# Do lumens add up?



## eyeeatingfish (Feb 12, 2008)

If you point two 100 lumen flashlights at a spot do you then get 200 lumens on target? Sometimes when doing wall bounce tests it seems that if i point one hot spot over the other flashlights hot spot it doesnt really seem to get much brighter. Lumens dont measure the spot though correct?
I know they have flashlights with say 3 crees that are rated 3x a normal cree though.
Is this some sort of trick to my eye?
Maybe i just dont know enough about how light is measured....


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## ttran97 (Feb 12, 2008)

eyeeatingfish said:


> If you point two 100 lumen flashlights at a spot do you then get 200 lumens on target? Sometimes when doing wall bounce tests it seems that if i point one hot spot over the other flashlights hot spot it doesnt really seem to get much brighter. Lumens dont measure the spot though correct?
> I know they have flashlights with say 3 crees that are rated 3x a normal cree though.
> Is this some sort of trick to my eye?
> Maybe i just dont know enough about how light is measured....



I'm no expert...but from reading on here, I think lumen do add up, but your eye won't perceive the difference linearly. Like in your example, two 100 lumen lights would equal 200 lumen...but your eye won't see a 2x increase in light. It takes a lot of lumen to see any actual difference with the human eye.


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## Fird (Feb 12, 2008)

Isn't it logarithmic, just like hearing? double the intensity to perceive a small change..


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## kongfuchicken (Feb 12, 2008)

Yes it is.
Perceived intensity factors by 1 over r squared, r being the distance from your source to the reflecting object; meaning, twice the distance away, the intensity is four times dimmer. Three times further, the intensity falls down by 9.

Of course, that's for point sources (as in without optics/reflector) and that doesn't take in account collimation, loss in medium, eyeball millage factor...


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## joema (Feb 12, 2008)

eyeeatingfish said:


> If you point two 100 lumen flashlights at a spot do you then get 200 lumens on target?...I know they have flashlights with say 3 crees that are rated 3x a normal cree though...Is this some sort of trick to my eye?..


Yes two 100 "torch lumen" lights produce 200 lumens together. It doesn't seem 2x as bright for the same reason a 200 lumen single flashlight doesn't visually appear 2x the output of a 100 lumen light.

Your eyes are logarithmically sensitive, which is different from the inverse square law of diminishing brightness at a distance.

Rather your eyes are similar to your ears: e.g, a 20 watt audio amplifier isn't 2x the loudness of a 10x amplifier.

A 40% lumen increase appears as a very small, almost tiny, visual increase. 

The lesson is don't over-prioritize lumen numbers, even if properly measured.


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## NA8 (Feb 12, 2008)

Some Fenix lights are well designed for rubberbanding. The L2Tv2 is a good size for it. Instead of spending big bucks on a quad emitter, you can buy four L2T's and spread them around or rubberband them together.


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## CM (Feb 12, 2008)

ttran97 said:


> I'm no expert...but from reading on here, I think lumen do add up, but your eye won't perceive the difference linearly. Like in your example, two 100 lumen lights would equal 200 lumen...but your eye won't see a 2x increase in light. It takes a lot of lumen to see any actual difference with the human eye.



What you see and what you perceive are two different things. You WILL see a 100% increase in total light, but it will be PERCEIVED as a moderate increase. That's why I find it funny that people will pay ridiculous premiums to go from one Cree bin to the next when the overall increase is only 10%.


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## Crenshaw (Feb 12, 2008)

I was just wondering my self.If you point a 100lumen light at a spot in the distance, but its just out of reach of the beam ie: you cant see the beam, and you point exactly the same light with the same throw, does the throw of the combined beams increase?

Crenshaw


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## tvodrd (Feb 12, 2008)

Crenshaw said:


> I was just wondering my self.If you point a 100lumen light at a spot in the distance, but its just out of reach of the beam ie: you cant see the beam, and you point exactly the same light with the same throw, does the throw of the combined beams increase?
> 
> Crenshaw



Yes.

Larry


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## enLIGHTenment (Feb 12, 2008)

kongfuchicken said:


> Yes it is.
> Perceived intensity factors by 1 over r squared, r being the distance from your source to the reflecting object; meaning, twice the distance away, the intensity is four times dimmer. Three times further, the intensity falls down by 9.



You just described the inverse square law. It explains how light dims with distance. The OP was asking a different question.

As others have said, lumens (and lux and candela etc etc) are linearly additive but perceived brightness is logarithmic. E.G. adding an additional 500lux to an object already lit by 500lux means that the object will be receiving 1000lux, but the perceived brightness will not double.


