperature implies warmer (more yellow/red) light while high color temperature implies a colder (more blue) light.

Daylight has a rather low color temperature near dawn, and a higher one during the day. Therefore it can be useful to install an electrical lighting system that can supply cooler light to supplement daylight when needed, and fill in with warmer light at night. This also correlates with human feelings towards the warm colors of light coming from candles or an open fireplace at night.

Standard unit for color temperature is Kelvin (k).

(The kelvin unit is the basis of all temperature measurement, starting with 0 k at the absolute zero temperature. The "size" of one kelvin is the same as that of one degree Celsius, and is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, which positions 0° Celsius at 273.16 k.)

Some typical color temperatures are:

1500 k	Candlelight
2680 k	40 W incandescent lamp
3000 k	200 W incandescent lamp
3200 k	Sunrise/sunset
3400 k	Tungsten lamp
3400 k	1 hour from dusk/dawn
5000-4500 k	Xenon lamp/light arc
5500 k	Sunny daylight around noon
5500-5600 k	Electronic photo flash
6500-7500 k	Overcast sky
9000-12000 k	Blue sky

At this place it shouldn't be forgotten that a color temperature value, though expressed as a single number, doesn't describe a simple property. In reality, it only summarises the spectral properties of a light source. Two light sources with the same light color can differ widely in quality, eg. when one of them has a continuous spectrum, while the other just emits light in a few narrow bands of the spectrum. Some of the qualitative aspects of such a spectrum can be summarised by means of its color rendering index.