The Tiffen 2x2" Decamired Blue Set of Glass Filter - Takes the guesswork out of determining color temperature. Acts as a conversion filter for color temperature adjustments from any point to any other point in the color temperature scale. Can be used to create "proper" coloration in unusual situations and to make creative alterations.
Applications include keeping color temperature constant during the course of the day and producing the effect of different times of day. Available in two series, reddish filters that warm the light and bluish filters that cool the light. Each series contains 4 densities: 1.5, 3, 6, and 12. Filters in the same series can be combined.
The decamired system has an advantage over the Kelvin system in that the Kelvin scale is not linear
Decamired readings provide more accurate/controllable readings than Kelvin
The difference between the Mired value of the film and the Mired value of the light is the Mired value of the correction filter that correctly tailors the light to the film
A simple formula
A positive value indicates a reddish KR filter, a negative value indicates a bluish KB filter
Thus, the filter required to adapt the 3000K halogen light (the slightly low value results from the mains voltage being lower than its nominal voltage) to a 5600K daylight color film is a blue Conversion Filter