# Make your own 97 CRI LED light



## Anders Hoveland (Jul 12, 2015)

Not sure if this will be of interest to anyone, or how practical it would actually be.

As we all know, great advances have been made in LED phosphor development, and today high CRI LEDs have become much more available than they were in the past. There are now even a few companies selling 3000K LEDs rated at 97 CRI— that seems to be as high as the CRI can go (at least with ordinary LED technology using a blue emitter and phosphor). However, these 97 CRI LEDs made by Bridgelux, LEDengin, and Mole-Richardson tend to be very expensive, and are not available in the usual small LED formats that are convenient for D.I.Y. enthusiasts to use.

So what other options are there? You can make your own 97 CRI LED light! Yes, that's right.
It is important to mention here there are a few caveats though.

I was playing around with the Osram color calculator app. By simply starting with a normal white LED and supplementing with some additional red and cyan, of the proper wavelengths, the tool indicated that the CRI could be raised to a value as high as 97.(see note at end)​

The following emitter combination was used to give a spectrum with 4063K, 97 CRI, 96 R9.

Verde 505nm 40 lumens
Red 635nm 18 lumens
4000K white (85 CRI) 460 lumens

The calculated results are remarkable for such a simple combination.

Some notes though if you are thinking about trying to substitute other wavelengths for the red and cyan. Very small deviations from the above given wavelength values will throw off the CRI, down to 96 or 95. For the cyan, the wavelength can go as low as 500nm, maintaining the 97 CRI, but the lumen intensity needs to be decreased. For the red, it is fine to use 640nm. What I was surprised by is that using a 635nm wavelength could give such a high R9 color rendering value.

I have been doing experimentation and can verify that that the spectrum of a regular CRI white LED can be enhanced with red cyan, greatly improving color rendering. However, it has been hard to achieve the exact optimal mixing ratios, so from what I have seen, I do not believe I was actually able to achieve a CRI higher than about 94. That is still remarkable though, for a haphazard throwing 3 LEDs together. I would probably have had better luck with variable resistors to be able to carefully adjust each wavelength emitter.

I want to emphasize the combined white light here still "feels like LED light", it just had great color rendering. So do not be expecting anything magical that feels just like natural sunlight. This is a great quick way to throw some cheap LEDs together to substitute for a high CRI emitter though. Again, not entirely sure how practical it may be to anyone, but at the very least it is very interesting.




* One more possibly important point to mention, I suspect the color calculator tool is using wavelengths with wider FWHM wavelength distribution values than a real emitters have, so not sure how much this could be affecting CRI calculation results.


----------



## calipsoii (Jul 12, 2015)

Osram put out an LED a while ago that already does that while also allowing CCT to be changed from 3700-5000k on the fly. Hard to find emitter and not used in any lights that I'm aware of.


----------



## Anders Hoveland (Jul 12, 2015)

calipsoii said:


> Osram put out an LED a while ago that already does that


That multi-emitter chip claims only 95 CRI. Maybe it is because it is using an amber emitter instead of a slightly longer red wavelength?
Or maybe the exact balance of different wavelengths could be more optimized?

It is one thing for it to be theoretically _possible_ to achieve 97 CRI with the combined separate emitters, it is another thing to actually achieve it.
In my experience, combining multiple separate wavelengths to try to achieve a color-balanced white light source that also has proper color rendering is not an easy thing to do.
95 CRI might be as high as this approach is commercially feasible to go.


----------



## degarb (Jul 15, 2015)

A tunable light excites me more than just high cri. It should offer best of both worlds, if the user can dial what they want, which may vary upon environment and task.


----------



## Anders Hoveland (Jul 16, 2015)

degarb said:


> A tunable light excites me more than just high cri. It should offer best of both worlds, if the user can dial what they want, which may vary upon environment and task.


There are many tunable color LED bulbs on the market.

