CRI vs. alternatives with measurement examples

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maukka
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CRI vs. alternatives with measurement examples

Hi all, I just bit the bullet and bought a highly regarded BabelColor CT&A analysis software that provides much more information than I’ve been able to provide regarding color rendering. Research has shown that CRI is quite inaccurate in representing the color fidelity with LED lights. CT&A is also able to extract some replacement indices, such as CQS, CRI2012 and MCRI.

I gathered here some information on metrics.

CRI: Color Rendering Index (1995)
The only widely used standard for quantifying color rendering.
Ra: Average of the first 8 (pale) color samples in the test
R9: Saturated red sample, difficult for LEDs and often negative
R10-R14: More saturated color samples

CRI is not very accurate with LED light sources and it often penalizes lights which enhance chroma but are actually preffered by human observers. The problem can be alleviated by calculating the average of Ra and GAI (Gamut Area Index). The system can also be gamed, since a light can be optimized for high Ra, while the actual color rendering is bad.

CQS: Color Quality Scale
NIST proposed replacement for CRI to better estimate color fidelity with modern light sources.
Qa: General index (RMS average of 15 samples, Q1-Q15)
Qf: Color fidelity, takes into account only hue, ignores saturation effects
Qg: Gamut area, saturation of colors

CQS Qa is calculated from 15 samples and makes the gaming of the calculation difficult. Research shows that lights with high Gamut Area Scale (Qg) are often preferred. CQS takes into account the error of increased chroma but penalizes less from it than CRI.

CRI2012 (Ra,2012)
Another proposed CRI replacement that comprises of 17 color samples, including several darker tones. Still actively worked on.

MCRI
A color rendering index based on the memory of colors in which a person is shown a set of 9 familiar objects. Research has shown that MCRI is a better indicator of color fidelity than CRI(Ra) or CQS.

Sources:
http://vision.uni-pannon.hu/LeCV/docs/2012_P01_Rajendra_Correlation_betw...
http://cie2.nist.gov/TC1-69/Princeton/Explanatory%20notes%20on%20CQS.pdf
http://babelcolor.com/index_htm_files/CT&A_Help.pdf
https://lirias.kuleuven.be/bitstream/123456789/261922/1/CRIVienna_KevinS...

Here’s a sample screenshot of all the data BabelColor CT&A’s CRI function provides.

This is the data I measured out of my own lights with an X-Rite i1Pro in a table and graphical form. I will update these into my existing reviews later.

Edited by: maukka on 01/31/2016 - 03:03
ReManG
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Where do I send you donor LED’s for testing?

Your testing is some wonderful stuff for those of us that are wanting more data on color reproduction and appearance. I am glad the Astrolux A01 scored so well, this shows that they are using genuine Nichia 219B LED’s…

Do you have the ability/skills to reflow LEDs for testing, or are you just testing lights that you currently have as purchases?

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djozz
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I agree with ReManG, this data are missing from what is done on led-testing thusfar (I sure do not have the equipment to add this to my output tests), and is interesting to know. 

THE_dAY
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Thank you maukka for the excellent work.

I wanted to note that there looks to be an update to the old CRI standard coming because of the LED inaccuracies you mentioned.
http://www.cie.co.at/index.php/index.php?i_ca_id=981

Basically, it hasn’t been updated since 1974.
“However, with the rapid uptake of LED lighting, which has greater freedom in spectral design, the need to update the CRI has significantly increased.”

maukka
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ReManG wrote:
Do you have the ability/skills to reflow LEDs for testing, or are you just testing lights that you currently have as purchases?

I have never done that so can’t promise anything. I would have to look up what’s required in terms of skills and equipment.

These are all just lights I have bought myself so the amount of data is naturally limited.

Gunga
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Wow. Amazing. Can you state what LED is in each light too? Great info.

maukka
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Updated table with manufacturer info about emitters.

higbvuyb
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Thanks for the great quality posts. Looking forward to more.

Evgeniy
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Thanks, excellent article

The Miller
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Nice work!!!

Can I send you some stars with a LED on them?
Then all you need is a cell and two wires to light them up.

maukka
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The Miller wrote:
Can I send you some stars with a LED on them?.

Sent a PM.

1dash1
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Nice work. I don’t claim to understand any of the test methodologies (or what goes on behind the numbers), but I am surprised that the average rating of all the test items correspond directly with their CRI (Ra) index. It seems like you could just do that one test and skip the rest.

Rule 1-1 as it applies to life, take it as it comes.

djozz
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1dash1 wrote:
Nice work. I don’t claim to understand any of the test methodologies (or what goes on behind the numbers), but I am surprised that the average rating of all the test items correspond directly with their CRI (Ra) index. It seems like you could just do that one test and skip the rest.

I see what you say here, but there’s a few differences here and there. It tells you that CRI is is a working indicator for colour rendering. But it costs nothing extra to use an indicator that is even better, and it helps the industry setting the right target for the products instead of a slightly wrong target.

In general, when you start testing things you have not tested before, often after a while you see a pattern: already from initial observation you can predict how it most likely will test. From that moment on, the tests will be less informative, but if you stop actual testing you will also miss the surprises: occasions where specimens test very different from your prediction.

