# Layman's Terms: Efficacy vs. Efficiency...?



## LEDAdd1ct (Jan 26, 2013)

I searched CPF first and found a few threads that touch on the subject, but nothing that easily came up in a search. Part of the purpose of starting new threads is not simply to satisfy one's own desire for information, but to provide an easily searchable database for others.

In that spirit, can someone please answer the following questions in layman's terms?

The best I can find is that "efficacy" means lumens that come out per watt. So, if 100 watt incandescent light bulbs generally produce 1600 lumens, then the efficacy is 16 lumens for each watt of power the bulb receives:

1 watt in yields 16 lumens; 
100 watts in yields 1600 lumens

But what does efficiency mean, then? I am not an electronics expert, so please keep it simple. I trust the ability of those on CPF to put things in simple language much more than the usual data Google churns up.

1) What is efficacy?
2) What is efficiency?
3) What is the key distinction between the two?

Thank you!

(NOTE: Started the thread here because it is neither a "Flashlight Electronics" question nor is it strictly an "Incandesent" or "LED" question. It is a question about lumens and lights in general, and would apply to any lighting technology or system that uses power to spit out lumens.)


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## brickbat (Jan 26, 2013)

The distinction is that efficiency is unitless. By that, it needs to be a ratio of one thing to another, where both things can be represented by the same unit. And thus efficiency is just a number with no units. For example, the efficiency of a car's transmission might be thought of as horsepower out/horsepower in. Maybe you put 100 HP in, and get only 95 HP out. The efficiency is thus 0.95, or 95%. There are no units - it's just a number.

Or think of a cow - maybe you put in 50 pounds of food and water and get out 24 pounds of milk. Again the input and output are in the same units (pounds) so you could calculate the efficiency of a cow to be 48%.

Contrast that with the lumens/Watt idea. Here the output is Lumens, which doesn't convert to power in any exact way. Watts OTOH represent power. So, there is no way to eliminate the lumens/Watt bit. It'll never be just a number.

A lot of people might not know the distinction, and maybe think that efficacy is just a hoity toity way of saying efficiency, but there is a difference...


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## jtr1962 (Jan 26, 2013)

Efficacy for a light source is how well it turns input power into the desired output, which is lumens. Efficiency is the actual percentage of power in which comes out as photons. The only way to calculate efficiency if you know the lumens per watt is to look at the spectrum of the light source in question. A typical cool white LED spectrum with medium CRI will have an spectral luminous efficacy of perhaps 330 lm/W. Therefore, it you have an LED which is 100 lm/W, it will have an efficiency of 100/330, or 30.3%. For warm whites and higher CRIs, the spectral luminous efficacy is less, perhaps even as low as 230 lm/W for high CRI, low CCT. A warm white, high CRI which is 100 lm/W will therefore actually be more efficient in terms of light energy out versus power in than a 100 lm/W cool white, lower CRI LED. Both however will produce the same amount of visible light per watt, just the warm white LED will make less waste heat. An incandescent light bulb only has a spectral luminous efficacy of around 200 lm/W because it emits all visible wavelengths, even deep reds and blues to which the eye isn't very sensitive. Here is some more reading on the subject. Note that the paper concludes that acceptable white light sources can have spectral luminous efficacies of around 250 lm/W up to about 370 lm/W (I've seen 400 lm/W elsewhere). Those numbers represent upper limits. A practical light source, even LED, is probably unlikely to achieve more than about 80 or 85% of that. I also find it interesting that you can truncate the spectrum and still get very good CRI numbers (see figures 5 and 6 on pages 3-4). For example, you can cut off everything over 650 nm and under 420 nm, yet still achieve a CRI of 95.


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## LEDAdd1ct (Jan 27, 2013)

Okay, forgive me, let me give it a shot:

1) Luminous Efficacy means power in vs. lumens out, and that is it.

2) Efficiency is a general term, but when we use it to speak about LEDs, we can say "an S2 bin is this much more efficient than an R5 bin" or "the Cree XM-L2 T6 is more efficient than the Cree XR-E Q5," with the key point being that it is without units.

3) Efficiency means "How good is it at turning watts into lumens?" or "what percentage of the power that goes in comes out as light."

Is that right?

I realize I am restating, but it is how I learn (and from observation, how many others learn as well).

A lot of this stuff is way over my head, so I am trying to gain a basic understanding.


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## BigRiz (Jan 27, 2013)

I'm no expert (so correct me if I'm wrong), but I would summarise the difference as:

Efficacy is the rating given in lumens per watt

Efficiency is stated in percentage - based on the ratio of total energy in vs useful energy out - so if an LED is 30% efficient and it's consuming 30W, it's outputting 3W of light (useful energy) and 7W of heat (wasted energy).


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## LEDAdd1ct (Jan 27, 2013)

I think that makes sense, too.


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## brickbat (Jan 27, 2013)

BigRiz said:


> Efficacy is the rating given in lumens per watt



Exactly.



> Efficiency is stated in percentage - based on the ratio of total energy in vs useful energy out -



Yep. It's useful output divided by total input.




> so if an LED is 30% efficient and it's consuming 30W,



I think you meant consuming 10W rather than 30W?



> it's outputting 3W of light (useful energy) and 7W of heat (wasted energy).



Here's the tricky part. It's the 3W of useful light. And the assumed fact that a human is the one 'using' the light.

The thing is human's eyes vary in their ability to see different wavelengths of light. So, if we are talking about a 'white' LED, there are an infinite number of ways to get that 'white' color. And a very large pool of humans that might be observing that light.

So, there are curves of the 'typical' human eye response, and this forms the basis of the lumen - it's been designed to measure light way a typical human might see it. But as you can imagine, it's inexact. So IMHO references to 'efficiency' of white light sources are a bit inexact, and would need to be carefully stated with lots of footnotes and asterisks.

