# Electrical efficiency (watts->photons) of an LED



## mux (Dec 5, 2011)

I'm busy with feasibility on an SSL project and I'm trying to do it as scientifically valid as possible. I know that basically the most knowledgeable people when it comes to LED tech live on this forum (and AVSforums), so as a change from my normal lurking behaviour I decided to register and ask my question here.

I want to calculate the electrical _efficiency_, not efficacy, of a Cree XM-L LED. Actually, I did it already. It's 37.16%. I think it's not right, but I don't know where it might have gone wrong. Or maybe I do. I'll walk you through the process. It's about the cree neutral white (5000K CCT) XM-L LED. Here's the datasheet.

Cree provide in their datasheets a graph of relative spectral power density (page 3, top). It's not unambiguously telling me, but I assume this is in (normalized to 100%) W/m. 

This means that I should be able to scale this to the CIE photopic curve and get photopic lumens. Again I assume photopic, but I'm pretty damn sure they're not using scotopic or hybrid lumens. So I extracted the spectral power density for neutral white as well as the CIE photopic curve and read them into Matlab. Here they are:






Red=XM-L datasheet curve, blue=CIE photopic curve

Then I multiplied them together to get the (unscaled/relative) photopic luminous spectral density:





Now I reasoned that I should scale this curve by a factor such that the integral of the curve becomes the amount of lumens quoted in the flux bin, e.g. I should scale it such that it sums up to 260 lumen for a T5 bin. 

After that, I can go back: if I divide again by the CIE photopic curve, taking into account that the peak should be 683.002 lm/W and not 1 as I showed in the first picture (yes, I took that into account), I will get a scaled curve for the spectral power density in absolute W/m. Right?

So when I integrate that curve, I should get photon watts. The actual photons leaving the device, and their associated power. Well, if I do that for the XM-L neutral white T5 at 700mA, I get a photon power of 0.7543W. The current is 700mA and if we look at forward voltage vs. current, you can see the forward voltage at 700mA is about 2.9V. That means the input power is 2.03W. I can now draw the conclusion that 0.7543/2.03=37.16% of electricity is radiated as photons and 2.03-0.7543=1.2757W is dissipated as heat (62.84%, but that seems to obvious to write down (damn, I did it anyway)). Mind you, I'm really just talking about efficiency, not efficacy, obviously a UV LED may be very efficient but not effective because we can't see any of it's radiation.

Is this the correct method and the correct outcome? If so, this is absolutely remarkable. Also, thanks to Cree's datasheets we can actually derive this. 



I'd like to extend this derivation to maximum theoretical efficiency and other general silicon-oriented performance metrics (and thus a way to calculate the internal droop/extraction efficiency/etc.) but first I want to know if this is correct.


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## samgab (Dec 5, 2011)

I wish Cree would just include their emitters' _efficiency_ data in the datasheets.
I can't affirm or deny your calculations, but I have a suspicion that temperature has an impact on the efficiency also. 
Did you factor that into your calculations? Or did you calculate it all as being at 25 degrees C?


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## mux (Dec 5, 2011)

I have a separate worksheet that takes into account degradation due to current droop and temperature to get 'true' luminous efficacy at higher drive currents. This one is just at 25C though.

What I want to do, is to plug this efficiency-calculator into a separate matlab function, and use it to do thermal calculations in the aforementioned worksheet. Right now, I just assume _all_ electrical power goes into heat dissipation, because I was under the impression that radiative power was not significant (<10%). However, if it is actually 40+% at low drive currents, I should take this into account and it will improve my lighting fixture calculations quite a bit.


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## mux (Dec 5, 2011)

I have a separate worksheet that takes into account degradation due to current droop and temperature to get 'true' luminous efficacy at higher drive currents. This one is just at 25C though.<br>
<br>
What I want to do, is to plug this efficiency-calculator into a separate matlab function, and use it to do thermal calculations in the aforementioned worksheet. Right now, I just assume <em>all</em> electrical power goes into heat dissipation, because I was under the impression that radiative power was not significant (<10%). However, if it is actually 40+% at low drive currents, I should take this into account and it will improve my lighting fixture calculations quite a bit.


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## samgab (Dec 5, 2011)

This info may help:
T5 and T6 bins:





and T5 bin, at 25, 50, and 100 degrees C respectively:





so the _efficiency_ would vary dependent on junction temperature, and current, and bin.
If you take the theoretical limit for perfect visible white light _efficacy _as being an average of ~330Lm/W, and divide the chosen point of temp/bin/current LED _efficacy _by it, you should get a ballpark efficiency. Eg: T5 XM-L @700mA @25 C tj =128.1 Lm/W / 330 = ~38.8% _efficiency_.


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## samgab (Dec 5, 2011)

Actually, the theoretical max for white LEDs might have been incorrect based on this chart:





As the datasheet indicates that a T5 cool white XM-L can be anywhere from 5000-8300K, this chart seems to indicate that the theoretical max possible luminous efficacy for that colour temp range is from about 400-450 Lm/W.
So that changes the equation somewhat.
128.1/400= 32%.
128.1/450= 28%.
So this indicates that the efficiency of the T5 cool white XM-L, @5-8.3K, @700mA, @25 degrees C tj, @2.9V FV =28-32%?


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## mux (Dec 5, 2011)

Terribly sorry about the double post there, but as these are my first posts and something went wrong during submission, I don't have any way to correct it...

Anyway, that type of estimate doesn't work. There's a big difference between CCT and blackbody color temperature that will throw that kind of ballpark estimate off. As the spectrum between manufacturers and individual LEDs varies appreciably, you cannot say that one 5000K CCT 120lm/W LED has the same efficiency as the other. Early white LEDs produced a significant amount of UV because of the high bulk losses causing locally high voltages on the die. That's light completely ignored by lumen estimates, but still lots of photon energy (and thus less heat output than you'd expect - higher efficiency).

However, that figure of 28-32% is interesting. It is low compared to my calculation (possibly meaning I did something wrong), but that may very well be explained by the huge blue peak (high energy photons, and lots of them) in the XM-L LED spectrum. 

(this makes me think that it would be *very* interesting from a heat output point of view to find a way to convert the energy that is normally dissipated as heat into low-energy photons, so that the LED inherently stays cooler)


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## bose301s (Dec 5, 2011)

The vast majority of efficiency is lost in the phosphors used to make white LEDs, the bare blue die are much more efficient.


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## mux (Dec 5, 2011)

Yes. Cree XT-E puts out 550mW @ 350mA (1.05W) at 75 degrees, or even 625mW at 25 degrees die temperature. That means that _most _of the electric power is converted into light. Of course, the basis of a white LED is that exact same blue die but instead of the extracted photons radiating outward immediately, the phosphors absorb most of those photons and emit them at a lower energy, converting the rest of that energy into phonons (heat). We can actually calculate how much loss that causes and that is certainly something I would like my program to automate, but first I want to know if what I did was correct.