The Miller
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Nice then I will send you
A Nichia 219C
A LatticeBright with not so bad color
A star with a 1w warm white led we have used in the kitchen and I wanna mod into a flashlight
All on stars no soldering needed.
Nice to have the 219C measured for a more complete overview.

The Miller
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Now wait when we are at it I will also send the XHP50 and XHP70.
Really appreciate that this is possible and you are sending them back afterwards.
Let’s build a comprehensive list right?

djozz
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Every individual emitter will test a bit different, so there will not be ultimate results. You could detect false claims by sellers though.

sixty545
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Another (uknown) factor is the applied reflector. The reflector will only direct a part of the light rays into the hot spot. The direct (bluish) rays Will miss the reflector and go into the spill. A measurement of the LED without some (standard?) reflector can give misleading results.

WalkIntoTheLight
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Fascinating. I’d be interested to see the values for a regular neutral-white XML2 when you test one, since it’s used on so many of my lights.

R(9) looks so bad on all those lights, with the exception of the Nichia 219B. What kind of Nichia 219 5000K emitter gives the bad R(9) value? Is that just because it’s a bit cooler, and so there’s less red in its spectrum, or is it a 219C that is generally less good at red?

What does a negative R(9) value actually mean?

The Miller
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Well testing some will give more data then testing none Wink
Maybe each emitter is slightly different, but there must be enough similarities in tint that is close enough in the same bin, otherwise all reviews of leds have little use right?
For weight sake I will not send the S70 (also removing that is more difficult then the easy to access S50 XHP50) and for my eyes the tint of the two is similar, of exactly the same.

maukka
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Here are the results for the bare unspecified emitters The Miller sent me. I was surprised how cool white the XHP50 and Latticebright turned out to be without a reflector.

I didn’t have any specs for the emitters so the voltage and current weren’t set for maximum brightness, because I didn’t want to destroy them and there was no cooling except for the stars. I did test them on different output levels and didn’t notice significant change in CRI or tint after reaching a certain threshold of usable output. The XHP50 was run at 6 volts and 1,5 amp and it was blinding.

The Miller
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Nice!
Thanks
I will certainly use that household spot led

That LB is not so bad (cursing in the church here, but some of the LB emitters do produce a nice tint (sorry sorry but they do))

maukka
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Here’s the newest color rendering method, IES TM-30-15 (lots of interesting information in the pdf), which is supposed to be much more reliable when measuring leds. The basic workings are similar to the original CIE CRI but with several improvements. I’ll include most of the metrics specified in the method in my future tests.

djozz
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That was a good read! The method provides a CRI-like general colour rendering qualification, as well as a wealth of more specific data about a given light source that is really helpful for general lighting applications, but may be a little overkill for a mere flashlight Wink . But that never stopped us from wanting it anyway Smile

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Thanks for another interesting thread.

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Great! I'll take a good glance at the IES TM-30-15 document later (an amazing improvement it seems).

I always wondered how much emphasis did the CRI(Ra) standard set on violets, seems to me typical white led light sources may suck a bit in this regard. 

 

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Great work. Which metric is best, in your opinion?

My gut tells me gai+cri for best vividness and naturalness. . I am suspecting/hopeing that cqs is better in this respect. I believe it was proven that people can’t remember color. I prove this daily, watching people get confused by shades seen a mere minute ago. So, puzzled at the memory cri metric.

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Spectrograms of LEDs look quite disheartening…

Looking for:

5” parabolic reflector (for recoil light)

degarb
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Barkuti wrote:

Great! I’ll take a good glance at the IES TM-30-15 document later (an amazing improvement it seems).


I always wondered how much emphasis did the CRI(Ra) standard set on violets, seems to me typical white led light sources may suck a bit in this regard. 


 


Cheers Party

Just read the tm-30-15 specs. . This is the holy grail. http://yujiintl.com/tm-30-15-high-fidelity-full-color-gamut-led-lighting

LED magazine had good article.

The CIE had better adopt this tm-30 standard! This is what I am after. If I only shot photos, I might not care. But, I use lights with my eyes to hunt subtle hue shifts.

Make your own spectrometer, from stuff you got at home. Led looks identical to the eye as an Incan or sun. You need special equipment to cut out the bottom 80 percent of illumination and make a graph. . I have seen led graphs that match the sun, close enough. Also, surprised, even the choppy CFL, does well in a 140 hue chart discrimination battle. Rbg led lights have the oddest graph, yet best for store fronts.

This tm-30 standard is what we need. Too bad engineers love numeric naming and it wasn’t called the 99 luft balloon proposal.

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degarb
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http://www.cie.co.at/index.php?i_ca_id=981

I am trying to research the 2017 status of the tm-30 It appears supported, but voluntary. Mandatory, only after everyone volunteers.

Am I getting right pulse?

Runtime=Usetime

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all spectral images thanks to maukka’s posts:

N219b:

Nichia 219B-V1:

XPL:

CW XM-L2:

TiTool

CuTool

maukka
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A wealth of information with real life examples about color rendering and the TM30-15 and how the average index is not often very useful, be it CRI(Ra) or TM30(Rf).

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