I have seen a teardown of a LIFX bulb and it uses a RGBW emitter approach. The Philips Hue uses a completely different combination of emitters: amber, phosphor-converted lime, and blue. 
The RGBW approach used in the LIFX bulb gives it an average CRI of 88-89, peaking at over 92 between 3100-3400K, according to a comment from a representative of the company which was posted on Amazon.
The specs for the Philips Hue bulb says it has a CRI above 80 between 2000-4000K, and it has a CRI of 91 at 2700K. This might not sound as good in terms of CRI, but the choice of emitters does allow the light to have better color rendering when the bulb is strongly color-tinted (which is the whole reason people buy a color tunable bulb in the first place). That LIFX bulb is going to make the surroundings in your room look pretty monochromatic if it is set to a yellowish-green lime color, or is set to a heavy pink rose tint. Most people do not stop to think about what the color rendering is going to be like in off-white color mode.


----------



## degarb (Jul 16, 2015)

That is great. For me, a color dial on a fixed light is a luxury, while headlamp dialing is a need. I personally would have great use for a headlamp that would be tunable, but no (or little) compromise on efficiency or throw over existing leds. I do mobile color matching, inch by inch film inspection, and I might take up sewing and card making (It could happen!): all, things that could use a color shift.


----------



## Anders Hoveland (Jul 23, 2015)

A thought occurred to me. You know some traffic intersection lights use 635nm red and 505nm cyan-green, and many of the streetlights built into them for general illumination are now using LED too, often 5000K (probably around 72CRI). I know the light is a very uneven mix, with different colors flashing all over the place, but I would wager to guess that that the combined illumination from all these different LEDs has a CRI over 90 in some spots.


----------



## Marcturus (Jul 24, 2015)

Anders Hoveland said:


> A thought occurred to me. You know some traffic intersection lights use 335nm red and 505nm cyan-green.


335nm red ... is this still a typo, or is it the missing *Unvrared* we need to finally synthesize that "perfect" color rendering?


----------



## Anders Hoveland (Jul 24, 2015)

[previous error was fixed. yes, obviously I meant 635nm red, not 335nm. I make these typing mistakes all the time]



Marcturus said:


> 635nm red ... is this still a typo, or is it the missing *Unvrared* we need to finally synthesize that "perfect" color rendering?


I was surprised too, I had previously assumed it would take a longer wavelength to properly render saturated reds and achieve high CRI, but the Osram color calculator indicated otherwise. This is in line with spectral reflectance graphs of various red objects I have seen. Experiments I have conducted also seem to agree. When I used 660nm to generate white light, reds were indeed very saturated, but it threw off skin tones, making them appear too pink, and forcing me to throw in other red-orange or amber wavelength emitters to try to compensate.

It does not have to be 635nm though. The program showed that 640nm works just fine too, and 645nm can be used, but there is a narrower margin for error.
I am not saying a 635nm wavelength is absolutely the most optimal for creating perfect color rendering across the span of red color hues, but if you are forced to use only one red wavelength in your spectrum to supplement a regular white LED, this would be it.

If anyone was curious, I was able to obtain 98 CRI with a 99 R9 red value (at 4032K) with the following combination on the program:

505nm 30 lumens
535nm 12 lumens (note: it was not possible to achieve 98 CRI using 530nm)
619nm 7 lumens
650nm 11 lumens
4000K (85CRI) 460 lumens

But even slight variations from these values throws the CRI and R9 value back down, so in reality it would likely be fairer to label the spectrum *97-98* CRI. 
And again, I am not entirely sure how exact this program correlates to reality.

I also want to quickly point out that 505nm is probably not the most ideal if you are trying to design a real full spectrum light source, but unless there are also shorter royal blue wavelengths in the spectrum, it is about as close to cyan as you can go without throwing off the R12 blue color rendering values. 505nm is much more of a cyan-green than a true cyan.

In terms of what is actually used in traffic lights, it depends on the traffic light, they can use all sorts of different wavelengths. I think the most commonly used one is 650nm, but some of them use a slightly more orange-tinted red at 635nm (slightly brighter one would assume).