By the way, these datasheets are astonishingly complete. I can quantify almost all losses directly from the data. Bulk losses, current droop, extraction efficiency, it's marvelous. KUTGW, Cree.


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## jtr1962 (Dec 5, 2011)

mux said:


> Right now, I just assume _all_ electrical power goes into heat dissipation, because I was under the impression that radiative power was not significant (10%). However, if it is actually 40+% at low drive currents, I should take this into account and it will improve my lighting fixture calculations quite a bit.


Radiative power for LEDs is quite a bit less than 10% under ALL operating conditions. Remember that radiative power is proportional to absolute temperature to the 4th power. Let's say a lamp filament with the same area of an LED die emits 100 watts of radiative power at a temperature of 3200K. An LED die with the same surface area operating at 60*°C (343K) will only emit around 0.01 watts. For all intents and purposes radiative power of LEDs is totally negligible.


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## Kinnza (Dec 5, 2011)

Your procedure is essentially correct, mux. You just made a false assumption, that the SPD graph on the datasheet is scaled to W/nm. It is just a relative SPD, as it is just a "typical" one, give or take in the center of the color range: each color bin have a different SPD. Using that graph is a good way of having an idea of the typical efficiency of those LEDs (and color rendering, etc), that's all. The exact output in W/nm varies depending of the radiant efficiency of the LED, so higher bins scores higher, but the relative SPD continues being the same. You need to normalize SPD figures in a way its integral is equal to 1 (Watt).

Actually, what you obtained when multiplying photopic curve and SPD is the LER (Luminous Efficacy of Radiation) of such SPD (more exactly, LER is the integral of such curve given an SPD of area=1W). Then you just need to divide luminous Efficacy in lm/W (which will vary depending of the flux bin you analyze) by the LER figure, and you get the radiant efficiency, the figure you are looking for (Wout/Win).

An explanation of the logic behind it:

ηv=Φv/P

where 

ηv=Luminous Efficacy (lm/W)
Φv=photometric flux (lm)
P=power (Input Watts=Vf*If)

Multiplying by Φe/Φe, where Φe is the radiometric flux, you get

ηv=(Φv/Φe)*(Φe/P)

as Φv/Φe=k=LER (lm/Wout), constant for a given SPD

and Φe/P=ηe= radiant efficiency (radiometric output/input power, Wout/Win), the figure you want to know. So ηe=ηv/k

When you know the SPD, you can calculate LER of it, by the procedure you used: multiply SPD of integral=1 by photopic curve and by 683,002. It mathlab does it directly, great, if not you need an Excel sheet with the coefficient of the photopic curve in 1nm increments and multiply it by the SPD in 1nm wavebands (you can use whatever figures you have for the relative SPD, and normalizing them to 1, just dividing each waveband by the total).


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## slebans (Dec 5, 2011)

mux said:


> However, that figure of 28-32% is interesting. It is low compared to my calculation (possibly meaning I did something wrong), but that may very well be explained by the huge blue peak (high energy photons, and lots of them) in the XM-L LED spectrum.



Kinnza explained in detail how to calculate LER and then WPE(Wall Plug Efficiency).
Here are some basic ballpark numbers:

XM-L Cool White - T6 or higher
Junction temp 25 C
350 mA
CCT 5000K or higher

Underlying Royal Blue LED pump - 60-65% efficient
Phosphor conversion losses 20-24%
XM-L Cool White LED - 50% efficient
LER range(bin dependant) 280-330 lumens per watt

Stephen Lebans


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## mux (Dec 5, 2011)

jtr1962 said:


> Radiative power for LEDs is quite a bit less than 10% under ALL operating conditions. Remember that radiative power is proportional to absolute temperature to the 4th power. Let's say a lamp filament with the same area of an LED die emits 100 watts of radiative power at a temperature of 3200K. An LED die with the same surface area operating at 60*°C (343K) will only emit around 0.01 watts. For all intents and purposes radiative power of LEDs is totally negligible.



I think we're talking about different definitions of radiative power. I mean radiation in the exact meaning of the word: radiated photons. All light emitted. You mean purely blackbody radiation, which indeed is negligible and totally invisible.



Kinnza said:


> Your procedure is essentially correct, mux. You just made a false assumption, that the SPD graph on the datasheet is scaled to W/nm.


Well, no, I did recognize that the SPD graph in the datasheet was relative. However, because only the luminous flux is given I need to first convert to luminous density, then scale that appropriately, then convert back to the actual SPD, which I believe is exactly what you're telling me to do. Yay. 

Is there any way to analytically/numerically predict or estimate the change in spectral density over different tints and CCTs? Also, you say that essentially the spectral power density 'shape' does not vary with flux bins (given a chromaticity and CCT) but it does change with drive current, right? As the die voltage inevitably scales with forward current, there will be some shift towards blue going from low to high drive currents. Plus there will be some parasitics like fringe effects (green halos on some dies). Can you say anything meaningful about that?

@slebans: thank you very much. So cool white LEDs are 50% efficient eh... that's a very compelling thought. Is this including extraction losses and optical losses (just the on-package optics, not external optics)?


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## IMSabbel (Dec 5, 2011)

jtr1962 said:


> Radiative power for LEDs is quite a bit less than 10% under ALL operating conditions. Remember that radiative power is proportional to absolute temperature to the 4th power. Let's say a lamp filament with the same area of an LED die emits 100 watts of radiative power at a temperature of 3200K. An LED die with the same surface area operating at 60*°C (343K) will only emit around 0.01 watts. For all intents and purposes radiative power of LEDs is totally negligible.



Not all radiated power is blackbody - and he is right that he should only consider the thermal fraction of the power consumed.


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## jtr1962 (Dec 5, 2011)

IMSabbel said:


> Not all radiated power is blackbody - and he is right that he should only consider the thermal fraction of the power consumed.


I was only referring to blackbody radiation in my post. Now that LEDs have gotten much more efficient, visible radiation obviously needs to be accounted for when calculating the true amount of waste heat which needs to be dissipated, or you can end up overdesigning the heat sink.


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## Kinnza (Dec 5, 2011)

mux said:


> Is there any way to analytically/numerically predict or estimate the change in spectral density over different tints and CCTs? Also, you say that essentially the spectral power density 'shape' does not vary with flux bins (given a chromaticity and CCT) but it does change with drive current, right? As the die voltage inevitably scales with forward current, there will be some shift towards blue going from low to high drive currents. Plus there will be some parasitics like fringe effects (green halos on some dies). Can you say anything meaningful about that?
> 
> @slebans: thank you very much. So cool white LEDs are 50% efficient eh... that's a very compelling thought. Is this including extraction losses and optical losses (just the on-package optics, not external optics)?



Yes, LER only depends of spectra (SPD) (as it result for multiplying it by photopic curve*683,002 on finite wavebands or integrating the resulting curve, when starting SPD is 1W power). A given SPD has fixed chromaticity coordinates and CCT, of course. All that are just spectral characteristics, which only depends of SPD. But beware, although a given SPD has an unique CCT, LER and chromaticity, you can find same figures for SPDs very different. You can achieve same chromaticity coordinates with different SPDs, or same CCT, or same LER. The three at the time is difficult, but not impossible.

