Mired

Unit of reciprocal color temperature
Close up of the Planckian locus in the CIE 1960 color space
Isotherms in mireds
Isotherms in kelvins
Note the even spacing of the isotherms when using the reciprocal temperature scale. The even spacing of the isotherms on the locus implies that the mired scale is a better measure of perceptual color difference than the temperature scale. The range of isothermal color temperatures for both diagrams is from 1000 K (1000 MK−1) to 10000 K (100 MK−1).

Contracted from the term micro reciprocal degree, the mired (/ˈmaɪrɛd/[1]) is a unit of measurement used to express color temperature. Values in mireds are calculated by the formula:

M = 1 000 000 K T , {\displaystyle 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 1000000 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.[2]

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.[3]

Examples

Common color temperature and mired equivalents[4]: 40 
Light source Temp. (K) Mired
Skylight (clear, blue) 15000–27000 40–70
Shade, illuminated by skylight 10000–12000 80–100
Skylight (hazy) 7500–8400 120–130
Overcast 6700–7000 140–150
Electronic flash 6200–6800 150–160
Sunlight (hazy) 5800 170
Daylight (average) 5500–6000 170–180
Daylight (morning / afternoon) 5000–5500 180–200
LED (cool white)[5] 3100–4500 220–320
Professional tungsten 3200 310
Incandescent bulb (100 W) 2900 340
Incandescent bulb (40 W) 2650 380

A blue sky, which has a color temperature T of about 25000 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.

Applications

Photographic filter and gel

Mired difference can be quickly approximated with a nomogram

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

10 6 5700 − 10 6 3200 ≈ − 137   MK − 1 . {\displaystyle {\frac {10^{6}}{5700}}-{\frac {10^{6}}{3200}}\approx -137~{\text{MK}}^{-1}.}

This corresponds to a color temperature blue (CTB) filter.[6][7] 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.[8]

Color description

Apple's HomeKit uses the mired unit for specifying color temperature.[9]

References

  1. ^ How Many? A Dictionary of Units of Measurement (PDF). Pasig City. 20 March 2013. p. 239.{{cite book}}: CS1 maint: location missing publisher (link)
  2. ^ Ohta, Noboru; Robertson, Alan R. (2005). Colorimetry: Fundamentals and Applications. Wiley. p. 84. ISBN 0-470-09472-9.
  3. ^ Priest, Irwin G. (February 1932). "A proposed scale for use in specifying the chromaticity of incandescent illuminants and various phases of daylight" (abstract). JOSA. 23 (2): 41–45. doi:10.1364/JOSA.23.000041.
  4. ^ Smith, Robb (1975). The Tiffen practical filter manual. American Photographic Book Publishing Co., Inc. ISBN 0-8174-0180-6. LCCN 75-21574.
  5. ^ "Understanding color temperature". Westinghouse. Retrieved 18 August 2024.
  6. ^ Brown, Blain (2002). Cinematography: Theory and Practice : Imagemaking for Cinematographers. Focal Press. p. 172. ISBN 0-240-80500-3.
  7. ^ "Mired Shift Gel Table" (PDF).
  8. ^ Robertson, Alan R. (November 1968). "Computation of Correlated Color Temperature and Distribution Temperature". JOSA. 58 (11): 1528–1535. Bibcode:1968JOSA...58.1528R. doi:10.1364/JOSA.58.001528.
  9. ^ "HMCharacteristicTypeColorTemperature". Apple Developer Documentation.