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:
where T is the colour temperature in units of kelvins and M denotes the resulting mired dimensionless number. The constant is one million kelvins.
The SI term for this unit is the reciprocal megakelvin (MK<sup>âÂÂ1</sup>), shortened to mirek, but this term has not gained traction.
For convenience, decamireds are sometimes used, with a 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 directly related to the difference of the reciprocals of their temperatures, rather than to the difference in their temperatures.
A blue sky, which has a color temperature T of about , 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.
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
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.
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 unit internally.
Apple's HomeKit uses the mired unit for specifying color temperature.