Specifically, you may think that once you has measured an spectra of a given color bin, all LEDs of that bin is going to be the same. But its not true. Due the way phosphor converted white LEDs works, it strongly depends of the peak wavelength of the blue LED used as pump. Manufacturer may use blues with different wl peak but with different phosphor (usually, thickness or density), resulting on a SPD falling on the same area but different from the other. Usually they are not big differences, but they could be so.

Any thing that changes SPD changes spectral derived figures as those. Drive current and operating temperature affects SPD, so affects spectral derived figures. It is almost impossible to derive the effect theoretically, as drive current affects SPD on some different ways, some working on one direction and some in the opposite (at least, for PC white LED). Al lower current,for example, the blue peak is at slightly shorter wl, but shape of the blue emission is narrower, and so on. Check this thread with lots of detailed info about it, old but highly informative: http://www.candlepowerforums.com/vb/showthread.php?70073-Constant-Current-vs.-PWM-dimming-Revealed

Ring and halos are other question. All depend of the spectra you are studying. If you get it using an integrating sphere, SPD recorded correspond to the average of the whole emission. If you record it with a probe, it will vary, depending on how much homogeneous is the light beam. Although this has improved largely along the last years, still the best white LEDs show some heterogeneity along the beam. But for studying the radiant efficiency, you need to consider the full emission and the average SPD of it.

But although SPD changes with current and temperature, fortunately LER dont change very much due it, so you can still derive radiant efficiencies at different drive currents with a decent margin error. Of course, the best way of doing it is with an spectrometer and IS, measuring both emission and SPD.

About the 50% efficiency of best white LEDs, always take in mind that a given LED efficiency depends of the drive current (and temperature). That figure is running them soft, and you can get higher efficiencies at lower currents. Once they are running hotter or at higher currents, efficiencies drops. But yes, top while LEDs can get it. Thinking in light output/input watts at the whole package.


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## David_Campen (Dec 5, 2011)

slebans said:


> ... Underlying Royal Blue LED pump - 60-65% efficient ...
> Stephen Lebans



Which LEDs are these? Looking at the datasheet for XT-E Royal Blue, I see about 48% to 55% efficiency.

http://www.cree.com/products/pdf/XLampXT-E_ROY.pdf


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## samgab (Dec 5, 2011)

Would this practical experiment allow for a calculation of efficiency/power wasted as heat:

Take 2 jugs of exactly 1 litre of water. Temperatures of water in both measured and logged.
Connect up 1 emitter and seal it with epoxy resin to make the electrical connections waterproof.
Put the emitter in 1 jug of water and turn it on.
Measure and log the current drawn and the forward voltage.
Log the temperature increase in the jug with the emitter in it over time, until it reaches equilibrium.
Calculate the power needed to increase 1 litre of water by that amount, with the temperature logged from the other jug as a control ambient temp.
Calculate the power drawn by the emitter over the time period using the logged current and forward Voltage.
From those calculations, measure the power wasted as heat, and therefore the efficiency of the emitter.

Would that work?
I use water and 1 litre, because they are easy to calculate, known constants.

The same test could be repeated at various constant currents, and using different emitters to get a comparison.


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## David_Campen (Dec 5, 2011)

The jugs have no insulation so you would lose a lot of heat to the environment which would be difficult to account for accurately. If you were going to try something like this then the second jug would have to have a resistive heater in it and then you would adjust power to the resistive heater until you found the power level that kept this reference jar at the same temperature as the test jug, this power rate would approximately match the power being put into the test jug.

But I think that the insulated black cylinder apparatus that I described the other day in another thread would be much more accurate.

http://www.candlepowerforums.com/vb...ED-intensity&p=3810764&viewfull=1#post3810764


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## slebans (Dec 5, 2011)

David_Campen said:


> Which LEDs are these? Looking at the datasheet for XT-E Royal Blue, I see about 48% to 55% efficiency.
> 
> http://www.cree.com/products/pdf/XLampXT-E_ROY.pdf



Cree does not sell the Royal Blue XM-Ls used in the production of White XM-L LEDs.

Stephen Lebans


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## David_Campen (Dec 6, 2011)

slebans said:


> Cree does not sell the Royal Blue XM-Ls used in the production of White XM-L LEDs.
> 
> Stephen Lebans



So where did you get the efficiency value?


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## MikeAusC (Dec 6, 2011)

samgab said:


> Actually, the theoretical max for white LEDs might have been incorrect based on this chart:
> 
> 
> 
> ...



The Planckian Locus is for radiators that have the spectral distribution of a Blackbody Radiator - and White LEDs certainly do not have that spectral distribution. So it's invalid to assume that you can derive the luminous efficacy of a white LED by this method.


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## samgab (Dec 6, 2011)

It's from a book about LEDs...

I should explain my point: Yes, the Planckian Locus is a constant, fixed curve.
Yes, it describes a locus in the xy space that is only relevant to black body radiators.
But the OTHER loci, the curved lines that divide up into lm/W, those can be put where ever you like on the xy chart.
And for the purposes of that chart, they are dividing it up into segments that are relevant to LEDs, and where they fall on the xy chart.
The averaged visible light output of a "white" LED can be approximated to a point on that chart.
But I see now why that would be incorrect or misleading, because of the irregular spectral colour curve of a white LED.


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## mux (Dec 6, 2011)

That seems like an uncorrelated remark  The planckian locus and very obviously (because of the 683 lm/W figure) the luminous efficacies plotted in that graph refer exclusively to blackbody radiators. LEDs have spectra that are appreciably different, so they will move differently on the plane with increasing CCT, and both luminous efficacy and radiative efficiency will be different. Because the spectrum of most LEDs is at least reasonably well-behaved, you can get a ballpark estimate, but it's absolutely no good for anything beyond that.

edit: noooo, not exclusively blackbody radiators, those loci refer to _truncated_ BB radiators. Anyway, it doesn't matter, see Kinnza below.


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## Kinnza (Dec 6, 2011)

The other day I was going to link that graph from Schubert's page, but finally I didnt because it may be very misleading. It shows the maximum possible achievable LER for each given chromaticity coordinates x,y on the 1931 CIE color space. Along the almost infinite ways (SPDs) of achieving a given x,y, it points the one which could produce more lm per optical watt. But it dont mean any SPD resulting on those x,y have such LER. Actually, almost always practical SPDs, with at least decent color rendering have way lower LER, because it implies emission on wavebands with lower photopic coefficients (blue and red).

A low pressure sodium lamp, for example, has a LER well over 400 lm/W, but it wont render colors. Blackbody radiator SPD's are perfectly defined and their LERs are way lower than the band indicated in Schubert's graph. For example, perfect BB radiators have these figures:

-2000K-65 lm/W (compare with graph with more than 500lm/W)

-3200K-139 lm/W

-4000K-163 lm/W

-5000K- 174lm/W

-6500K-173 lm/W

-10000K-158lm/W


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## samgab (Dec 6, 2011)

Fair enough. Is it because a white LED is just an approximation of a desired colour temperature by emitting blue or UV light and converting it through phosphor, rather than actually radiating the given colour temperature to start with, as a black body would?

