Mired

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Close up of the Planckian locus in the CIE 1960 color space
File:Planckian-locus-in-mireds.png
Isotherms in mireds
File:Planckian-locus.png
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/[citation needed]) is a unit of measurement used to express color temperature. Values in mireds are calculated by the formula:

M=1000000KT,

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.[1]

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.[2]

Examples

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.

Common color temperature and mired equivalents[3]Template:Rp
Light source Temp. (K) Mired
Skylight (clear, blue) class="mw-no-invert" style=Template:Linear-gradient background | 15000–27000 40–70
Shade, illuminated by skylight class="mw-no-invert" style=Template:Linear-gradient background | 10000–12000 80–100
Skylight (hazy) class="mw-no-invert" style=Template:Linear-gradient background | 7500–8400 120–130
Overcast class="mw-no-invert" style=Template:Linear-gradient background | 6700–7000 140–150
Electronic flash class="mw-no-invert" style=Template:Linear-gradient background | 6200–6800 150–160
Sunlight (hazy) class="mw-no-invert" style="background:#Template:Color temperature" | 5800 170
Daylight (average) class="mw-no-invert" style=Template:Linear-gradient background | 5500–6000 170–180
Daylight (morning / afternoon) class="mw-no-invert" style=Template:Linear-gradient background | 5000–5500 180–200
LED (cool white)[4] class="mw-no-invert" style=Template:Linear-gradient background | 3100–4500 220–320
Professional tungsten class="mw-no-invert" style="background:#Template:Color temperature" | 3200 310
Incandescent bulb (100 W) class="mw-no-invert" style="background:#Template:Color temperature" | 2900 340
Incandescent bulb (40 W) class="mw-no-invert" style="background:#Template:Color temperature" | 2650 380

Applications

Photographic filter and gel

File:Color temperature - mired shift.svg
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

1065700−1063200≈−137MK−1.

This corresponds to a color temperature blue (CTB) filter.[5][6] 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 unit internally.[7]

Color description

Apple's HomeKit uses the mired unit for specifying color temperature.[8]

References

  1. ^ Page Module:Citation/CS1/styles.css has no content.Ohta, Noboru; Robertson, Alan R. (2005). Colorimetry: Fundamentals and Applications. Wiley. p. 84. ISBN 0-470-09472-9.
  2. ^ Page Module:Citation/CS1/styles.css has no content.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.
  3. ^ Page Module:Citation/CS1/styles.css has no content.Smith, Robb (1975). The Tiffen practical filter manual. American Photographic Book Publishing Co., Inc. ISBN 0-8174-0180-6. LCCN 75-21574.
  4. ^ Page Module:Citation/CS1/styles.css has no content."Understanding color temperature". Westinghouse. Retrieved 18 August 2024.
  5. ^ Page Module:Citation/CS1/styles.css has no content.Brown, Blain (2002). Cinematography: Theory and Practice : Imagemaking for Cinematographers. Focal Press. p. 172. ISBN 0-240-80500-3.
  6. ^ Page Module:Citation/CS1/styles.css has no content."Mired Shift Gel Table" (PDF).
  7. ^ Page Module:Citation/CS1/styles.css has no content.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.
  8. ^ Page Module:Citation/CS1/styles.css has no content."HMCharacteristicTypeColorTemperature". Apple Developer Documentation.
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