----------



## SemiMan (Jul 24, 2015)

Anders Hoveland said:


> [previous error was fixed. yes, obviously I meant 635nm red, not 335nm. I make these typing mistakes all the time]
> 
> 
> I was surprised too, I had previously assumed it would take a longer wavelength to properly render saturated reds and achieve high CRI, but the Osram color calculator indicated otherwise. This is in line with spectral reflectance graphs of various red objects I have seen. Experiments I have conducted also seem to agree. When I used 660nm to generate white light, reds were indeed very saturated, but it threw off skin tones, making them appear too pink, and forcing me to throw in other red-orange or amber wavelength emitters to try to compensate.
> ...



By far the most common would be 630-635nm for traffic lights. 

The OSRAM program is very close to reality within the confines of what you feed it. I.e. if you feed it the spectra of real LEDs then the output will be quite accurate. If you use a narrow band approximation for the LED, the your results will be off. Problem with real LEDs is they are not consistent.

Don't get too hung up on Ra/R9 either. Look at R10-R15 as well. Also keep in mind these are color swatches (somewhat properly chosen) with their own unique spectral characteristics as well.

Last, I would not get too hung up on 100CRI at 4000K. A 4000K blackbody radiator for all intents and purposes does not exist in nature for any period of time. That said, it does, based on high CRI LED samples look very good and Dr. Ohno has shown that 4000K is one CCT that people actually perceive as white. He has also shown though that people "prefer" the look of points below the blackbody curve, so 100CRI is perhaps not ideal as a visual, not technical light source.


----------



## Anders Hoveland (Jul 24, 2015)

Much of this may be of interest to those interested in custom reef lighting for aquariums. Not really many reefers in this forum, but there are several other reef forums on the net with members very interested in LED spectrums.

It is amazing that such high CRI values can be obtained even when they have a big blue spike in their spectrums. I think this further goes to show that CRI is not inherently the same as light quality. These light sources are still going to feel very much like "LED light", even if they do have extremely good color rendering.

(I think an argument can be made for adding in 487-492nm true blue-cyan to make the light feel more 'natural' even if it is detrimental to deep blue saturation and results in a lower overall CRI value)


----------



## SemiMan (Jul 24, 2015)

Anders Hoveland said:


> It is amazing that such high CRI values can be obtained even when they have a big blue spike in their spectrums. I think this further goes to show that CRI is not inherently the same as light quality. These light sources are still going to feel very much like "LED light", even if they do have extremely good color rendering.



Except all those curves are radiometric, not photopic. A big blue radiometric spike is a relatively small photometric spike.

It's also a "spike" and represents a single point (hypothetically), where it's power under the curve that matters. The photometric power under the curve in that blue "area" is small on a relative basis.


----------



## Anders Hoveland (Jul 24, 2015)

I know that, I meant when all the blue light in the spectrum is a single wavelength, without any longer wavelength true cyan/azure, or shorter wavelength royal blue/indigo. Amazing that the overall CRI value can be so high when the R12 value is not particularly excellent.

By the way, if anyone is tantalized by the idea of these super high CRI values, but wants something with better cyan/azure coverage (I mean where the dip in the spectral graph between blue and green is not too low), the Xicato Artist series LED module is a great way to go. It can achieve 98 CRI at 3000K, despite not having much royal blue wavelength coverage in its spectrum (has an R12 value of 88). Of course, violet-emitter LEDs from Soraa or Yuji have much higher R12 values, as the violet wavelength diodes can be used in conjunction with broad-spectrum blue phosphors.

I have not had the opportunity to actually see the Xicato Artist series in person, but judging by the spectrum I believe the light would probably feel much more natural than the improvised 97-98 CRI combination LED being discussed in this thread.
I'm not sure, perhaps some member in this forum who has actually seen them will be able to comment.

Xicato is becoming popular in large institutional museums, since the shorter wavelength violet-emitter LEDs can cause damage to older paintings.

