The 138mm Decamired Blue 12 Cooling Filter from Tiffen is for those who want a sure-fire way of getting accurate color temperature readings. Using this formula, you can transpose temperatures from one point to another easily and quickly. This system is also useful when you wish to bring a certain amount of creativity to color photography.
Decamired filters are used to maintain consistent color temperature during the day's shooting. Decamired filters are available in two series - reddish filters that warm the light and bluish filters that cool the light with each series available in four densities: 1.5, 3, 6, and 12. When filters from the same series are used, they can be combined.
Decamired filters can be readily combined to create almost any required color correction. In measuring the color temperature of the light source, and comparing it to that for which the film was designed, required filtration can be easily calculated.
A filter that produces a color temperature change of 100K at 3400K will produce a change of 1000K at 10,000K. This is because the filter relates to a visual scale of color. It will always produce the same visible difference. A color change of 100K at the higher temperature would hardly be noticed.
To use this concept, subtract the mired value of the light source from that of the film. If the answer is positive, you need a reddish filter; if negative, use a bluish filter. Mired-coordinated filters are termed as decamireds. Mired value divided by ten yields decamireds. The 60 mired shifts, above, would be produced by an R6 filter, where the higher values were that of the lighting.
Sets of such filters generally come in values of 1.5, 3, 6, and 12 decamireds in both B (bluish) and R (reddish) colors. These numbers are additive; that is, a pair of R3's produces an R6. An R6 plus a B6 cancel each other out to produce a neutral gray.