# LED effiency Watts=> light and Watts => heat



## The Dane (Feb 25, 2009)

So when i apply 3,75Volts and 1000mA to a Cree XR-E it euqates 3,75Watts of consumption from the source, BUT:

What amount of the 3,75Watts ends as light that exits the front of the light and how much ends as heat?

I tried to search these forums, and because I'm not the sharpest knife in the drawer, i ended up without an usefull answer :shrug:


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## jtr1962 (Feb 25, 2009)

A rough but useful guide for cool-white phosphor white LEDs is ~330 lumens per watt of emitted output. High-CRI LEDs will be somewhat less than this, perhaps in the high 200s. Anyway, if for example your LED consumes 3.75 watts at one amp, and emits 250 lumens, then light output will be 250/330, or 0.76 watts. Since heat = power input - light emitted, then this means the LED will give out 3.75 - 0.76, or 2.99 watts of heat.

Note that up until when the Cree XR-E came out a few years ago, most people didn't even bother taking the light output into consideration. Most LEDs were less 10% efficient so it didn't affect calculations by much. Now the best LEDs are 30% efficient or more. It's starting to get to the point where it makes sense to take this into account. It's also interesting to note what happens as efficiency rises further. Suppose we manage to make LEDs which are 80% efficient. That means 80% light, 20% heat. Now one might say it doesn't pay to try to get to 90% as that would only mean 12.5% more light for a given power input. However, note that this increase cuts your heat in half compared to 80%. And going from 90% to 95% cuts it in half again! Practically speaking, this means smaller heat sinks, or more light output for a given size heat sink.


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## LukeA (Feb 25, 2009)

jtr1962 said:


> A rough but useful guide for cool-white phosphor white LEDs is ~330 lumens per watt of emitted output. High-CRI LEDs will be somewhat less than this, perhaps in the high 200s.



Source?


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## jtr1962 (Feb 25, 2009)

LukeA said:


> Source?


I read it in a technical paper from Nichia I think. Also, I remember seeing this info in some LED technology roadmap papers.


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## LukeA (Feb 25, 2009)

jtr1962 said:


> I read it in a technical paper from Nichia I think. Also, I remember seeing this info in some LED technology roadmap papers.



You forgot to put the link in your post.


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## jtr1962 (Feb 25, 2009)

LukeA said:


> You forgot to put the link in your post.


Didn't forget-it was a while ago and I just don't remember exactly where I read it.


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## LukeA (Feb 25, 2009)

jtr1962 said:


> Didn't forget-it was a while ago and I just don't remember exactly where I read it.



Then your prior assertions are false.


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## carbine15 (Feb 25, 2009)

LukeA said:


> Then your prior assertions are false.



This is not a conclusion you can logically draw from his statement. If you know otherwise, then inform us.


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## jtr1962 (Feb 25, 2009)

LukeA said:


> Then your prior assertions are false.


Why? Because I don't time to hunt through the Internet for hours for my source(s), assuming they even still exist? I read lots of things online. Some articles I save on my hard drive, others I don't. I generally don't bother saving links because more often than not they disappear within months. This 330 lm/W figure I alluded to has been common knowledge around here for quite some time. Do a search of CPF and maybe you'll find someone who already linked to a source if that's really so important to you. Or better yet stick a white LED in an integrating sphere where you'll get a nice output of the spectrum, and can use that to calculate the lumens per watt of emitted light. I bet it comes pretty close to the 330 lm/W figure I posted. In fact, someone around here a while back integrated the area under the spectrum of a Luxeon and ended up a figure in the neighborhood of 330 or 340 lm/W.

On another note, one of my pet peeves are people constantly hounding others for "sources". Sometimes an online source doesn't plain exist. Other times maybe the person _is the source_ as they originated the knowledge in the first place. Besides all that, when I'm online I'm usually more concerned with actually learning things rather than building a directory of where this knowledge came from. I'm not a librarian.


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## IMSabbel (Feb 25, 2009)

LukeA said:


> Then your prior assertions are false.



Well, you are not helping either.
He is right, btw. But i wont give YOU the link, pal.


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## monkeyboy (Feb 25, 2009)

The Dane said:


> What amount of the 3,75Watts ends as light that exits the front of the light and how much ends as heat?
> 
> I tried to search these forums, and because I'm not the sharpest knife in the drawer, i ended up without an usefull answer :shrug:



The Dane, this is a common source of confusion and certainly *not* a stupid question. The answer is that it doesn't make sense to quote a percentage efficiency for a light source.

