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November 3rd, 2011, 07:11 AM
 Doug Kerr Senior Member Join Date: May 2006 Location: Alamogordo, New Mexico, USA Posts: 8,598
The units "mired" and "mirek"

We have recently seen here, in connection with colorimetric matters, references to the unit mired. Not everyone here may be familiar with it, so I thought I would expound a little.

The unit (typically pronounced "MY-r'd") is a way to express color temperature (or correlated color temperature), but we most often find it used to express a certain kind of chromaticity shift (perhaps one we want to make in an image, in connection with white balance color correction, or just for some artistic reason).

The unit name is a contraction of micro reciprocal degree. The "degree" referred to is the degree Kelvin, the older name for the unit of temperature now called the kelvin.

The concept is most rigorously applicable to "Planckian" chromaticities; that is, the chromaticities of the light emitted by a luminous "black body" at different temperatures. If we plot the points corresponding to all Planckian chromaticities on a chromaticity chart (perhaps with the CIE x-y coordinates, or maybe the CIE u-v coordinates), they form a curve called the Planckian locus (or blackbody locus).

The location of a point along that curve depends on the temperature of the black body that would emit light of that chromaticity. That temperature, ordinarily denominated in kelvin (formerly, in degrees Kelvin), is called the color temperature of that chromaticity.

Note that only Planckian chromaticities have a color temperature. For other chromaticities, it is often useful to note what Planckian chromaticity is "closest to it" (that actually means geometrically closest to it if we are working on a chromaticity chart using the CIE u-v coordinates). The color temperature of that chromaticity is called the correlated color temperature (CCT) of the chromaticity of interest. (Sadly, it is often called just the "color temperature", thus leading to various misunderstandings.)

It was noted by early color researchers that the visual impression of chromaticity seemed to vary not with the color temperature (or correlated color temperature) but approximately with the reciprocal of the color temperature (or correlated color temperature).

Thus, it was seen to be useful to quantify this visual impression in a unit suited for the reciprocal of a temperature in "degrees (Kelvin)": the reciprocal degree.

But quantities in reciprocal degrees were very small (a temperature of 6500K corresponds to about 0.000154 reciprocal degree). So a smaller unit was introduced, the micro reciprocal degree - one millionth of a reciprocal degree. Now, for a color temperature of 6500 K, that metric would be 154 micro reciprocal degrees.

The contraction mired, from micro reciprocal degree, was adopted as the short name for the unit.

Today, given that the unit of thermodynamic temperature is not the degree Kelvin but rather the kelvin, the proper SI name for this unit was established as the mirek (micro reciprocal kelvin), but this sees little use.

As I mentioned, we mostly see the unit mired in connection with chromaticity shift (in a color temperature perspective). It is only fully apt for a shift of chromaticity in the direction (on a chromaticity chart) along, or parallel to, the Planckian locus.

Sometimes, however, we find changes in the perpendicular direction (along a line perpendicular to the Planckian locus in the region where we are working), or in some other direction, denominated in the unit mired. That usually is taken to mean a unit of distance (on the CIE u-v chromaticity chart) equal to the distance (in that region) corresponding to a color temperature (or correlated color temperature) change of one mired. That usage is deprecated.

Well, I see Carla has our lovely breakfast almost ready. I need to go shoot 14 IU* of short-acting insulin so as to maintain my blood glucose level in an appropriate trajectory.
*Note that in handwritten written prescriptions or orders, the notation "IU" is discouraged out of fear that the "I' will be mistaken for "one". Rather, only "U" is used.
Best regards,

Doug

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