Mired, derived from the term Micro Reciprocal Degree, is a unit of measurement used to express color temperature. The mired value is calculated using the formula: Mired = 1,000,000K/T, where K stands for kelvins and T for the color temperature. For example, if the actual light condition is 4800K, you would calculate 1,000,000/4800K = 208.3. If your desired color temperature is 5500K, you calculate 1,000,000/5500K = 182. The difference between the mired values is 26, meaning you need to decrease the mired value by -26. Using this formula, you can determine the appropriate filter. In this case, the C2 filter with a -20 mired value is the best choice.