You can quote a percentage efficiency for radiated power to power consumption but this has no relation to what the human eye sees and also includes radiated heat; infrared radiation (as opposed to heat conducted from the LED).

The lumens per watt rating is the best way of quoting efficiency as the rating is weighted with the sensitivity of the human eye to a particular wavelength or spectrum of wavelengths. This means that the maximum possible lm/w is dependent upon the spectrum of the light as jtr1962 pointed out. 

Some examples:

A pure IR source that was 100% efficient in terms of radiated power would have zero (0) lm/w and similarly for a pure UV source.

The human eye has a peak sensitivity at 555nm with a maximum possible 683 lm/w. 

Wikipedia quotes percentage efficiency based on the max 683 lm/w value which is pointless and confusing IMO.


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## SemiMan (Feb 25, 2009)

Nice thread...... first question was good and valid and was followed by a fairly accurate reply. Not sure what happened after that, but it was not very productive........ oops, I just did it....


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## likeguymontag (Feb 25, 2009)

SemiMan said:


> Nice thread...... first question was good and valid and was followed by a fairly accurate reply. Not sure what happened after that, but it was not very productive........ oops, I just did it....


Indeed. I found the question and the answer interesting and useful. Then this happened; dunno where the aggression came from. :thumbsdow



LukeA said:


> You forgot to put the link in your post.





LukeA said:


> Then your prior assertions are false.


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## LukeA (Feb 25, 2009)

likeguymontag said:


> Indeed. I found the question and the answer interesting and useful. Then this happened; dunno where the aggression came from. :thumbsdow



If you can't separate intellectual rigor from aggression, then I know what list you belong on. :thumbsup:


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## Marduke (Feb 25, 2009)

LukeA said:


> Source?





LukeA said:


> You forgot to put the link in your post.





jtr1962 said:


> Didn't forget-it was a while ago and I just don't remember exactly where I read it.





LukeA said:


> Then your prior assertions are false.



Just because a fact is not referenced explicitly does not make it false.



LukeA said:


> If you can't separate intellectual rigor from aggression, then I know what list you belong on. :thumbsup:



There is a fine line between intellectual rigor and being a *******. I'll let the reader decide which side the above posts are. 

BTW, typically the burden of proof falls on the accuser, so why don't YOU post a link citing your source for doubt. :thumbsup:

PS, I have seen a paper as described above, so I do know the statement is TRUE. For those who are not simply lazy, 5 minutes of Googling will give you at least half a dozen such papers.


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## likeguymontag (Feb 25, 2009)

Marduke said:


> There is a fine line between intellectual rigor and being a *******. I'll let the reader decide which side the above posts are.


I respectfully disagree. I believe that the two are totally independent of each other. That won't stop me from quoting from _The Big Lebowski_,


> *Walter Sobchak*: Am I wrong?
> *The Dude*: No you're not wrong.
> *Walter Sobchak*: Am I wrong?
> *The Dude*: You're not wrong Walter. You're just an asshole.
> *Walter Sobchak*: Okay then.





Marduke said:


> BTW, typically the burden of proof falls on the accuser, so why don't YOU post a link citing your source for doubt.



In science, at least when we're talking about _new_ information, the burden of proof falls on the person presenting the new information. Clearly that standard can't apply when the information is (or could be) common knowledge.

Aah, well. I probably shouldn't have revisited this thread anyway.


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## Marduke (Feb 25, 2009)

likeguymontag said:


> I respectfully disagree. I believe that the two are totally independent of each other.



True, I guess you can also be both.



likeguymontag said:


> In science, at least when we're talking about _new_ information, the burden of proof falls on the person presenting the new information. Clearly that standard can't apply when the information is (or could be) common knowledge.



In the LED world, this is very OLD information, and is in no way "disputed". Common knowledge indeed. Saying it's false is basically calling basic physics "false", such as if someone writes a basic equation (think V=IR) without also providing the proof for that equation.


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## likeguymontag (Feb 25, 2009)

Marduke said:


> Saying it's false is basically calling basic physics "false", such as if someone writes a basic equation (think V=IR) without also providing the proof for that equation.



I feel a Wikipedia "citation needed" joke coming on.


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## BillyNoMates (Feb 26, 2009)

OK. it appears to me to be useful to put a stake in the ground. By typing 'led lumens per watt' into a well known search engine, I found this press release close to the top of the list...

http://www.cree.com/press/press_detail.asp?i=1227101620851

This says 161lm/W it is dated as Nov 2008, so it is reasonably recent stuff.