So what would the maximum theoretical lumens per Watt figure be for a white LED if it could hypothetically be 100% efficient?
And how can one work that out?

My thinking was, you take the maximum theoretically possible achievable LER for a given colour temp LED, as Kinnza said that chart gives for LEDs, and take the actual luminous efficacy, and the result of dividing the one with the other would give the efficiency as a percentage. 

Where am I going wrong with that train of thought?
Is it because a certain amount of the light actually emitted is outside the visible spectrum? Or is it because of losses due to the Stokes shift?


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## MikeAusC (Dec 6, 2011)

According to Tom Murphy from UCSD a Blackbody at 5800k puts 93 lm/w counting the total radiation spectrum, or 251 lm/w if you truncate the radiation spectrum measured to 400-700nm.


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## MikeAusC (Dec 6, 2011)

samgab said:


> Fair enough. Is it because a white LED is just an approximation of a given colour temperature by putting out UV light and filtering it through phospher, rather than actually radiating the given colour temperature to start with, as a black body would?
> So what would the maximum theoretical lumens per Watt figure be for a white LED if it could hypothetically be 100% efficient?
> And how can one work that out?



The spectrum from a white phosphor LED is nothing like a blackbody radiator - they have a massive peak of blue light, even after the phoshor, where the eye's sensitivity is only 7% of its peak sensitivity.

The maximum theoretical overall luminous efficacy for a white phosphor LED is estimated to be around 250 to 300 lm/w.


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## samgab (Dec 6, 2011)

MikeAusC said:


> The spectrum from a white phosphor LED is nothing like a blackbody radiator - they have a massive peak of blue light, even after the phoshor, where the eye's sensitivity is only 7% of its peak sensitivity.
> 
> The maximum theoretical overall luminous efficacy for a white phosphor LED is estimated to be around 250 to 300 lm/w.



Cheers for that... Wow, in that case they are much more efficient than I'd previously worked out.
In the T5 XML example @25ºC/700mA/2.9V/2.03W/260Lm/128.1 lm/W, that would be possibly as good as 50% _efficient_?


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## slebans (Dec 6, 2011)

David_Campen said:


> So where did you get the efficiency value?



65% Royal Blue XM-L - 22% Stokes Losses = 50% WPE White XM-L

Stephen Lebans


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## jtr1962 (Dec 6, 2011)

samgab said:


> My thinking was, you take the maximum theoretically possible achievable LER for a given colour temp LED, as Kinnza said that chart gives for LEDs, and take the actual luminous efficacy, and the result of dividing the one with the other would give the efficiency as a percentage.


See here, pages 9 to 11. The theoretical maximum efficacy for LED sources of various color temperatures is discussed. You can in fact get near perfect color rendering ( CRI 98+ ) with an emitted efficacy in excess of 320 lm/W by using 4 or 5 emitters. Practically speaking, if we assume LEDs will eventually peak at about 90% efficiency, that means a very high CRI source of >280 lm/W. If a CRI of 80 is acceptable, then a practical LED light source might one day exceed 330 lm/W. Blue plus phosphor whites will probably never exceed about 250 lm/W due to Stokes losses.


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## MikeAusC (Dec 6, 2011)

samgab said:


> Actually, the theoretical max for white LEDs might have been incorrect based on this chart:
> 
> 
> 
> ...



You can only achieve 683 lm/w for a monochromatic 555nm light - ANY light that appears white must always involve a spectral distribution that achieves less than 683.

I expect that the lm/w numbers shown on the Planckian Locus are for the *theoretical best possible spectra* that produce the range from cold *white *to warm *white *light. 

The maximum shown is 530 lm/w. The predicted best for white phosphor-conversion LEDs is 300 lm/w - not surprising since the spectrum is way different to the most-efficient eye-response curve.


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## MikeAusC (Dec 6, 2011)

David_Campen said:


> So where did you get the efficiency value?



The Cree XT-E Royal Blue emits the same colour as used in Cree's XM-L.

At 350mA they put out between 475 and 500 milliwatts of *Radiant* power (Page 2)

The electrical input is 2.85 x 0.35 = 1.0 watt

So radiant efficiency is 48% to 50%.

There is no radiation at >550nm so all waste energy (52 to 50%) will be escaping as heat *conducted *through the base or *convected *from the surface.


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## MikeAusC (Dec 6, 2011)

jtr1962 said:


> See here, pages 9 to 11. The theoretical maximum efficacy for LED sources of various color temperatures is discussed. . . . .



Interesting that 9 years ago the focus was on using three-colour LEDs rather than phosphor conversion to deliver high powered white LEDs.

The table on page 11 shows that *for three-colour LEDs* with a FWHM line width of 20nm (a very "notchy" spectrum) and a target of *80 CRI*, the best *theoretical * efficacy is 419 lm/w at 2900K, 0r 378 lm/w at 5000K.

Still less than the 510 and 470 shown on the Planckian Locus above for a *best possible* spectrum.


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## Kinnza (Dec 6, 2011)

MikeAusC said:


> According to Tom Murphy from UCSD a Blackbody at 5800k puts 93 lm/w counting the total radiation spectrum, or 251 lm/w if you truncate the radiation spectrum measured to 400-700nm.



LER figure depends of the spectral range you consider, specially for BB radiators which have a very wide spectral emission, with often lots of energy (not light) emitted on IR or UV (depending of temp). The figures I provided are the official ones, calculated for the spectral range 360-830nm following CIE protocol.

If you consider an smaller spectrum, LER figure is higher because it lets out the equation energy which add very little photometric flux.

For that example, I have figures for 5802K BB:

-LER (360-830nm): 174lm/W

-LER (400-700nm): 250 lm/W

-LER (380-780nm, the old "visible" range of CIE): 197 lm/W

If the official protocol is for 360-830 is due it is the wider visible range possible. There is no sense of calculating LER along wider ranges for any application beyond astronomy. Electromagnetic energy emitted outside the visible range is not "light" (by definition, light is the part of the electromagnetic spectrum into the visible range), but it is considered wasted energy for lighting purposes. LEDs have an special characteristic, that they almost dont emit any energy apart of the waveband associated with their bandgap. Just a very small emission in the IR due the operating temp about 350-400K, negligible for any practical purpose. So all the waste energy is generated as heat on the chip, and we can divide input energy in just two parts: light emitted and heat load. For any other light source, energy loss is dissipated by more ways, not only heat (by conduction and convection), but as waste electromagnetic radiation (UV and IR basically).

So except for an application designed to emit on not visible wavelength, LER is calculated between 360 and 830nm. For LEDs, often using narrower ranges changes very little the LER, because the almost not energy emitted outside a narrow range. Just for white LED it makes some difference, but usually small.

