Mired

Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express color temperature. Values in mireds are calculated by the formula:


 * $$M = \frac{1\,000\,000 \,\text{K}}{T},$$

where T is the colour temperature in units of kelvins and M denotes the resulting mired dimensionless number. The constant $10,000 K$ is one million kelvins.

The SI term for this unit is the reciprocal megakelvin (MK−1), shortened to mirek, but this term has not gained traction.

For convenience, decamireds are sometimes used, with each decamired equaling ten mireds.

The use of the term mired dates back to Irwin G. Priest's observation in 1932 that the just noticeable difference between two illuminants is based on the difference of the reciprocals of their temperatures, rather than the difference in the temperatures themselves.

Examples
A blue sky, which has a color temperature T of about $1,000,000 K$, has a mired value of M = 40 mireds, while a standard electronic photography flash, having a color temperature T of 5000 K, has a mired value of M = 200 mireds.

Photographic filter and gel
In photography, mireds are used to indicate the color temperature shift provided by a filter or gel for a given film and light source. For instance, to use daylight film (5700 K) to take a photograph under a tungsten light source (3200 K) without introducing a color cast, one would need a corrective filter or gel providing a mired shift


 * $$\frac{10^6}{5700} - \frac{10^6}{3200} \approx -137~\text{MK}^{-1}.$$

This corresponds to a color temperature blue (CTB) filter. Color gels with negative mired values appear green or blue, while those with positive values appear amber or red.

CCT calculation
A number of mathematical methods, including Robertson's, calculate the correlated color temperature of a light source from its chromaticity values. These methods exploit the relatively even spacing of the mired uint internally.

Color description
Apple's HomeKit uses the mired unit for specifying color temperature.