Notice the spectral graph in that link has much more longer azure-blue wavelength coverage than normal LEDs, but there is still a dip between the 450-455nm blue peak and these azure-blue wavelengths, which presumably allows the 450nm peak to render deeper blues with adequate saturation, without too much interference from the longer blue/cyan wavelengths. In high-end art museums it is extremely important for the cyan and azure colors to be well rendered.

(and yes I do realize the coloration was not perfectly overlaid with the wavelength values in that graph, since 500nm is more green than blue, but what I stated above in this post is still true)


----------



## SemiMan (Jul 24, 2015)

Anders Hoveland said:


> I know that, I meant when all the blue light in the spectrum is a single wavelength, without any longer wavelength true cyan/azure, or shorter wavelength royal blue/indigo. Amazing that the overall CRI value can be so high when the R12 value is not particularly excellent.
> 
> By the way, if anyone is tantalized by the idea of these super high CRI values, but wants something with better cyan/azure coverage (I mean where the dip in the spectral graph between blue and green is not too low), the Xicato Artist series LED module is a great way to go. It can achieve 98 CRI at 3000K, despite not having much royal blue wavelength coverage in its spectrum (has an R12 value of 88). Of course, violet-emitter LEDs from Soraa or Yuji have much higher R12 values, as the violet wavelength diodes can be used in conjunction with broad-spectrum blue phosphors.
> 
> ...




Keep in mind the following:

- Most natural things are somewhat broad-band reflectors. I.e. they will reflect somewhat the same at 420nm as at 450 as at 480nm. Not exactly the same .... but close.
- You eyes have 0 ability to differentiate wavelength

When you keep that in mind, holes and spikes in the spectrum don't make much impact on color perception with real world samples. There are exceptions, of course.


... and the 4000K Xicato Artist series modules are probably the nicest overall light source I have.


----------



## Anders Hoveland (Jul 24, 2015)

SemiMan said:


> Most natural things are somewhat broad-band reflectors. i.e. they will reflect somewhat the same at 420nm as at 450 as at 480nm. Not exactly the same .... but close.


That is very true. I have looked at several spectral reflectance graphs for different colored objects.




SemiMan said:


> - You eyes have 0 ability to differentiate wavelength


Theoretically one would think that was true, but with actual observation I have found that not to be true in the blue-green region of the spectrum.
I have tried mixing 470nm and 495nm in an attempt to get the same feel as 488nm laser light, and it does not feel the same. It's almost as if my eyes can sense it is a mix of blue and green light rather than real bluish-cyan light.
The same is true of some "485-490nm" LEDs I bought from China. They are the same color as 485-490nm wavelength light would be, but they actually seem to be 470-475nm emitters covered with some green phosphor. And they feel "harsher" on my eyes than some special 480-485nm bin LEDs I have. If it was completely all about color, then the more blue colored LEDs should be the ones that feel a little harsher, but that was not the case. 

This is the reason I feel the presence of longer wavelength blue and bluish-cyan wavelengths are so important to light quality, but unfortunately there is simply no source of true 485-490nm emitters I can find, and I have done much looking, and ordered many different LEDs from China, only to be disappointed every time.




SemiMan said:


> When you keep that in mind, holes and spikes in the spectrum don't make much impact on color perception with real world samples. There are exceptions, of course.


I agree. A smooth spectrum is not as absolutely important as many may think, although generally higher CRI light sources do tend to have smoother spectrums. A high CRI light source can have spikes, so long as those spikes are close together, or well placed.

An* RGBA* LED source, which contains only four wavelengths to make white light is capable of 92CRI.




SemiMan said:


> ... and the 4000K Xicato Artist series modules are probably the nicest overall light source I have.


I am envious. Too bad light quality does not show up very well in pictures.


----------



## SemiMan (Jul 24, 2015)

Anders Hoveland said:


> Theoretically one would think that was true, but with actual observation I have found that not to be true in the blue-green region of the spectrum.
> I have tried mixing 470nm and 495nm in an attempt to get the same feel as 488nm laser light, and it does not feel the same. It's almost as if my eyes can sense it is a mix of blue and green light rather than real bluish-cyan light.