Link to formula for calculating BB SPD's.

PS: BTW, I believe the Schubert graph refer to 380-780nm LER, official at that time, but it could be possible it refers to the short 400-700nm, and that could explain the large difference. I need to check that on the referenced text.


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## Kinnza (Dec 6, 2011)

MikeAusC said:


> Interesting that 9 years ago the focus was on using three-colour LEDs rather than phosphor conversion to deliver high powered white LEDs.
> 
> The table on page 11 shows that *for three-colour LEDs* with a FWHM line width of 20nm (a very "notchy" spectrum) and a target of *80 CRI*, the best *theoretical * efficacy is 419 lm/w at 2900K, 0r 378 lm/w at 5000K.
> 
> Still less than the 510 and 470 shown on the Planckian Locus above for a *best possible* spectrum.



The holy grail of LED lighting has been always to get white light by mixing pure color LEDs, not getting it by phosphor mixing. But the "green hole" has prevented it for reality in practical lighting applications.If you check the current roadmaps, you can see how many effort is being done at developing efficient green LEDs and improving amber-orange ones.

What have improved on the last years is the understanding on color appearance and color rendering. Several studies has shown that 3 and, specially, four colored chips can obtain an high quality of light at LER still higher than projected in 2002. The use of the very limited CRI metric prevented to develop other solutions. CRI actually correspond badly to color quality perceived by humans. Since for the old lighting technologies it achieved a decent correlation, at least on rank order of preference (not in the score difference), it worked fine. But LED lighting has shown CRI shortcomings, by giving low scores to spectra than in blind test scores very high and are often preferred by persons to spectums with way higher CRI.

As what limits practical SPDs to obtain high LER values (thus, higher luminous efficacy at same radiant efficiency) are the color rendering constrains, realizing that color quality is a concept way more complex than CRI opens the door to white LED lighting of higher efficacies than we could imagine just some years ago. We just need to close the green gap.


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## David_Campen (Dec 6, 2011)

MikeAusC said:


> The Cree XT-E Royal Blue emits the same colour as used in Cree's XM-L.
> 
> At 350mA they put out between 475 and 500 milliwatts of *Radiant* power (Page 2)
> 
> ...



Yes, that is what I said. I wanted to know where slebans got the value of 65% efficiency


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## samgab (Dec 6, 2011)

Kinnza said:


> ...I believe the Schubert graph refer to 380-830nm LER, official at that time, but it could be possible it refers to the short 400-700nm, and that could explain the large difference. I need to check that on the referenced text.



Here is a link to the chapter that chart was taken from, out of the book  “Light Emitting Diodes, Second edition” by E. F. Schubert (Cambridge University Press, 2006).


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## mux (Dec 7, 2011)

David_Campen said:


> Yes, that is what I said. I wanted to know where slebans got the value of 65% efficiency



If you look at the datasheet, radiant output for temperatures lower than 75C is more than 100% of the quoted value. At 273K, I can imagine 65% to be attainable (the graph seems to be fairly linear over temperature, which is to be expected electrically).


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## BvanderG (Dec 7, 2011)

MikeAusC said:


> I expect that the lm/w numbers shown on the Planckian Locus are for the *theoretical best possible spectra* that produce the range from cold *white *to warm *white *light.


That's right, in fact the calculation of these maximum LERs (done by David MacAdam in 1950) is based on the theoretical optimum spectrum: a mix of _two monochromatic peaks_. For instance, a spectrum of 0.1368 W/m2​ at 445 nm and 0.8632 W/m2​ at 580 nm and zero at all other wavelengths has a color that's exactly on the black-body locus, at 2750 K. Its LER is wonderful, at 516 lm/W. Of course such a spectrum is not very useful due to its horrible CRI, where Ra​ = -38 and R9​ = -500. It is white, but it can hardly be called _lighting_.


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## Dr.Jones (Dec 7, 2011)

I once calculated a cool white XP-G's LER:
I read the XP-G spectrum c(lambda) from the data sheet in 10nm-steps (smaller steps at the blue peak),
took the photopic CIE standard observer V(lambda) somewhere from the internet
and (numerically, trapezoid) calculated this expression:

LER = 683 lm/W * INT( c(lambda) * V(lambda) ) / INT( c(lambda) 

with INT meaning Integral over lambda from 400nm-750nm (the range of the spectrum in the data sheet) 
and INT( c(lambda) ) the normalization factor for that spectrum.

The result was 324 lm/W. Finer spectral resolution would have helped I guess.
mux, how did you get that XM-L spectrum?

Taking fluorescence losses into account (the simple way), it shouldn't be possible to get higher than 261 lm/W with that method and spectrum.

Sometimes I wonder if one should consider other luminosity functions than the standard CIE 2° observer.


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## mux (Dec 8, 2011)

The XM-L datasheet gives a relative SPD. So exactly the same formula as you used. 10nm steps is pretty coarse, I used 1nm steps. Still, the shape is very smooth so it shouldn't matter too much. 

Also, I'd like to comment on the mixing-many-monochromatic-sources. Seeing as phosphorus conversion efficiency is well over 70% for useful CCTs (kind of hand-waving here, but let's say 4500-6500K) it seems to me like it would be an impractical matter _electrically_ to make a light source operating at many different voltages. Because that's what you have to do for multi-color super high efficiency lighting: blue needs 3V, red needs 1.8V, etc. Every color would need its own power supply. I don't think it is possible to have one constant current supply running through all the different colors, because that would mean every color will produce the same amount of photons (in an ideal LED, every electron produces a photon) and the SPD would look like a slanted line from high W/nm in blue to low in red. 

Maybe the number of power supplies can be reduced by bunching a few colors together, but I'm not sure whether that will ever be a practical (or cheap) construction. That's an interesting proposition by the way: if you would want to do that, how much could you bunch colors together before color rendering starts to suffer noticeably?

Also, one question that I haven't really found an answer to on these forums (not sure how much offtopic I can go): I get the general sense that some people are skeptical as to the color rendering quality of LEDs when compared to incandescent. I don't understand this: LEDs seem to be the light source with pretty much the best color rendering properties of all non-incandescents. There are no missing colors in the spectra, the spectrum is very closely matched to the eye's sensitivity and very little is radiated outside the visible spectrum. I understand that there are some teething problems like CCT/chromaticity lotteries, the still-present blue peak and chromaticity shift on different driving levels, but in general - in my opinion - LEDs are or at least will be perfectly fine for any lighting application. Are certain people's objections purely those what I would consider infancy problems, or is there a more fundamental problem that I have yet to recognize?


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## Dr.Jones (Dec 8, 2011)

XM-L spectrum: Sorry, I meant how did you get it in tabulated form for calculation? I'm interested in that data for cool white and neutral white, too...

The best spectrum is more or less flat from 380nm to 750nm, going down a bit at the violet and red; sun's spectrum falls about 20% to the 400nm and 700nm edges. It should not match the eye's sensitivity curve.