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## kongfuchicken (Feb 13, 2008)

enLIGHTenment said:


> You just described the inverse square law. It explains how light dims with distance. The OP was asking a different question.


Correct; I wasn't answering the OP's question directly, but rather the third post.
The OP was answered as early as the second post and I felt it redundant to restate the fact that lumen output values are additive (define irony).



enLIGHTenment said:


> adding an additional 500lux to an object already lit by 500lux means that the object will be receiving 1000lux, but the perceived brightness will not double.



Now, to clear out confusion, it has a lot to do with the definition of perceived brightness.
1000lux is by definition twice as bright (at one point) than 500lux since, after all, it is the unit to measure brightness. However, eyeballs tend to compensate for excessive light and are therefore bad instruments to judge brightness (as well as color temperature and overall output for that matter).

So in short, shining two identical flashlights onto the exact same place from the same distance will double the lumen output as well as the brightness at that spot; however, it might not look like it to the naked eyes, especially if it is brighter than what your eyes are comfortable looking at.


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## elgarak (Feb 13, 2008)

Just to make sure:

Lumen are unrelated to spots. Lumen measure the TOTAL light output -- NOT the amount of light reaching a certain spot. 

The amount of light at a certain spot is measured in lux, which adds up the same way as lumens.


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## jbviau (Feb 13, 2008)

:twothumbs to all of you. I love this kind of thread. 

tvodrd, care to expand on your answer to Crenshaw? I wasn't sure what he was asking. My best guess: Do 2 identical lights together throw better than just one? If so, then I guess sheer output can compensate for poor throw to some extent. To what extent exactly? Is a 2x brighter light just as good a thrower as a light with a 2x deeper/bigger reflector?

If this doesn't make sense, I'm sure someone will let me know...


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## p1fiend (Feb 13, 2008)

May not be the best place to ask this question (I don't mean to steal the thread) but:

I have noticed it is easier to see the difference in brightness between 50 and 100 lumens (light) than between 100 and 150 lumens.

It seem obvious that it would be easier to notice a difference between very little light and moderrate light, and harder to notice the difference between moderate light and high light, but can this be explained "scientifically".

Do the eyes' perception of light plato at a certain output? Is it non-linear? Am I even making any sense?


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## mudman cj (Feb 13, 2008)

The reason for the noticeable difference from 50 to 100 compared to 100 to 150 is because of the geometrical progression of perceived intensity vs. light intensity (lux). Since the two are not linearly related, 50 lumens at the low end of the scale is perceived as much more of an increase than at the high end.

Perception of light intensity follows Stevens' Power Law with an exponent that depends upon the amount of your field of view occupied by the light. For a 5 degree spot the exponent is about 0.33 but for a point source it is about 0.5. This means that for a 5 degree spot the source needs to increase by a factor of 8 to seem twice as bright and a point source needs to increase by a factor of 4 to seem twice as bright.

In the case of your examples for a 5 degree spot: (100/50)^.33=1.257 for a perceived increase of 25.7% compared to (150/100)^.33=1.143: a perceived increase of 14.3%. 
And for a point source: (100/50)^.5=1.414 for a 41.4% increase compared to (150/100)^.5=1.225: a 22.5% increase.


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## tvodrd (Feb 13, 2008)

jbviau said:


> :twothumbs to all of you. I love this kind of thread.
> 
> tvodrd, care to expand on your answer to Crenshaw? I wasn't sure what he was asking. My best guess: Do 2 identical lights together throw better than just one? If so, then I guess sheer output can compensate for poor throw to some extent. To what extent exactly? Is a 2x brighter light just as good a thrower as a light with a 2x deeper/bigger reflector?
> 
> If this doesn't make sense, I'm sure someone will let me know...



Emitting twice the photons by adding a second identical beam will put twice the photons on the target, and might make the difference in being able to discern the target from the return to your eyeballs. And yes, sheer output can render beam profile non sequeter with respect to "throw!" Some of the 250W incans are jaw-droppers!

Larry


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## Hitthespot (Feb 13, 2008)

Crenshaw said:


> I was just wondering my self.If you point a 100lumen light at a spot in the distance, but its just out of reach of the beam ie: you cant see the beam,
> 
> Crenshaw


 
Your just looking in the wrong direction. Walk out to where the beam stops and look back at the flashlight. You will see the beam just fine.


