The mired is a unit that measures colour temperature, just like Kelvin does. Only, almost everybody knows degrees Kelvin while fewer people know what the mired stands for. That’s not surprising, given that most light and strobe colour temperatures are expressed in Kelvin. So why use mired instead then — and perhaps more importantly — when do you use the mired unit instead of Kelvin?
Mired is a unit of measurement that is calculated using colour temperature in… degrees Kelvin. The formula to go from Kelvin to mired is very simple — even I understand it: Mired equals 1 million over Temperature in Kelvin. If your light meter measures a colour temperature of 5600K, then that equals to roughly 179 mired (1,000,000/5600K). A temperature of 2700K equals 370 mired, etc.
In very simple terms, we think of the colour spectrum going from left to right in terms of red going to blue. As tungsten light bulb white has a colour temperature that’s much lower than that of bluish white — which we associate with sunny days and blue skies — we find the Kelvin scale quite an attractive psychological representation of a physical phenomenon (at least if these psychological observations stick across cultures, which I honestly can’t tell).
In reality, though, it’s not as attractive as we might want it to be. The colour temperature representation in degrees Kelvin, when plotted across a Planckian locus, is not a linear representation. The difference between two illuminants humans can only just tell apart cannot be represented by a linear jump in Kelvins. For example, the difference between 2100K and 2300K is quite visible to us, while the difference between 6000K and 6300K is much less obvious.
As a result, this exponential character of the colour temperature as expressed in Kelvin is not terribly helpful when we want to correct for temperature shifts in photography and video/film making. What we need is a nice linear scale and mireds do increase in linear fashion when plotted across the Planckian locus.
That’s the reason that photographers learn about mireds and use them. When to use them is a different matter. When we say a light is set at 5600K, we do think of a more or less overcast day, while when we imagine a sunny day, we do think of higher degrees Kelvin. Mireds, on the other hand, are counterintuitive to express colour temperature. They are, however, very useful to express differences in colour temperature.
So, if you want to match one light to another, the best way to express the difference will be in mireds. And so, as a result, a gel or a filter for photographic use is always expressed in mired shift.
For example, if you have a light set up that outputs 3000K and you want that to match another light that is set up for 3600K, you will need to calculate 1mio/3600 – 1mio/3000, which gives -55.55. That translates into installing a blue filter as negative values translate into blue or green filters, while positive values translate into amber or red filters.
To find a gel, you can calculate the mired value using the Lee filters online calculator. That one makes it very easy and simple to find the closest gel.