Convoy S21D w/ legendary Nichia 219b. Review & comparison w/ Nichia 519a, B35AM, FFL351A Rosy & "tint-snob" LEDs. (Summary & measurements on P. 1) LATEST: Firefly E07X with VERY rosy 351A LED and Lume1 Driver.đź‘Ť

LED Flashlights – General Info 21700 Flashlights
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To calculate the resulting CCT of a mix it’s best to convert to “mireds”, do the average then convert back to CCT.

Mired unit: Mired - Wikipedia

Essentially, CCT is not linear (e.g. 3000K -> 3500K isn’t same amount of change as 4500K -> 5000K)
Mired is linear. I learned that in this thread: Preferred mired (colour temperature)?

So, to convert the tint mix of 2700 and 5700:
2700K = 1000000/2700 = 370.4
5700K = 1000000/5700 = 175.4
Average it: (370.4 + 175.4) / 2 = 272.9
Convert back to CCT: 1000000 / 272.9 = 3664K

More general formula: T1 * T2 / ((T1 + T2) / 2)
OR
T1 * T2 / average(T1, T2)

Tint mix 2700K and 5700K = 2700 * 5700 / ((2700 + 5700) / 2)
2700 * 5700 / 4200 = 3664K

Edit: Oh neat, found the mathematical name for it: “harmonic mean”