I should have been a lot more clear with what I wrote.

Any given photo-receptor in your eye has 0 ability to differentiate wavelength.

Mix the right amount of stimulus and you can "stimulate" a perceived color ... with the exception of some anomalies where there is uneven overlap in response spectrums.


----------



## Anders Hoveland (Jul 24, 2015)

For the life of me, I am not able to tell the difference between the light given off by real violet wavelength LEDs and those fake "black light" LEDs that use a 445-450nm emitter covered by a little red phosphor.
The only difference is that the fake "black light" LEDs are only able to cause fluorescent orange and pink pigments to fluoresce, not the green ones.

Of course, I could also hold up a CD and use it as an improvised diffraction grating to see the lines in the spectrum. 450nm by itself is not the same color as 405nm violet.

I think the light from real violet wavelength LEDs might also be even harder to visually focus on too, but 450nm light does not make things look entirely in-focus either, if you have ever seen the original Philips Ambient LED bulb with the orange phosphor panels removed. Things look a blurry and out of focus, it is a strange effect.


One more thing to mention. It is possible for a trichromatic white light source (composed of only 3 discrete wavelengths of light) to reach 82CRI, but the wavelength values have to be carefully chosen, and even small deviations from these values result in a drastic decrease in CRI. In the Osram color tool calculator, these theoretical wavelength values are 455, 545, and 609 nm. 
In reality the CRI of typical RGB LED lights are much lower (69 CRI), in part because a 545nm green LED emitter would be too inefficient and impractical with current technology.


----------



## hakkikt (Aug 9, 2015)

Anders Hoveland said:


> One more thing to mention. It is possible for a trichromatic white light source (composed of only 3 discrete wavelengths of light) to reach 82CRI, but the wavelength values have to be carefully chosen, and even small deviations from these values result in a drastic decrease in CRI. In the Osram color tool calculator, these theoretical wavelength values are 455, 545, and 609 nm.
> In reality the CRI of typical RGB LED lights are much lower (69 CRI), in part because a 545nm green LED emitter would be too inefficient and impractical with current technology.



As I'm sure you're aware this highlights just how unwise it is to rely on just one metric like CRI. Discrete wavelength mixes like this have very low or even negative R9 and R12 values and CQS in the 60s. They also render anything that falls between these wavelengths as black, e.g. a strong cyan reflector, a phenomenon you can trivially observe by wearing red or blue clothes under an LPS streetlight. I think the best color matching metric in the Osram tool to cross-check is the TLCI which has a totally different sensitivity to the CRI and CQS and contains a cyan test patch. If both CRI and TLCI are above 90 then you probably have decent color rendering. At least you will be able to take photos of it and expect to see the same colors as you did by eye. The various papers where researchers show that N discrete wavelengths produce a good CRI source should be taken as a warning against relying on CRI alone. Fortunately only lasers and single-metal vapor lamps produce this kind of unsavory output.

Using the TLCI index we see the effect of low blue-cyan content in a typical 92-95CRI LED, both the Nichia 219B and the Oslon GW-PSLRS1 have TLCI indices in the mid 80s while a Xicato Artist 3000k has a TLCI of 99 to go with its CRI and CQS of 97-98 based on the datasheet spectrum. Like all simplified metrics the TLCI can be gamed, with just four common LED colors I can get a TLCI of 81 at ~16000k with a CQS of 85 despite having an R9 of -23 and 4 other R metrics in the 60s. The best TLCI I have got with 5 selected color LEDs is 86 and thats too sensitive to bin variation to be buildable even if you were happy with < 29lm/W.

Within the range of sane CCT values the TLCI is fairly resistent to gaming with real sources but when it looks suspicious there is yet another metric that can be tested against. The COI or Cyanosis Observation Index tests how easily a person can distinguish oxygenated blood from deoxygenated and both of these from the other skin tones with the ideal being 0 difference from daylight at the calculated CCT. If we want more there's also the GAI-E and the Class-A illuminant test which directs us towards purple for CCT < 4000k as being preferred by viewers and should be between 80 and 100 for a good light. The tool gives us enough metrics that if they are all good it's highly unlikely the light will be unpleasant, after which it's simply a matter of finding a design that is tolerant of bin variation.