The LED spectrum lacks deep violet, cyan and deep red and has a too prominent peak in the blue.


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## mux (Dec 8, 2011)

Oh, I extracted the image from the PDF (nothing fancy, just enlarged it 1:1 as big as I could on my screen, screencaptured->png), cropped it to exactly the inside of the axes and read it using matlab's image manipulation tools and a bit of programming. AFAIK Cree does not have tabulated data on these graphs so you have to do it the bodgy way.

Anyway, how desirable is it to have a light source that emits a lot of light in deep red and violet? Or to put it differently, how objectionable are the shortcomings you name to general lighting applications? Is it just personal preference (see the whole CRI and CCT debate) or, when I start dabbling in more serious DIY SSL projects, should I compensate for this? I've seen a lot of people basically say 'it's not the same as sunlight/incandescent' - but is this just preference or something that can, in practice, hinder lighting conditions and perception?

Maybe I should put this in a different topic, because I'm really hinting at a solid state lighting project here and it doesn't have anything to do with lighting efficacy and efficiency anymore.


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## samgab (Dec 8, 2011)

I think it's akin to the CDs/MP3s vs Vinyl records debate.
CD/MP3 people say that - whilst the signal is cropped, compressed, and converted and deconverted - the bits that are cropped out are the bits that aren't audible to the human ear anyway.
The Vinyl record people say, well, I can hear the difference, whether science says it's out of the range of human hearing or not, the analogue sound is fuller, richer, fatter...
And so the debate rages on.
I think the same sort of thing is happening in the world of LEDs (digital) vs black body light (analogue).
The LED people say, well it's more efficient and lasts longer, and the spectrum is almost as good in the regions where you can actually see it.
The incan people argue that they miss the richness and depth and fullness of the real deal...
And so the debate rages on.


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## mux (Dec 8, 2011)

And as with those debates (although I've been in audiophile debates and have done enough research/debating to have formed my own opinion) it's a bit murky when you first step in: I like to be the science guy and I want to have proof. It's obvious to me that, for example, earlier CFLs simply didn't render some colors, and rendered other colors (the well-known mercury spikes) very differently from 'natural'. Back when I was a very young kid and played with legos, I had some blocks that were very differently colored from others show up as nearly black under my desk CFL. That's what I want to avoid: very practical, real problems with spectral response. 

As with audio, the rest is just our brain. When we get used to it, we don't listen to the sound anymore and just hear the tune. But before I say such things about LED lighting, I might as well check in with people who actually know something about it. I mean, otherwise it's really just religion.


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## Kinnza (Dec 8, 2011)

BvanderG said:


> That's right, in fact the calculation of these maximum LERs (done by David MacAdam in 1950) is based on the theoretical optimum spectrum: a mix of _two monochromatic peaks_. For instance, a spectrum of 0.1368 W/m2​ at 445 nm and 0.8632 W/m2​ at 580 nm and zero at all other wavelengths has a color that's exactly on the black-body locus, at 2750 K. Its LER is wonderful, at 516 lm/W. Of course such a spectrum is not very useful due to its horrible CRI, where Ra​ = -38 and R9​ = -500. It is white, but it can hardly be called _lighting_.



Thanks for the tip. Yep, its a plot of LER obtained by two pure monochromatic peaks (as laser diodes, not as LEDs), so actually it is irrelevant the spectral range considered, as there is no energy considered apart of the peaks. I read the max CRI on the full plot is 34. Nobody would use it for lighting.

However, there are simulations for 3 and 4 peaks which get very high LER and very good color rendering. Start with this one, very recent:

http://www.led-professional.com/tec...ite-light-produced-by-four-color-laser-source

http://www.ledsmagazine.com/news/8/11/2

Both articles based on this study: http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S4-A982

There are several more studies on this topic, many of them involved of the current work at CIE to redefine the color metric (CRI, currently). CRI fails completely on giving an score representing well the color quality when several narrow spikes spectra are analyzed. Still for LEDs with FWHM of 20-30nm it strongly underscore perceived color quality. I have them saved but not the links, I could post tittles so people interested could read them in detail.


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## MikeAusC (Dec 8, 2011)

If we're looking at continuous spectrum rather than three monochromats, what spectral shape will give the highest lumens per radiant watt ?


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## Kinnza (Dec 9, 2011)

Dr.Jones said:


> Sometimes I wonder if one should consider other luminosity functions than the standard CIE 2° observer.



Actually, you should. Standard CIE 2º observer is clearly obsolete and not represent well perceived luminance, specially on the blue and violet range. Often 10º function represent better perception when not performing focusing tasks, and Vm(lambda) is way more accurate than standard 1931 function representing perceived brightness. For anything beyond reverse engineering lm flux, as official lm uses the standard function, Vm function is better, as the own CIE recognizes officially. There are newer functions based on the latest studies which should work better than Vm yet.

There is a clear discrepancy between official lm (based on 1931 function, standard observer) and luminosity actually perceived, larger as larger the blue/violet fraction of SPD considered and larger as shorter wl involved.

I have selected some studies and articles for those who want to go deeper on the topic of color rendering vs LER of the light. Some refers to optimization of LED lighting spectrum in order to get higher LER while getting an higher CRI, but I focused more on the topic of optimizing LER vs light quality, a more complete concept than CRI score. For understanding it, you need some knowledge of what is "light quality" and the different metrics used to measure it, so I selected a few articles about it:

"Color Rendering and Luminous Efficacy of White LED Spectra" by Yoshi Ohno (NIST). Ohno is one of the best experts on the topic discussed here, so it is good to start with an article from him. Excellent starting because article is aimed to optimize white LED spectrums for both multicolor and phosphor converted approaches. Good and complete tables and graph of LER vs CRI (Ra) of several spectra at different CCTs, and compared to conventional light sources. At the end, it shows how Ra is not a good predictor and the problems associated with its use.

"Improving the color spectrum to increase LED efficacy", by Y. Ohno. Similar content than previous, but graphic (powerpoint presentation for a conference). Way easier to follow and more meaningful because the color comparisons and pics, but there are no explanations, found in the article above. You can start with this and then complete it reading previous one. CQS scale (Qa) is already used together with Ra, it is explained on next article.

"Rationale of Color Quality Scale" by Yoshi Ohno and Wendy Davis, NIST. Careful explanation of the shortcomings of CRI and how the new color metric (CQS) try to overcome it. With very good graphic info.

"Lighting with Color and the Color in White" by W. Davis. Similar content, but as ppt presentation. Brief presentation of the alternative new color metrics proposed at CIE.

"Light and Color" by Yiting Zhu (LRC). Wonderfull ppt presentation (in pdf format, as all linked). I thought to start the Tour with this because it explain things from the beggining, as simpler as possible but being very complete and accurate. But as it is more complete and explore more concepts than the previous, it's here. But it is *a must read for anyone interested on understanding light and color*.

