Why does LED binning rating also changes Guaranteed CRI level ?

Kivatch

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I was inspecting the Cree XHP70.2 datasheet when I noticed two things :

1. Some high CCT LEDs have high CRI levels. I thought that "Cool" tinted LEDs were emitting exclusively in blue so that the CRI was staying below 75. Also, some "warm" tints such as the 4000k ones have the same lumen output as 6000k LEDs, how ? There is much phosphor on warmer LEDs so the output should be lower right ?

2. The binning "grade" of the LEDs impact the CRI. If you want a high binning grade, such as P2, CRI is 70+. Consequently, a high CRI means a low binning, why ?

That's a lot of questions but maybe I'm not the only one interested by the answers ? Thanks !
 

idleprocess

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Due to the quantities involved, making LEDs is a statistical process. There are variables such as the particular wavelength and flux intensity, of the blue die, quantity of phosphor, precise placement/pattern of the phosphor, and most likely numerous variables with regard to the phosphor formulation.

As a broad general rule, higher CCTs are more efficient because they down convert less blue to lower frequencies. Conversely, lower CCTs are apt to be higher CRI because they down convert more blue. But there are numerous exceptions no to this, presumably centering around the phosphor distribution and formulation.

I gather that getting tight, repeatable results from batch to batch of phosphor and even finished LEDs is difficult. Much like how semiconductors have their 'center of wafer' vs 'edge of wafer' variances, so too does LED production. Data sheets are educational, but looking at the bins available from a major distributor is likely to better inform one as to what the process best produces. High availability and low prices suggest that's what the process best produces.
 

Kivatch

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Lots of explanations here !

So there is no "higher grade" dies, because a "low quality" die usually has a better CRI if my understanding is correct ?
 

idleprocess

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Let's say the following options are available from a given manufacturer for a given model of LED:
  • 6500K, mid-low intesnity, <60 CRI : Easiest to hit because it's what the process naturally favors. The market will have them available in large quantities for cheap because it's easy to produce.
  • 4500K, mid-high intensity, 80 CRI : Obtainable at distributors like Digi-key or Mouser, low availability or out of stock, and always more costly than the previous option.
  • 3500K, high intensity, 90 CRI : Apt to be very hard to source at common distributors - it may in their catalog but perpetually out of stock - and is going to be expensive
  • 2900K, highest intensity, >90 CRI : May be best described as 'unobtanium' and may only be available directly from the manufacturer ... at a premium price

In reality, most manufacturers will spec a wide range of CCTs, outputs, and CRI bins (mercifully, they've generally eliminated forward voltage bins) in their specification sheets. What said spec sheets won't tell you is what's reasonably available from the broad spectrum of possibilities.

Lots of explanations here !

So there is no "higher grade" dies, because a "low quality" die usually has a better CRI if my understanding is correct ?

The general tradeoff is high CCT/output results in low CRI and vice-versa. While manufacturers can produce LEDs that better balance these traits, they tend to be rarer and thus more expensive. It could be low availability of blue LED chips that hit a precise frequency range, the random availability of really good phosphor mixes, some subtly-yet-impossible-to-nail-down-why better binning on one of the assembly lines, etc.

But your question depends on one's definition of quality. For most flashlight usage the priorities are output, CCT, then CRI; brightness/efficiency are primary selling and utility points, tint is an important perceptual point (cooler tints are perceived to be brighter for otherwise-same output), and CRI isn't terribly important for most users. For fixed-lighting usage (think light fixtures) it's different - CCT, CRI, output; CCT is crucial since the residential market prefers that incandescent-like 2700K while the commercial market is more about fluorescent-like 4000K, CRI is nearly as important since fixed lighting is for sustained use, while output is less important since pretty much all light fixtures will use scores of emitters to hit target output levels. These two cases are generalizations of course - specific products may have markedly different requirements: a dental instrument might need moderate output, high CCT, and highest possible CRI ... a special-use flashlight might need high output, neutral CCT, high CRI.
 

eh4

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Question for idleprocess:
Do I understand it correctly enough, that CCT corresponds to the dominant light frequency reported as Kelvin temperature?
 

idleprocess

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Question for idleprocess:
Do I understand it correctly enough, that CCT corresponds to the dominant light frequency reported as Kelvin temperature?

Lower CCT correlates with higher CRI, but the correlation isn't very strong since there's more to high CRI than down converting more blue.
 

Hunter8673

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Due to the quantities involved, making LEDs is a statistical process. There are variables such as the particular wavelength and flux intensity, of the blue die, quantity of phosphor, precise placement/pattern of the phosphor, and most likely numerous variables with regard to the phosphor formulation.

As a broad general rule, higher CCTs are more efficient because they down convert less blue to lower frequencies. Conversely, lower CCTs are apt to be higher CRI because they down convert more blue. But there are numerous exceptions no to this, presumably centering around the phosphor distribution and formulation.

I gather that getting tight, repeatable results from batch to batch of phosphor and even finished LEDs is difficult. Much like how semiconductors have their 'center of wafer' vs 'edge of wafer' variances, so too does LED production. Data sheets are educational, but looking at the bins available from a major distributor is likely to better inform one as to what the process best produces. High availability and low prices suggest that's what the process best produces.

thanks for posting this is very interesting.
 
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