On the topic of light source spectra I also found https://cs.joensuu.fi/~spectral/databases/download/daylight.htm which has daylight spectra from different times of day that could be useful for comparison.


----------



## Anders Hoveland (Aug 9, 2015)

hakkikt said:


> As I'm sure you're aware this highlights just how unwise it is to rely on just one metric like CRI. They also render anything that falls between these wavelengths as black, e.g. a strong cyan reflector,


While I do very much agree with that CRI is not always the most accurate metric, the point I was actually trying to make is that a light source does not actually have to be full spectrum to have decent color rendering, it can be composed of only a few discrete wavelengths, but those exact wavelength values have to be carefully chosen.

As tangible example, think about tricolor fluorescent tubes. With only 2 wavelengths, yellowish-green and reddish-orange, it is possible to achieve surprisingly satisfactory color rendering of the range of green, yellow, orange, and red colors. Of course it is not the same, and fluorescent lighting can feel like it sucks all the color out of the room, but still, the colors do not look too far off what they should look like.

This is just my speculation, but from everything I know about color rendering, I think it is very likely that it could be possible to achieve a 97 CRI light source with only 5 or 6 discrete wavelengths.


----------



## Anders Hoveland (Aug 9, 2015)

hakkikt said:


> They also render anything that falls between these wavelengths as black, e.g. a strong cyan reflector,


 That is true, LED lighting often tends to render cyan colors a little more dull than full spectrum lighting. 
(by "dull" I mean not as brightly illuminated, and in some cases less saturated or slightly greyish)







This inability of LEDs to render color accurately is very visible in tests performed by The Academy of Motion Picture Arts and Sciences (AMPAS) as part of their “Solid State Lighting Project Technical Assessment”. In one comparison a model was photographed wearing a bluish-turquoise colored dress. Film was shot with both a tungsten halogen source and a 3200K LED source. The tungsten-lit footage displayed the subtle blue tones in the fabric, while the LED-lit footage, lacking cyan output, showed just a blue-colored dress, without the same richness of hue. Since the light doesn’t put out much cyan, the camera/film simply can’t record it.

http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorHigh Output AC LEDs
http://www.cinematography.com/index.php?showtopic=67240



On the other hand, fluorescent tends to render cyan colors better because there is a small ~490nm spectral emission from the terbium phosphor. (That is really the only downside of LED, in all other ways the color rendering of an 85 CRI LED is the same or better than standard tri-color fluorescent tubes)


----------



## hakkikt (Aug 9, 2015)

I entirely agree that a handful of narrow-band sources can have good color rendering. I think we've both explored the possibilities offered by 4 and 5 LED mixes here, I was just disagreeing with the single-wavelength idea as being more than a curiosity. From what I've seen of their spectra triphosphor fluorescents have a non-trivial light output between their peaks, the peaks themselves are at least 10nm wide and you can't get much above 90CRI without fairly full spectrum coverage even if it is somewhat peaky e.g. a 5 LED mix of the right DWL bands is excellent.

As mentioned in the article with that photo the TLCI metric was created precisely because we care about more than just the limited abilities of the human eye. It just happens that cyan is one of the colors tested by this that is ignored by both the CRI and CQS metrics. Even if you manage to get a CRI over 5 with 6 discrete wavelengths I'm pretty sure you'd be slightly bothered by the color distortion using a webcam under it and family photos would be a source of much frustration. Certainly a quick check with using 7 wavelengths to give CRI93 has a TLCI of 56 which, according to the paper defining the metric means many colors will be visibly off. Not saying we need to aim for studio quality light, just that we should take this into account, a TLCI of 70 is probably perfectly acceptable for the home.

