Notes on Colour

mired values

Mired nomograph for light source conversion Kodak1

Colour temperature and mired values

A change in colour temperature will have a bigger effect on the colour of a light source with a low colour temperature than on one with a high colour temperature. For example, a change from 3000 K to 4000 K produces a greater change in colour than a change from 5000 K to 6000 K, even though they increase the colour temperature by the same amount—1000 K. For this reason, we use mired values instead of kelvins when we describe changes in colour temperature.

The mired value of a temperature is calculated by dividing one million by the temperature measured in kelvins. For example, 5000 K had a mired value of 1 000 000 / 5 000 = 200 A change from 5000 K to 6000 K is a mired shift of 1 000 000 / 6 0001 000 000 / 5 000 ≈ −33 but a change from 3000 K to 4000 K is a mired shift of 1 000 000 / 4 0001 000 000 / 3 000 ≈ −83

A photographic colour conversion filter or light balancing filter will produce a consistent mired shift in a light source. For example the Kodak 80B filter, which has a particular blue colour, produces a mired shift of −112. It was used when the subject of a photograph was lit by photoflood lamps but the film in the camera was designed for use in daylight. (A 500 W photoflood had a colour temperature of about 3400 K but daylight film was optimised for use with light sources of 5500 K.)

The illustration shows a nomograph that Kodak produced to help photographers decide which filter to use. To cope with special circumstances, filters could also be combined and the affect calculated simply by adding the mired shift values.

Now a photographer can make any of these adjustments by setting the white balance on a digital camera to match the colour temperature of the light source. The changes are still measured as mired shift values.

Close up of the Planckian locus in the CIE 1960 colour space with isotherms in mireds
from Wikipedia2