"High Quality Color Light Sources". Another excellent presentation about color quality and different metrics. Explain the logic behind the Memory Color Quality Index (MCQI, Sa), an study for its validation which includes an experimental setup and a comparison of the performance of 13 color metrics analyzed for Fidelity/Naturalness and Preference/Attractiveness. Resulting on CRI-GAI best for Fidelity and MCQI for Preference. Only tittle is in Dutch, rest on English.

"Correlation between color quality metric predictions and visual appreciation of light sources, by Kevin Smet, Wouter R. Ryckaert, Michael R. Pointer, Geert Deconinck, and Peter Hanselaer, very good experts from 3 institutions. Previous ppt is based on this article, probably the most complete of all linked in this post.

"Optimal colour quality of LED clusters based on memory colours" by Smet et al. Same authors now perform an study to optimize white LED spectrums for best LER vs Sa. With Sa,it is possible to get better Sa than reference illuminants used on CRI, CQS and other metrics, showing it is possible to obtain good color quality at LER near 450 lm/W and excellent one (best than sunlight and incans) around 400 lm/W.

"Embracing the ColorProperties of SSL", by W. Davis. Brief ppt for finishing the tour. Some concept about color rendering you should never forget.

Enjoy!


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## MikeAusC (Dec 9, 2011)

Kinnza said:


> . . . . . I have selected some studies and articles for those who want to go deeper on the topic of color rendering vs LER of the light. . . .
> 
> "Embracing the ColorProperties of SSL", by W. Davis. Brief ppt for finishing the tour. Some concept about color rendering you should never forget.
> 
> Enjoy!



Thanks for providing these excellent links. The last article and the link you provided previously to the 4-laser illuminant certainly dispelled the myth that spiky spectra are useless for lighting.


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## mux (Dec 10, 2011)

Thanks a bunch for the links, they are exceptionally helpful. Although I am a bit annoyed that light proves to be such a subjective matter that I can't just put numbers to it and be done. Still very confused as to how I can do optimizations for SSL when I have to take perception into account. Without express consensus on how perception actually influences the light's functionality.

For instance, one very important parameter of light to me is my ability to see a sharp image. As you all probably know, the eye just as any other optical system has a focal length and f-number. At some f-number and (corresponding) light intensity level, sharpness is maximal. This differs for scotopic and photopic vision (there are actually two peaks at different f-numbers). The eye _does not_ measure lumens and adjust it's aperture accordingly. The lumen and lux measurements and their relationship to perceived brightness fall apart when comparing different spectra. I'd really like to know how to optimize my lighting for perceived brightness and how to make lighting that allows me to do very fine inspection work (SMD soldering inspection, for instance) best (and to prove somewhat scientifically that it does). At the lowest power consumption.

When I started this thread I thought this could at least be reasonably predicted using some textbook light intensity (lux), CCT and CRI value, but I'm not that sure anymore.


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## slebans (Dec 10, 2011)

mux said:


> When I started this thread I thought this could at least be reasonably predicted using some textbook light intensity (lux), CCT and CRI value, but I'm not that sure anymore.


There are, of course, published guidelines that are task specific, for minimum lux values and corresponding suggested ranges for CCT and CRI. Ultimately, these guidelines are subjective, and like many things in life-one size does not fit all.If I was capable of building my own work light, I would aim for complete flexability. Target at least 50% more flux than the guidelines suggest for the specific task. Use a combination of Cool White XM-Ls and 40% or more efficient Red LEDs. Have the White and Red LEDs on separate dimmable circuits so that you could control both intensity and color temperature. This would give you the capability to determine if there is a specific lux level and color temp that works best for your task.I am currently experimenting with DMX controlled arrays of Cool White and Warm White LED bulbs to basically achieve the same goals(minus the highest possible efficiencies goal).Stephen Lebans


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## Kinnza (Dec 10, 2011)

mux said:


> Thanks a bunch for the links, they are exceptionally helpful. Although I am a bit annoyed that light proves to be such a subjective matter that I can't just put numbers to it and be done. Still very confused as to how I can do optimizations for SSL when I have to take perception into account. Without express consensus on how perception actually influences the light's functionality.
> 
> For instance, one very important parameter of light to me is my ability to see a sharp image. As you all probably know, the eye just as any other optical system has a focal length and f-number. At some f-number and (corresponding) light intensity level, sharpness is maximal. This differs for scotopic and photopic vision (there are actually two peaks at different f-numbers). The eye _does not_ measure lumens and adjust it's aperture accordingly. The lumen and lux measurements and their relationship to perceived brightness fall apart when comparing different spectra. I'd really like to know how to optimize my lighting for perceived brightness and how to make lighting that allows me to do very fine inspection work (SMD soldering inspection, for instance) best (and to prove somewhat scientifically that it does). At the lowest power consumption.
> 
> When I started this thread I thought this could at least be reasonably predicted using some textbook light intensity (lux), CCT and CRI value, but I'm not that sure anymore.



Yep, color perception is so complex that it is almost impossible to build an scientific model taking all parameters into account. Color metrics try to be an average "good enough" score representing perceived light quality, given light source spectrum is just a part of the equation.

But when you have an specific task in mind, it would be possible to develop an index more accurate than general ones. For example, you are looking specifically for discrimination ability and contrast. Exploring the Feeling of Contrast and CRI-GAI metrics could provide results with high correlation with the quality of both characteristics obtained using a given spectrum and luminance. Although most metrics uses a two dimensional color space and normalize luminance for simplicity, you can use a gamut volume index using a 3D space (being luminance the 3th dimension) and obtain figures taking luminance level into account. Math algorithms, more or less accurate, for calculating how luminosity perception varies with luminance have been developed and available.

Metrics explained in my post are candidates for describing quality for general lighting, but there are the analytic tools and models based on psychophysical studies required to obtain more accurate data for predicting what happens when there are specific conditions. 

Optic characteristics of human eye are well studied, so it is possible to model how light reach the photoreceptors. And brain processes after it are subject of intense research and modeling.

But for sure it is not a few days task.


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## mux (Dec 10, 2011)

Apart from (arguably) quantifiable optical characteristics like - let's use photographic terms this time - MTF, there's something else I'm afraid of when I would start doing absolutely ridiculous amounts of science on this topic. Here's a concrete application of the above discussion:

In my new house, I have two areas designed to perform work: the garage (yay) and the 'office' (which I, for social reasons, split with my girlfriend). Let's take the office as a case study.

My desk has two sections, one for my computer and one for my soldering equipment. When I'm at my computer, and in general in that room, I need to have a light level that is comfortable for screen viewing. Necessarily this will be close to my screen brightness, i.e. 100-300 lux. When I'm soldering, I need a light level that enables me to inspect fine-pitched SMD soldering work, which will certainly be something like 1000-5000 lux. Way higher. Well, maybe I'm getting ahead of myself.

The light level at my SMD station would be what I was talking about in my previous post: I'd like to have maximum visual acuity under those circumstances. I have absolutely no data to support this claim, but I suspect that the optimal light level for this work will be waaaaay higher than my room lighting. For instance the figure above.

