Luminus SST-20-W 4000 K CRI95 color and output test

The 4000K on their own are quite pleasant, at least under TIR (didn’t try in a reflector).
What I really like is that they have a small die (like XP-G2) so the throw is improved compared with 219C

Thanks for the link djozz! It explains why some experienced members tend to say that tint-mixed beams are so good.

Or you could have two greenish emitters, and as long as they are sufficiently far apart CCT wise, it’ll work as well.

That's very interesting! So "average tint" is always somewhere on straight line between CCT1 and CCT2?

By using logic, it seems it's better to use two LEDs with large CCT difference, at least when below BBL is desirable.

3000K+5700K seems to be best combination for most LEDs.

I wonder what is the procedure for 3+ different CCTs?

With three emitters you could theoretically cover the whole triangle they create by adjusting the output.

With the Zanflare T1 and its infinitely adjustable CCT, the effect on two emitters is clear as well.

Yes, it's very easy to see from that graph why is better to use 3000K+5700K emitters than 4000K+5000K emitters to get ~4500K tint, first combination has much better chance to be under BBL.

Yes, the 2700K to 6500K tint mixing should be done with an additional set of 4000K leds in between - I tried to tell this in the BLF lantern topic weeks ago for the exact reasons, which your last figure shows.

Thanks for those helpful graphs. My quad E21A 2500k with 2x 2000k and 2x 3000k is much rosier than just 4x 2000k or 4x 3000k variants. My quad E21A 3500k with 2x 3000k and 2x 4000k is rosier than just 4x 3000k or 4x 4000k.

From my observation, comparing emitters of the same CCT, below the BBL tint does not seem to tint objects as much as above the BBL tint so white objects look whiter with below the BBL tint where as white objects look yellowish/greenish with above the BBL tint. I’m guessing this is why members here said they feel mixing CCT improves color rendering eventhough all it does is give you more below the BBL tint.

The perceived tint deviation must have to do with the eye/brain relative sensitivity to the different frequencies of the visible spectrum:

Source: Light Measurement @ TutorVista.com

:-)

Wow…
Surprised to see it works like that.

But why CRI drops, it doesn't make sense?

Hmmm… i don’t understand that either…

Sometimes it does, sometimes it doesn’t. Usually not any meaningful amount in either direction if the emitters are similar WRT CRI to begin with.

what is WRT ?

Maybe your spectrum has a lot of blue and a lot of red but a hole in the middle?

1 Thank

I think, it does make sense, since a spectrum, which exactly matches a black body radiation is considered as 100 CRI.

(For now, let’s assume, that a given spectrum is perfect for a given color temperature - we don’t want to mess with correlated color temperature /which is calculated for suboptimal light sources with uneven spectrum/)

Once the tint goes above or below black body line, it is not an accurate black body radiation anymore - so it cannot be 100 CRI in any circumstances.

The reason why many people tends to prefer ‘under BBL’ tints, that it gives more saturation to the scene.

Please correct me, if I know/remember it wrong, but the latest LED tests now work with two summarizing value instead of single CRI (Ra).

The first one is R-fidelity (Rf) - which would mean the accuracy of the color rendering - which can be max. 100.
The second one is R-gamut (Rg), which actually can be more than 100.

If above 100, we speak of adding saturation to the scene (this is when ‘under BBL’ tints), as we get more light for the parts of the spectrum, where human eye is less sensitive, thus color perception gets boosted somewhat (as a ‘side effect’, tint becomes rosy) - this effect can be regarded as pleasant and can be felt as a ‘superior’ light source - making the CRI/Rf drop seem to be of no sense, despite the measurements.

If under 100, we speak of desaturating the scene (when using ‘above BBL’ tints), where we tend to miss blue and red colors from the scene, and we talk about a tint being ‘greenish’ in most of the cases - these tints sometimes can be felt less useful/’inferior’ even with relatively high CRI/Rf readings.

Look at any measurement, which is recently provided by maukka. You can see a graph on the right with Rf and Rg coordinates. You can see a triangular area, which has white background (or another with lighter gray) - this shows, that how much under/oversaturation (difference to 100 on the Rg axis) is possible for a given value on the Rf axis - of course, less under/oversaturation is tolerated, if one would go for a higher Rf value.

Now I hope, this makes sense to you :slight_smile:

This is why I love that red dot in that graph (and the 99 color samples as well, but for a different reason).

Im going to try and mix 2x Luxeon MZ 3000K and 2x 5700K so i hope these results are similar. Someone should start a color mixing thread (maukka)

Here is why the CRI drops when mixing different LED tints.

The 100% curves for the CRI are taken from the perfect black-body radiation at different (color) temperatures. Each curve has a different maximum point and slightly different shape. 5000 K has its maximum at 580 nm and 2700 K at 1070 nm.

(Discaimer: The CRI uses black-body curves only for a CCT below 5000 K. The ‘Illuminant D’ data used for higher CCTs are too ugly to show here.)

After standardizing the curves (divide all values by the maximum value, so maximum is always 1; easier to compare) and zooming into the area of visible light you get this:

For each wavelength your LED has to get near the curve to get a good calculated CRI value. (These curves are the blue ones in maukka’s diagrams.)

The curves look relatively similar to each other, just shifted left and right so each shows a different part of itself in the visible light range.

Now lets mix a high CRI 2700 K and a 5700 K LED.
Their spectra almost fit the 2700 K and the 5700 K curve. You mix their tints by summing up the values of the two curves for each wavelength. For the same power output on both LEDs that is the dashed line.

If you make one LED brighter and the other one dimmer the curve will only change slightly. Only the maximum and the slope will get closer to the curve of the stronger LED.
See the dotted 0.4/0.6 and the slash-dotted 0.8/0.2 curves for some mixtures. The slash-dotted line would not be too bad for 3500 K but with too much blue.
I could not mimic the shape and position of the 4200 K curve.

The more the mixed tint gets away from one LED tint the worse the mixture gets. Without a curve similar to the 100% CRI curve the calculated CRI value will be bad.
To really change the color temperature of an LED you would have to shift the wavelength, not just modify the amplitude.

The penalty in CRI will be lower with less distance between the tint of the LEDs.

Thanks TBone, great explanation!

Nice post Tbone!