Edit:
Maybe I'm wrong, checking standard illuminant F11, that has a TLCI of only 45 and it doesn't do too badly on webcams although I wouldn't really call it a light quality to strive for. I feel we should be aiming to do consistently better than F9.


----------



## Anders Hoveland (Aug 9, 2015)

The problem with discrete wavelength spikes, I have found in my experiments, is that it usually results in undesirable color shift of other nearby colors, and needs to be counterbalanced with other wavelengths. However these wavelengths can then create problems with other colors, and require yet another wavelength added to counterbalance them. Unless you carefully calculate exactly what wavelengths you will need before hand, you will end up requiring at least 7 separate emitters just to try getting something with half-decent color quality, and it will be hard to adjust and balance out all these multiple emitters to get the proper ratio.


----------



## Canuke (Aug 14, 2015)

Anders Hoveland said:


> For the life of me, I am not able to tell the difference between the light given off by real violet wavelength LEDs and those fake "black light" LEDs that use a 445-450nm emitter covered by a little red phosphor.



I can, but it's sort of cheating: the chromatic aberration in my vision acts as a sort of mini-prism and splits the components. True violet remains a homogenous blob/dot, while the red and blue component on the phosphor blue-red source separate quite readily - usually to a red dot and a blue blob.

I've gotten pretty good at using crappy vision to estimate spectra, in particular identifying LED versus HID headlights on cars.


----------



## hakkikt (Aug 14, 2015)

I've been approaching this same target from a base LED of >90CRI. This has two major advantages, the first being that such LEDs already have a strong red spectrum so they don't require very much of the horribly inefficient pure red or deep red and secondly the broad spectrum they start with usually gives enough room to accommodate the bin groups you can reasonably buy without significant loss of quality. Another nice advantage is CCT tuning if you're looking for it(it's a nice-to-have for me).

Right now I'm doing a bulk combinations run to see just how much variation can be got from a 3500K Nichia 219B (the datasheet spectrum) before CRI drops off and also how wide the CCT range is at 97+ CRI. I've constrained it to 95+ CRI 80 +R9 and the Class A illuminant model. These runs take several hours in the VM I'm using to run the tool but from a first look at the results I'm looking at what I think will be a buildable 98+CRI from 3800-4100K dropping to 96-97CRI from 3700-4400K, all with >95 R9 and R12 and no R value less than 89. A lot of the time I can get a CRI of 99 and my CQS scores range from 95-99. The red is only needed for CCTs below 4000K.

Given how much CPU time these runs take I thought I should share my results so I've created a repository on Github where I'll upload them along with my spectra files https://github.com/hakkikt/target97cri. Anyone who wants to contribute or just take what I've done for their own experiments is welcome to.

My binning targets are: blue (Cree XPEBBL-L1-0000-00X05 470-480nm DWL), cyan (this run is Oslon Signal LV-CQBP 505±7nm, next I will be checking Luxeon Z at 500±10nm) and deep red (any of Cree XPEPHR-L1-0000-00X01, Oslon or Lumileds approximately 660±10nm). All of these are straightforward order codes so if I can get a result that works across the whole range of DWL values for a passable CCT range I can actually build it. Can anyone here suggest where I can buy Nichia 219s in sample quantity for a test light? I'll need to talk to Nichia about changes in the upcoming 219C R9050 (don't want my design to be broken by a major phosphor remix in the LED I hope to buy for the final build) so I can ask them if there's not a known recommended source.

Edit: An annoying discovery, after 2 hours of runtime I find that the tool does DWL + step not DWL ± step so I only have half the data I need. Still I can run again and combine the results to get what I thought it was calculating.

Edit 2: Now with the full range calculated I can say that regardless of which bin combination I get I can almost always achieve 97CRI and 97CQS at 4100K using just blue and cyan with the white, occasionally dropping to 96CRI for particularly poor combinations. If the red is in the 660-670nm range I can get a small increase in R9 but it's not worth it at this CCT. I still need to do a proper analysis to characterize the light, I've just done a visual check for holes in the data over the result CCT range. Depending on wavelength the supplementary blue ranges from 1-10% and the cyan from 1-8%. Of course this result doesn't take into account variation in the white LED since I have no way to characterize this and Nichia only promises to supply within a 3-step MacAdam ellipse for each CCT. It is entirely possible that the worst case CRI would drop to 95 and each array will definitely need to be tuned to achieve these results.