So that's the theory, but what happens in practice: a 5000 lux workstation in a 100 lux room would be very uncomfortable for me and my girlfriend. Increasing light levels in the entire room would be prohibitively expensive and causes equal discomfort between the office and other rooms in the house. How the hell do you optimize this kind of a situation? It's not just the physical adaptation to go between orders of magnitude different luminance levels, it's also the fact that while I'm doing SMD work, I need to be able to view datasheets on my screen.

Mixing science, practical considerations and psychovisual considerations makes this a big mess. I'm still committed to finding some kind of optimum, but there is a very strong chance that this will necessarily be much more empirical than something I can design in Matlab.

So thanks, Kinnza, for all those articles. They made everything more difficult


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## Kinnza (Dec 10, 2011)

I thought you want to develop a new type of lamp for SMD soldering and inspection. But if you just want to tune your office lighting, it is more an empirical question. On a work place, usually the luminance level suggested is 300lux, up to 500. As you want a comfortable ambient, looking for a in working plane luminance of 300lux is a good target, and using an slightly warm tone, say 3500K (but it would depend of the personal preference of both). Too warm tones arnt advised at work places (reduces productivity) and usually more cool tones are used, but for an office at home, where you want to work but more relaxed, it makes sense to use a neutral or a slightly warm tone. 

Install a general lighting for that purpose, and a working lamp over the SMD soldering area, which mainly lights up that area. A dark color on the soldering table strongly reduces the reflection and increases contrast, so that trick can achieve the way larger luminance level on that part of the room. You dont need many lm, just focusing them well on the soldering area. 

When you design the lighting for a room of known characteristics, you have way more info than when you just work with the light source spectrum and need to build it in a way that works in a variety of scenarios. You can run software simulations including the color of the walls, tables and main objects in the room, so you can find a lighting setup which achieves the desired levels of luminance on different parts of the room. Try Dialux, it is free on the pages of several lighting companies, and at least Osram provides plug in of the ray tracing for the Golden Dragon models. You can know the luminance level at any point/plane of the room in cd/m2 or the illuminance in lux. That if you want to plan it well before making the lamps, but likely you will solve the problem you face with a general lighting setup and an special lighting for the soldering area.

For changing viewpoint from SMD soldering area and screen you need some time of adaptation, thats all (likely, brief). A working lamp dimmable would be very helpful, of course. For better contrast, use a cool tone on the working area, and better an spiked one, which saturates main primaries.


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## bshanahan14rulz (Dec 20, 2011)

Found an interesting dataset. 
http://spreadsheets.google.com/pub?key=rD0zIkgz3u1N28U9kPpl_hg

correlate it to the spectral content of the LED and integrate?


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## Kinnza (Dec 20, 2011)

I uploaded an OpenOffice spreadsheet which calculates it:

https://docs.google.com/open?id=0B7d2cuescDSaNzUwNWNlNTctZDRjNy00OWZhLThjODAtZDhlN2QyODEwOTE2 (5nm version)

https://docs.google.com/open?id=0B7d2cuescDSaNzNhMmJiODgtYWNiNS00ODY3LWE0ZWMtMDMwMzBlYjA3ZjVl (1nm version)

Photopic coefficients are in page "Calc", column G (tittled "1931 y_(λ)"). Sheet calculates efficacy of light and color indexes.

In this page you can get the coefficients of all photopic functions, and some functions themselves. 1931 2º Standard Observer is here: http://cvrl.ioo.ucl.ac.uk/lumindex.htm. One time I found a function able to give the exact coefficients, but it was really complicated so I got the official coefficients (those used on the 5nm version of my sheet were downloaded from CIE page directly).

You can visit too this page with excellent content, easier to navigate: http://www.brucelindbloom.com/index.html?CIESpectralCalcHelp.html


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## PFR84 (Feb 19, 2013)

I've downloaded this spreadsheets and I have no idea what's wrong, if I trying to compare results from this spreadsheet (5nm version) with results from spectrophotocolorimeter I see some discrepancies in CCT and all 15 Ri indexes, however all coordinates (x, y , u', v') are exactly the same. So other values like CQS, GAI could be wrong too. Maybe someone can test this spreadsheet and find what is wrong (see attached files - https://docs.google.com/folder/d/0B1Vu0GZMaW5RWDNzYnZXOWlmVEU/edit?usp=sharing). I found only one error in original sheet CRI_GAI on test point 8 - mismatch in formula.


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## mux (Sep 11, 2014)

Complete necropost on this, but I recently saw the announcement of Cree's XM-L2 (XMLBWT) LEDs and decided to try and find out how much more efficient I could make a lighting fixture with these devices. I got a bit tired of Matlab's slowness, so I decided to rewrite my program as an interactive javascript application. This may be of use to other people on the forum, so feel free to use it:

http://www.enijssen.com/publish/XM-L2 calculator.html

It takes the heatsink surface temperature (i.e. the surface that the LEDs sit on), amount of LEDs in series, CCT and efficacy bin and drive current and calculates, using digitized datasheet graphs, the efficacy and some other stuff.

Of course, suggestions for improvement are welcome. I was kind of hoping it would be possible to build a 200lm/W general purpose LED light with these new devices, but it seems like that's a bit too optimistic. The only LEDs that get near 200lm/W are the cool whites, and those are unfit for GP lighting (and if you include electrical conversion losses and optical losses in the fixture it's damn near impossible anyway).


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## slebans (Sep 11, 2014)

mux said:


> I was kind of hoping it would be possible to build a 200lm/W general purpose LED light with these new devices, but it seems like that's a bit too optimistic. The only LEDs that get near 200lm/W are the cool whites, and those are unfit for GP lighting (and if you include electrical conversion losses and optical losses in the fixture it's damn near impossible anyway).



Hopefully Philips will be able to deliver a product that lives up to their marketing claim of 200 lumen per watt. It is supposed to be released in 2015.
http://www.newscenter.philips.com/m...-200-lumens-per-watt-barrier.wpd#.VBHFWGx0wyU

I was reading an Intematix press release yesterday and it mentioned Royal Blue LEDS are available with a 75% WPE. 
http://www.ledinside.com/news/2014/...ncreased_efficiency_solutions_for_led_systems


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## mux (Sep 11, 2014)

so 75% WPE as in: from AC mains to photons? Only 25% needs to be dissipated as heat TOTAL? 

Combine that with remote phosphors and thermal design becomes so much easier!

In the meantime I really need a replacement for one of those 58W round fluoros that's on almost every day. I think I'll open a new topic on that, because I have some design specific questions about that.


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## WhiteRabbit (Sep 17, 2014)

mux said:


> Of course, suggestions for improvement are welcome. I was kind of hoping it would be possible to build a 200lm/W general purpose LED light with these new devices, but it seems like that's a bit too optimistic. The only LEDs that get near 200lm/W are the cool whites, and those are unfit for GP lighting (and if you include electrical conversion losses and optical losses in the fixture it's damn near impossible anyway).



What about the "warm white" CREE MC-E?


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