Sorry, my phone cam isn’t good for beam shots unfortunately. I’ll try it though and I’ll post if it’s good.
If you happen to have a light with 519a 3500K to compare, my guess is that the combined temps of the S21F at 50/50 will be close to that, but a hint rosier.
I have noticed in beam shots in the past that the lower temperature LED affects the finished mixed temperature a lot more than the higher one, at least perceptually. So rather than a mixed temperature being (2700 + 5700) / 2, which would be 4200K, it is perceptually more like (2*2700+5700) / 3, which is 3700K.
Just to add some “mystery” following is Simon’s typical men-of-few-words response to my questions. I haven’t decided yet what it means. 
:+1:
Me: …Is there a way to set to the mix to 50/50 from each LED pair?
Simon: You can adjust the color temperature to 4500K and let the brightness drop
The good thing is with my Opple and the Texas Ace lumen tube, I could test output at 4500k vs other temps and maybe have some better idea of what he was referring to.
Hmmm yeah maybe he uses Alibaba translator and something got lost in translation lol
I noticed one of his messages was translated to me
I don’t think the UI is sophisticated enough to do a 50/50 reset like Anduril 
It seems when using moonlight mode, the LEDs are 50/50, no matter what temp ramp is at. Maybe we could use that to try to calibrate the settings
“Arrived at local delivery center!” I could almost feel it in my hands.
Kidding aside, for me, an interesting comparison would be against my S21D with 519a 4500k (measured 4000k). How is the new light different wrt:
Output
Beam profile
Beam color and tint - the supposed advantage of lowering Duv
And also against 519a 5000k in my Wurkkos TS32, which I like VERY much, more than the 519a 4500k version as it is less yellow. Hopefully I would find a reason other than market demand that leads Simon to spring this surprise of a mixed LED design on us. 
Delivery tomorrow?! 
Hopefully my S21F might be delivered Friday (it’s a public holiday here on Thursday), otherwise it’ll be early next week. Will publish the review here.
Might be! Tracking even states name of my city now. I have the day off so this would be perfect timing.
EDIT-UPDATE: “Out for delivery” :exclamation:
Please do. I’ve always enjoyed reading your reviews.
YAY! Did it arrive yet
A picture speaks a thousand words. I was somewhat skeptical but it seems like the “mixing theory” above is true.
The light came with CCT 3700k, so I decided to test at this temp. Can’t believe it but somehow this combination of two Nichia 519a’s has lowered the Duv significantly, both photographically and by measurement.
At –0.0066 it probably has among the lowest, or THE lowest, Duv at this CCT range. The beam accordingly as you could see has a strong rosy tint. I would say it “out-rosy” a 219b like Voodoo magic. Output otoh is low, only 900 lm at this CCT (vs 1500 for D4V2 w/ 219b 3500k and 1700 for S21D w/ 529a 4500k).
Thx for the pics!
What optic are you using?
60 for both S21 lights for now.
I also plan to check outputs at other CCT’s later.
How do you like the light so far? Would you recommend it over the S21D?
I don’t know how mixed emitter lights work. the pic you posted in the middle, is it a 50/50 amp mix between the 5700 and 2700 emitters in the S21F? and you say it is at 3700K?
So does the formula I wrote above actually describe reality? “I have noticed in beam shots in the past that the lower temperature LED affects the finished mixed temperature a lot more than the higher one, at least perceptually. So rather than a mixed temperature being (2700 + 5700) / 2, which would be 4200K, it is perceptually more like (2*2700+5700) / 3, which is 3700K.
Assuming that the higher and lower CCT channels have similar output levels, the lower output one will be weighted more in an averaged CCT calculation. On the CIE chart, distances between points 1000K apart become longer toward the low end and shorter toward the high end. You want to take the average of the two points on the CIE chart, not the average Kelvin values.
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”
Nice discussion. It’s interesting that the calculated result is similar to my estimate from drawing lines between the 2 CCT and take the mid-point. I wonder if it is coincidental or the line method is expression of the mathematical calculation.
I have three 219b D4V2’s with different CCT’s, and their measured outputs:
4500k = 1670 lm
3500k = 1500 lm
2700k = 1350 lm
From the above I suspect the 519a 2700k LED would have lower output at same current as the 519a 5700k. Wonder if it affects the resulting CCT at same current for both LED’s a little bit.
Well, in that case you’d want a weighted harmonic mean. Found an online calculator: Online Weighted Harmonic Mean Calculator
Lets throw out an example and use 2700/5700 with 2700 at weight 1 and 5700 at weight 1.3 (i.e. 30% more output than 2700)
Weighted harmonic mean = 3843K
I prefer fixed CCT lights so I still hope Simon would eventually release S21F that way.
But I also would buy this S21F again based on what I see so far. The mixing of CCT’s gives a unique low-Duv 3700k beam color. The Duv is actually lower than 219b 3500k.
Output at 3700k is low for my taste. I have changed the optic to 10 Degree Clear and now the improved central brightness is much more useful to me.
So, does the 21F have the same buck driver as the 21D….or none at all?
.