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## 2xTrinity (Feb 14, 2008)

> If you point two 100 lumen flashlights at a spot do you then get 200 lumens on target? Sometimes when doing wall bounce tests it seems that if i point one hot spot over the other flashlights hot spot it doesnt really seem to get much brighter. Lumens dont measure the spot though correct?
> I know they have flashlights with say 3 crees that are rated 3x a normal cree though.
> Is this some sort of trick to my eye?
> Maybe i just dont know enough about how light is measured....


First off, for clarity I'll give the definition of relevant units:

Lumens are the total power _output_ (not input/battery draw) of the light, weighted by how sensitive your eyes are to the color of the light. A UV light for example would produce zero lumens no matter what since it is invisible.

Candela/Candlepower is a measure of how concentrated your light is, so it's your lumens output divided by the beam angle. Narrower beam angles mean higher candela.

Lux, or lumens/square-meter is a measure of how much light falls onto a surface. This is the easiest way to measure light output using a simple meter, so it's often referred to on the forums. Our eyes have a logarithmic response to lux -- that is why it's possible for our eyes to work in such different situations as under moonlight (less than 1/10th lux), indoor lighting (100s of lux) or direct sunlight (as much as 100,000 lux). 

In the case you talked about, looking at the bright hotspot on the walls, there's another factor that comes into play. Because of the bright hotspot, your pupils will be likely to contract, limiting the amount of light they'll allow in. This is why it's not practical to use a concentrated light meant for throw to work up close. If you were to point two diffused lights at a wall instead, it would be easier to discern a difference.


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## nikon (Feb 15, 2008)

Crenshaw said:


> I was just wondering my self.If you point a 100lumen light at a spot in the distance, but its just out of reach of the beam ie: you cant see the beam, and you point exactly the same light with the same throw, does the throw of the combined beams increase?
> 
> Crenshaw


No. There may be a misconception regarding the word "throw". Assume you have a flashlight with low, medium, and high settings. While shining the light at a distant object and switching from high to medium, you may think your light is now not able to throw as far. On low, it appears to throw less far than either of the other settings. In actuality, your light is throwing equally far on all three settings.

Why? Because all photons travel equally as far as all others. Throw is a perception, not a reality. In order for us to perceive throw, we have to hit an object with a sufficient number of photons to be able to see them as light when they reflect off the object and back to our eyes. 

Sorry, I'm a Capricorn and therefore a stickler for details.


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## srvctec (Feb 15, 2008)

nikon said:


> No. There may be a misconception regarding the word "throw". Assume you have a flashlight with low, medium, and high settings. While shining the light at a distant object and switching from high to medium, you may think your light is now not able to throw as far. On low, it appears to throw less far than either of the other settings. In actuality, your light is throwing equally far on all three settings.
> 
> Why? Because all photons travel equally as far as all others. Throw is a perception, not a reality. In order for us to perceive throw, we have to hit an object with a sufficient number of photons to be able to see them as light when they reflect off the object and back to our eyes.
> 
> Sorry, I'm a Capricorn and therefore a stickler for details.



So what's being a Capricorn got to do with it- I'm a Gemini and a real stickler for details too.  :laughing:

OK, let me play the devil's advocate here. Following the above logic, if a person were to stand at the point where the above mentioned distant object is and the same low, medium, and high light (let's just say for the sake of argument the light's modes are low- 2 lms, medium- 40 lms and high- 130 lms) was shined at them, they should be able to see the light in all 3 modes since "all photons travel equally as far as all others", correct? If the person were to travel out to the point where they could only just barely see the light on high, they should also be able to see it on medium and low following the above logic. I don't see how this would be possible, since if the light is only barely visible on high, low wouldn't even be remotely visible- to the human eye.

But I suspect I already know the answer (or at least I can guess) to this. I'm guessing the photons travel that same distance in all modes, it's just the limitation of the human eye keeping the person from seeing the light on low.

I love discussions like this, since I get to learn stuff like this. CPF is a fantastic place!!


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## Jimmy H (Feb 15, 2008)

The Inverse Square Law is at work here. 

For a given amount of light, the illumination on a object, and therefore its brightness, is proportional to the inverse of its distance squared.

An object twice as far away appears 1/4 as bright. 

In terms of "throw" you need 4 times the output to make an object twice as far away appear to be the same brightness. 

You can achieve this with a source that has 4X lumens, or a reflector that concentrates the light to put 4X of the available lumens into a tighter beam.

Of course, the Inverse Square law works for light coming back at you, so lights look dimmer the farther away they are.


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