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Whenever I see color High Power LEDs (for example green 3W on Star) I see disappointing efficiencies: For 520 nm the theoretical efficiency is 683 lm/W * 0.88 (green) = 601 lm/W. But most are described with 110 lm / 3W (Example)= 37 lm/W which is a conversion of electricity to light of 6%. There aren't phosphor losses and we talk about the efficiency of the LED without power converters.

I wonder why they are so inefficient while white LEDs (not necessarily High Power) with phosphors often have a efficiency of 100 lm/W?

Transistor
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Hansebenger
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    People don't want to see everything illuminated at one wavelength. There's something called the CRI, which probably needs to be above 80 or more in order for people to want to use it for lighting. Given the distribution over wavelength, an efficiency of 350 to perhaps 390 lm/W might be possible - in theory discounting electric efficiency. With lower CRI suggesting higher values. There is also the simple conversion of electrical energy to LED emissions to account. Phosphors are quite efficient (QM is like that) and can be used to improve the CRI while using the better conversion bands for LEDs. – jonk Dec 02 '21 at 22:20
  • CRI is not relevant in the question. I guess conversion bands goes in the right direction. – Hansebenger Dec 02 '21 at 22:23
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    It's likely that both you and I may be conflating ideas and not necessarily, similarly so. You may need to do a clearer and more refined job of asking the question. I will say that I spent almost a decade working with Seimens-OSRAM (before the sale of OSRAM) working on the topic of human color and brightness perception and LED display binning for manufacturing. I also worked on manufacturing phosphors for temperature measurement (phosphor thermometry.) So I know a few things. But that doesn't mean I understand you. – jonk Dec 02 '21 at 22:27
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    @jonk More than a few! ;-) – Ed V Dec 02 '21 at 22:32
  • Maybe you are right. The concept of lumen/brightness does confuse here because I am only interested in conversion efficiency of electrical power to radiometric power. But since most data is provided with lumen I thought it would be easier to use lumen. I will ask another question tomorrow, only complicated thing then is to get radiometric power output of a white LED. – Hansebenger Dec 02 '21 at 22:34
  • "Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths." Source: https://en.wikipedia.org/wiki/Lumen_(unit) – Dampmaskin Dec 02 '21 at 22:36
  • Efficiency of semiconductors in the green part of the spectrum is very poor compared to blue. The best blue LEDs can have a wall plug efficiency of over 75%. Nothing green (or red) can match that. – user1850479 Dec 02 '21 at 22:43
  • @Hansebenger The ***only*** time I ever dealt with radiometry of LEDs (outside of calculating CIE values for hue and brightness) was during a moment of experimentation on the idea of creating "cheaper" *standard candles* using LEDs instead of the vastly more expensive varieties that also required continual calibration and replacement. And in this case, the focus was on their long term stability. (With that in hand, we can calibrate against NIST standards for accuracy.) No focus on efficiency, which was poor at the time anyway. (These weren't for lighting purposes, either.) – jonk Dec 02 '21 at 22:50
  • @Hansebenger I probably cannot help with LED radiometric efficiency. That will have to come from others. I do have the two seminal books on phosphors from my earlier activity days: *"Measurements of Phosphor Properties,"* CRC Press, by Yen & Shionoya & Yamamoto, 2007 and also *"Inorganic Phosphors: Compositions, Preparation and Optical Properties,"* CRC Press, by Yen & Weber, 2004. But it's likely there have been some advancements on those topics, as well, in the last decade. But the latter does include many many efficiency curves for phosphors. Over a thousand, I'd say, at a glance. – jonk Dec 02 '21 at 22:56
  • @Hansebenger I'm seeing some newer texts that I've not examined but seem good. In particular, *"Phosphors: Synthesis and Applications"* edited by Sanjay J. Dhoble & B. Deva Prasad Raju & Vijay Singh and the two volume set, *"Phosphors, Up Conversion Nano Particles, Quantum Dots and Their Applications"* edited by Ru-Shi Liu. The first one looks useful in terms of updates and efficiency curves. – jonk Dec 02 '21 at 23:57
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    Here are some useful optical equations, but more on the colorimetry side https://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/scripts/NLPIP_LightSourceColor_Script.m also https://www.lrc.rpi.edu/programs/NLPIP/publicationResults.asp?type=2 – Tony Stewart EE75 Dec 03 '21 at 01:22
  • The statement "phosphors have a good efficiency of 80% upwards and LEDs much worse with high variance" is sufficient for me. – Hansebenger Dec 03 '21 at 10:13
  • Related Q&A : https://electronics.stackexchange.com/questions/203264/why-are-the-three-component-leds-in-an-rgb-led-so-unbalanced/203273#203273 - there's a link in the answer to a paper showing the quantum efficiency of green LEDs is relatively low. Possibly also https://electronics.stackexchange.com/questions/204998/why-are-there-only-rgb-and-rgbw-leds-but-none-with-more-chips-inside-one-housin?noredirect=1&lq=1 for related background. –  Dec 03 '21 at 14:33
  • @Hansebenger But they aren't all "upwards" of 80%. Because they are ***curves*** and there will be peaks and dips in various places. So their efficiency curve is required to be applied to the direct LED emission distributions over angle from the die. (It's not a Lambertian distribution, either.) Plus, there is always some amount of transmission effect (unless the phosphors are *thick*.) Anyway, sure. A single number may be good enough for your needs. I can't say. – jonk Dec 04 '21 at 22:04

