Mired

Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express color temperature. It is given by the formula:

$M=frac\{1000000\}\{T\}$

where M is the mired value desired, and T is the color temperature in kelvins (former name degrees Kelvin or Kelvin degrees).

1 mired is equivalent to 10⁶ K⁻¹ or MK⁻¹. For convenience, decamireds are sometimes used, each decamired containing 10 mireds. The SI unit is the reciprocal megakelvin (MK⁻¹), shortened to mirek, but this term has not gained traction.

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

Examples

A blue sky, which has a color temperature T of about 25,000 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.

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 a tungsten light (3200K) in natural light (say, 5700K) without introducing a color cast, one would need a corrective filter or gel providing a mired shift of

$frac\{10^6\}\{5700\}\; -\; frac\{10^6\}\{3200\}\; approx\; -137\; mbox\{MK\}^\{-1\}$

This corresponds to a CTB (color temperature blue) filter.

References