I should also note that by using the Class-A model for filtering I've probably excluded some valid designs at lower CCTs where the GAI-E falls too far below the BBL.

Edit 3:
I have extracted the best values for each combination of CCT, and LED DWL from my huge data dump, picking the best CCT in each step of 50. With only 1100 rows I can get a better picture of what works. The most interesting thing is that there are practically no entries with the blue LED above 480nm. Examining this I find that the problem is requiring a GAI value above 80 has simply excluded a whole bunch of otherwise good results because they don't look enough like an equal-energy source and I can trivially get CRI and CQS scores of 98 or 99 with gamuts of close to 1 using a 484nm led and whatever verde I want, indeed I can chuck the verde completely and get 98CRI with just white and bluish-cyan. One thing that must now be decided is what metric to choose, GAI-E or GAI-BB. The choice has quite a lot of impact on the design of the light. Essentially I have to decide if Rea is correct that the black body locus is not the best target for low CCT lights, his 2007 paper leaves that question somewhat open, noting that GAI predicts our discrimination of blues better while CRI predicts better for red. Fortunately I can build and test to find my preference and if I really want to pull the GAI-E up I can add a 420-430nm LED without reducing the CRI by more than 1. Personally I think there aren't enough light sources in his experiment to form a solid judgement and this light I'm planning will help fill in one of the gaps.

Plotting selected columns I see that if I can accept 95 CRI (only 95!




) I can run the light anywhere from 3750 to 4300K and given the troubles the GAI-E has introduced probably a good bit further if I stay closer to the BBL.

Other data that is generally useful, verde decreases in utility above 505nm though it still adds up to 2 points of CRI so it's not useless. Not too surprisingly the very best results are 470-475+495-505nm which together leave very small notches in the spectrum and let you push the light around the locus more easily.

I have also added the Cree XP-E HEW 90 CRI led at 3000k to my repository for anyone wanting more spectra to work with. Having a second 90+ source is good for pushing the CCT around but this spectrum does show Cree's tendency to make green tinted lights.

EDIT 4:
While waiting for the announcement of the 219C-R90 I've been playing around with more designs. First, a fairly expectable result, Nichia uses the same red phosphor in their whole lineup so lower CCTs can be easily achieved by mixing with their 2000k 95lm/W "candle white".
Secondly, I was wrong about the efficiency of red LEDs, both Osram and LED Engin have 50% efficient red which is far better than the 30-35% of Cree or Lumileds red. This makes red a much more interesting choice for CCT shifting although from experimenting it tends to pull the CRI towards 90 rather aggressively unless you add reddish-orange and, for lower CCTs, yellow. This is where the CRI and CQS models cross over, even though the CRI has dropped to 90 the CQS is still around 95. 
Thirdly, the Osram Brilliant Mix system, a specialized variant on the idea being played with here does not offer much better CRI, efficiency or CCT tunability than a system starting with a good 90CRI LED. It can only exceed 110lm/w and 90CRI for a narrow range of CCT.

I've added the LUW-CR7P from the datasheet I used to my Github repo.

I also loaded the Nichia NCSE219B-V1 95Lm/W blueish-green to compare to the built-in verde spectrum. At 502nm it's an almost perfect match so the built-in model LEDs are pretty accurate.


----------



## JoakimFlorence (Dec 1, 2016)

I believe this may be how Indy's ChromaControl LED technology works. They are tunable across the whole range of different color temperatures and claim a CRI as high as 97 (although typically lower than this when the color temperature deviates from a set value)

https://www.youtube.com/watch?v=WhCvXr0xtGE

http://www.junolightinggroup.com/literature/LIT-INDYLED-CT-BD.pdf


----------