IOW, to an engineer interested in technical detail, an 'efficiency' rating of a white LED raises more questions than it answers...


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## Beniejeanly (Aug 26, 2013)

The efficiency of a car's transmission might be thought of as horsepower horsepower in.




____________________________________________
Every body is a swimsuits uk body.


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## Anders Hoveland (Sep 17, 2013)

jtr1962 said:


> Efficacy for a light source is how well it turns input power into the desired output, which is lumens. Efficiency is the actual percentage of power in which comes out as photons.


 So a 360nm red LED chip and a 335nm red LED chip may have the same efficiency, but the orangish-red 335nm LED chip has a higher _efficacy_ because the human eye is less sensitive to the deeper red color?


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## Bruce1957 (Jul 28, 2015)

Luminous efficacy is a ratio of lumens of output per watts of input.
In the above example where 100 watts yields 1600 lumens the luminous efficacy is 1600/100 or 16.

Luminous efficiency is a measure of the 'true' efficiency of a bulb.
By definition in the SI (metric system) 1 watt = 683 lumens.
Scaling luminous efficacy by 1/683 yields luminous efficiency.

Therefore the luminous efficiency (or true efficiency) of the 1600 lumen 100 watt bulb is 16/683 or about 2.34%.
2.34% is not very efficient at all - most likely an incandescent bulb!


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## brickbat (Jul 28, 2015)

Bruce1957 said:


> ... the luminous efficacy is 1600/100 or 16.



Well, we're only giving you half credit on that answer. You left off the units. Efficacy, unlike efficiency, needs units. And in this case, you left off the lumens/W part...

The answer is 16 lumens/W.

If we're going to split hares, lets do it right


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## broadgage (Jul 30, 2015)

Or perhaps we should split hairs !


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## tab665 (Jul 30, 2015)

dude, i never even knew they were two different words! never noticed the spelling


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## SemiMan (Aug 1, 2015)

Bruce1957 said:


> Luminous efficacy is a ratio of lumens of output per watts of input.
> In the above example where 100 watts yields 1600 lumens the luminous efficacy is 1600/100 or 16.
> 
> Luminous efficiency is a measure of the 'true' efficiency of a bulb.
> ...



Some of this is right ... Lots of missing details and the devil is in details.

Efficacy is lumens/watt but that watt can be either input power or output power. Both are used. The former...output power is the spectral efficiency of the output for visible means. The latter for a component level.

A watt is not defined as 683 lumens its defined as 683 lumens at a very specific wavelength.

Even discussing luminous efficiency you need to clarify. Radiant luminous efficiency is simply radiant energy in the visible band over total energy. Keep in mind that definition is from 1910 or before ... Ie PW ... Pre-wikipedia. 

The definition of luminous efficiency you stated is but one that happens to show up on Wikipedia now .... And even recognizes as but one definition. Websters defines differently (as do many other sources).

Efficacy is most useful just define spectral or source efficacy.

Semiman


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## greenlight (Aug 1, 2015)

Great concepts and also be careful splitting heirs.


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## brickbat (Aug 1, 2015)

greenlight said:


> Great concepts and also be careful splitting heirs.



...and I thought splitting hares was messy


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## en2oh (Aug 8, 2015)

brickbat said:


> Well, we're only giving you half credit on that answer. You left off the units. Efficacy, unlike efficiency, needs units. And in this case, you left off the lumens/W part...
> 
> The answer is 16 lumens/W.
> 
> If we're going to split hares, lets do it right



I think one important point that is missing in the number is the actual _meaning_ of that number.
While efficiency is dimensionless (no units), efficacy does have units associated with it but that really doesn't _define_ what efficacy is.

In healthcare, efficacy considers outcome, which may be measured in a variety of ways. 
I found the definition of Luminous Efficacy is the following:

*Luminous efficacy* is a measure of how well a light source produces visible light. It is the ratio of *luminous* flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source.

More specifically: 

The light sensitivity of the normal human eye has been studied extensively. *The response of the eye as a function of frequency is called the luminous efficacy of the eye*. It has been tabulated for both the light-adapted (photopic) case and the dark-adapted (scotopic) case.

So, a lamp which outputs more light in the green range of the spectrum is going to be perceived more intensely than one that produces the majority of it's output in the red range of the spectrum. That's why a green laser pointer of 1mw intensity will be perceived as significantly "brighter" than a 1mw red laser pointer. In the context of "visible efficacy", the green pointer would have a higher number than the red pointer.

I think "efficacy" must consider spectrum (frequency) as well as simple "lumins/watt" and what is actually involved in "perceiving" this output.

At least that's how I'd look at this question. Of course each discipline has it's own nomenclature. It isn't uncommon for something to be defined using the same terms but mean completely or nearly completely different things.

I just found this thread interesting in that the definition and the Units for that value we being used interchangeably. Commonly done, but the units can't be the definition. (although they certainly can be in the definition).

Anyway, that's my take on this.
Doug


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## SemiMan (Aug 8, 2015)

Lumens by definition takes into account spectrum


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## PointyOintment (Aug 11, 2015)

It's 2015…

Here you go:
https://en.wikipedia.org/wiki/Luminous_efficacy (particularly the "efficacy and efficiency" section)


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## SemiMan (Aug 11, 2015)

PointyOintment said:


> It's 2015…
> 
> Here you go:
> https://en.wikipedia.org/wiki/Luminous_efficacy (particularly the "efficacy and efficiency" section)



... except you will notice this page is a bit vague and/or contradicts itself a bit. Some would argue things it says are not what the industry follows. 

I once saw a Wikipedia page that was blank. There was a note, "This Wikipedia page deleted, thereby greatly increasing it's accuracy".


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