Ah, Kelvin. Abbreviated in degrees K. The unit we use to measure color temperature of light on set and the value we assign white balance to in camera. Named after Lord Kelvin.
Kelvin truly is the lord of color temperature on film sets. If you've ever been on one, you probably already know that Tungsten lights measure 3200° K and daylight varies around 5600° K. It's what we use to keep track of the differences in white balance. But what if there was another way to look at it? Let me introduce you to something you may not know. Buckle up: things are about to get nerdy.
Color Temperature Perception
When changing the color temperature on lights or in camera, have you ever noticed that going from say, 3000 to 3200 makes a bigger difference than going from something like 5600 to 5800? Well, that's because of the way that your perception of color temperature works. The lower you go on the Kelvin scale, the greater the perceivable difference is between the units of Kelvin, compared to the higher end of the Kelvin scale, where you notice the same difference less and less. It's a logarithmic scale. Not all color temperatures are created equal.
So 3200° to 5600° is a WAY bigger jump than 5600° is to 8000°, even though the gap is the same 2400° K) Try it yourself with a light or a camera. See?
If Kelvin isn't a consistent, linear unit, then is there one? I didn't even know about this until I started diving down a rabbit hole after wondering what a unfamiliar symbol was on my spectrometer.
Enter the Reciprocal Megakelvin. It's often abbreviated as "MK ⁻¹". You might also hear it called Mired or Mirek. And it's a measurement meant to represent differences in color temperature equally and linearly, unlike degrees in Kelvin.
This chart illustrates the relationship between Kelvin to Reciprocal Megakelvin. It shows you the difference in MK ⁻¹ from one color temperature in K (White Point A) to another (White Point B).
So to go from 4000k to around 2850k is a difference of 100 MK⁻¹, and 2000 to 10000 is 400 MK⁻¹.
Around 3200k to 5000 is about the same amount of change as 5000 to 10000 (200 MK⁻¹).
Math for Nerds
If you want to get the MK ⁻¹ difference between color temperatures, you can do the following math, where A is the first color temp in K, B is the second color temp in K. and Δx is the difference in MK ⁻¹.
Conclusion
I never knew about Reciprocal Megakelvins before I stumbled across the MK ⁻¹ symbol, nor did I really understand the perception differences in color temperature, so this really blew my mind. Did you know about any of this? Let us know in the comments.
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