I also found a reference in the luxeon Rebel data sheet stating 275mW of radiometric power for the royal blue led at 350mA drive. Now assuming that 350mA and a 'typical' voltage of 3.15V (value from data sheet http://www.philipslumileds.com/pdfs/DS56.pdf), the electrical input power is 1.1W. Efficiency of this LED is close to 25% (better than I thought it would be) therefore 75% (825mW) goes as heat.

#RANT
Now, that took about 5 mins and I hope I didn't annoy anyone in the process.....  ..... I only hope it is helpful, or even better stimulates some interesting discussion
#END RANT


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## The Dane (Feb 26, 2009)

Thanks alot guys.
Many fine comments.


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## ledstein (Feb 27, 2009)

I am sorry to burst your bubbles but this hole thread is full with physics nonsense. 

First you don´t measure heat in watts. Heat is measured in calories or BTU´s:

http://videos.howstuffworks.com/hsw/16788-exploring-heat-measuring-heat-energy-video.htm







Second: the efficacy (not efficiency) depends on electrical energy (watts) and luminous energy (luminous flux or lm). Its says how much energy (watts) you need to produce light (lm). It has nothing to do with heat and for a good reason: the heat produced by a light source depends on the technology used not on its efficacy: example- neon.


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## jtr1962 (Feb 27, 2009)

ledstein said:


> First you don´t measure heat in watts. Heat is measured in calories or BTU´s:


Strictly speaking you're right. Heat flux is measured in watts, but most people aren't used to using that term, so it ends up being "watts of heat". To me this isn't so bad-at least I know what the person is talking about. Now when someone says "watts of light" that really drives me crazy. I had a customer once that wanted "100 watts of light" in something. I think I came to the conclusion that the equivalent of a 400 watt metal halide lamp would be about right as far as emitting 100 watts of luminous energy. The customer asked how bright that would be (another misused term but that's beside the point). I said it would be about the same as those parking lot lights. He looked at me like I was nuts. I responded that this would indeed be 100 watts of light energy. No answer. Finally he decided to mention that he needed about the same light as a 100 watt bulb. I asked why didn't he say that in the first place. I also pointed out that 100 watts of light most definitely is not what a 100 watt bulb gives. It's actually closer to about 8 watts. Anyway, anybody using "watts of light" deserves to be drawn and quartered.



> Second: the efficacy (not efficiency) depends on electrical energy (watts) and luminous energy (luminous flux or lm). Its says how much energy (watts) you need to produce light (lm). It has nothing to do with heat and for a good reason: the heat produced by a light source depends on the technology used not on its efficacy: example- neon.


Well, yes, in general two sources with the same efficacy but different spectrums will produce differing proportions of waste heat. And around CPF we've fallen into using the term efficiency instead of efficacy for various reasons. For one, the efficiency of a light source can actually be defined two ways. One way is simply how much of the power in comes out as luminous energy of some sort (for example the best white LEDs emit about 0.3 watts of luminous flux per watt of input power). The other way is to take the efficacy in lumens per watt, and divide it by 683 (683 lm/W is the maximum possible efficacy of any light source). In the first case, raw conversion efficiency numbers are usually not readily available from manufacturers, making this definition of the term relative useless from an everyday standpoint. In the second case, the definition is just stupid and meaningless since you're comparing all types of spectra to a 555 nm monochromatic source. Indeed, a light source which converts 100% of electrical energy to white light of some sort might be at best manage 400 lm/W (and this is with gaps in the spectrum and a CRI of around 80). But by the strict definition of efficiency it would only be around 59% efficient. Like I said, it's just a stupid, nonsensical definition. Therefore, since both strictly defined meanings of efficiency aren't particularly relevant or useful around here, we've taken to using efficiency when referring to lumens per watt. To differentiate this from the efficiency definition referring to actual luminous energy, I personally use the term conversion efficiency. Here's a good example: A cool-white white LED with an efficiency of 100 lm/W will have a conversion efficiency around 30%. Most people will understand exactly what I'm talking about.


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## monkeyboy (Feb 27, 2009)

ledstein said:


> I am sorry to burst your bubbles but this hole thread is full with physics nonsense.
> 
> First you don´t measure heat in watts. Heat is measured in calories or BTU´s:
> 
> ...




1) Calories or BTU is an outdated non SI measure of heat energy. It is still used in some industries but not in physics. The SI unit is Joules. Expressed as a rate, this is joules per second or *watts*. Anyone can tell from the context, that this is what we are talking about here.

2) The watt is a unit of *power* not *energy*. power = rate of energy.

3) *Nobody* here said that lm/w has anything to do with heat.


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