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Due to the high demand in consumer electronics - especially lighting applications - both "blue" semiconductors (mainly GaN) and phosphor coatings have been developed further and further in recent years. Green LEDs are only used in niches. This is especially true for green high-power LEDs, because efficiency is much less important in low-power applications like LED displays or signalling.

Some time ago I developed a framework that allows me to convert all various photo- and radiometric units into each other based on the corresponding spectra. I can confirm that your conversions between luminous and radiant/electrical efficiency are correct.

However, I don't agree that green LEDs are as inefficient as you say. The LED you refer to is simply an example of a particularly bad LED. For the famous LM301B white LED (operated at 100mA, which is half abs max), I get a radiant efficiency of ~70% and a luminous efficiency of 220 lm/W. According to my framework, the GT QSSPA 1.13 (521nm) by Osram has a radiant efficiency of ~30% (140 lm/W) at Imax/2.

Even though this is obviously much worse than the LM301, it's still much better than what you assumed.

One should not forget that LEDs have improved greatly in the last years and the efficiency of less important colors is simply still at the level that was common a decade ago. When I look at the other LEDs I deal with, there is clearly a correlation between the efficiency of an LED and its demand on the global market. For example, the efficiency of red LEDs, which play a role in the automotive market (tail and brake lights) or in plant lighting, has improved greatly as well. On the other side: if you take a look at how other colors like yellow, far-red or UV perform, you'll see they are all more or less at the level of green ones.

So the simple answer is: a lot of effort has gone into improving certain LEDs.

Another point is, that cheap no-name LEDs generally perform much worse than those you get from reputable manufacturers which are specialized in LED manufacturing. A comparison is simply not fair.

Sim Son
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  • LOL, I thought LEDs have an efficiency of converting electrical to radiometric energy of at least 80% since the beginning of their time and only losses come from Phosphors and electrical power converters... – Hansebenger Dec 03 '21 at 10:05
  • My project to build the a efficient lamp with the spectrum of a CCFL with LEDs has gotten much more complicated... – Hansebenger Dec 03 '21 at 10:14
  • There are several reasons for losses: limited quantum efficiency, light absorption in the semiconductor material, ohmic losses in the package (solder joints, bond wires, impurities), absorption in the lense, etc... Though they are a common component nowadays, LEDs are far from trivial :D – Sim Son Dec 03 '21 at 10:16
  • Yeah, LEDs have some advantage over CCFLs, but efficiency is not necessarily one of them. – Sim Son Dec 03 '21 at 10:22
  • I knew that the efficiency is not always better, I like the simplicity of constant current + no mercury poisoning + color choosable – Hansebenger Dec 03 '21 at 10:51