# Is it possible to verify the claimed lumens using a lux meter?



## pwangdel (Oct 6, 2009)

Greetings:

Supposedly you bought a flashlight claiming 200 lumens. Is there a way to verify this claim approximately using a lux meter?

Thanks.


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## LuxLuthor (Oct 7, 2009)

Not as easily as you would think, unless you want to "ballpark" it. 

Technically the definition of lumens is very precisely defined, and to avoid all the variables that can affect the "lumen rating" there should be an integrating sphere, calibration of the photometer, proper scale used which varies by light source (LED, Incandescent, HID, Laser, etc.)

I posted some links and issues to consider recently in this thread.


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## pwangdel (Oct 7, 2009)

Thank you for the info.

Yes, I just want to "ballpark" it. How much variation could there be, +/-30%?

If you point a supposedly 200 lumens flashlight focus to a circle diameter at a lux meter at one meter distance. How many lux should I be expected approximately? In other word, is there any reference chart which gives you a "ballpark conversion" between lumens and lux?


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## Marduke (Oct 7, 2009)

There is no direct correlation between lux and lumens, as they measure two seperate things.


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## HKJ (Oct 7, 2009)

pwangdel said:


> Greetings:
> 
> Supposedly you bought a flashlight claiming 200 lumens. Is there a way to verify this claim approximately using a lux meter?
> 
> Thanks.



It is possible, but a lot of work. You simple need to sum lux*area, i.e. place the flashlight where all the light from it hits a plain surface, make a grid, measure each cell in the grid and sum it weighted by area. Remember that each cell must have uniform brightness for this to work.

To optimized the measurements, use the symmetry in the beam and instead of a grid uses concentric circles, then you only have to measure points on a line (But must do some more calculations).


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## gswitter (Oct 7, 2009)

pwangdel said:


> If you point a supposedly 200 lumens flashlight focus to a circle diameter at a lux meter at one meter distance. How many lux should I be expected approximately? In other word, is there any reference chart which gives you a "ballpark conversion" between lumens and lux?


You can't correlate lux meter samples with lumens using this method.

Re-read what LuxLuthor wrote and referred to above.


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## kramer5150 (Oct 7, 2009)

I have seen some people try... and I have always wondered how legit it is.

Their lux meter has a pretty big collecting area... like a 2-3 inch diameter plate.

They place the light bezel down on the plate, take a LUX measurement and compare light A and light B.

Am I right that this is still a flawed method, because the LUX meter still only measures the spot intensity... not the total emitted light.


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## qwertyydude (Oct 7, 2009)

The only reasonably easy way is to make yourself a lightbox perhaps the milk carton version then put in a couple known lumen light sources to calibrate then you'll have a better guesstimate of the lumens.


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## Bullzeyebill (Oct 7, 2009)

qwertyydude said:


> The only reasonably easy way is to make yourself a lightbox perhaps the milk carton version then put in a couple known lumen light sources to calibrate then you'll have a better guesstimate of the lumens.



Here is a link to Quickbeams lightbox setup for measuring approximate lumen values. http://flashlightreviews.com/features/lightbox_output.htm

Other members use a similar setup, selfbuilt being one. He also co-relates his lightbox figures with bounce with lightmeter. Do a search for selfbuilt and his reviews

Bill


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## LuxLuthor (Oct 7, 2009)

Bullzeyebill said:


> Here is a link to Quickbeams lightbox setup for measuring approximate lumen values. http://flashlightreviews.com/features/lightbox_output.htm
> 
> Other members use a similar setup, selfbuilt being one. He also co-relates his lightbox figures with bounce with lightmeter. Do a search for selfbuilt and his reviews
> 
> Bill



Nothing wrong with some of these "ballpark" constructions, but even under the best of circumstances they should be seen as comparing one light relative to another, rather than a "real" measurement.

Also recognize that in the beginning of the linked source (upon which a homemade light box was constructed), almost all of his introductory statements are completely wrong.


> *[FONT=Arial, Helvetica, sans-serif]Lux is the measurement of the maximum output from the brightest part of the beam (the center).  (False)
> [/FONT]*
> *[FONT=Arial, Helvetica, sans-serif]
> [/FONT]*
> ...




​To his credit, he does clearly state


> *"[FONT=Arial, Helvetica, sans-serif]this little apparatus [is] to measure RELATIVE TOTAL OUTPUT from a light (NOT LUMENS!!!)" [/FONT]*


*[FONT=Arial, Helvetica, sans-serif][/FONT]*


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## Bullzeyebill (Oct 7, 2009)

LuxLuthor said:


> Nothing wrong with some of these "ballpark" constructions, but even under the best of circumstances they should be seen as comparing one light relative to another, rather than a "real" measurement.
> 
> Also recognize that in the beginning of the linked source (upon which a homemade light box was constructed), almost all of his introductory statements are completely wrong.
> 
> ...



Yes. Quite right, just for comparative info, but still good info. Results are useful to me, and I find selfbuilts info very useful. Quickbeam was able to adjust his little formula for light output after this CPF pass around several years ago. https://www.candlepowerforums.com/threads/94232 LSI did some lumen testing on the pass around lights, also lux measurements. I am also aware that different labs can have different results.

Bill


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## csshih (Oct 7, 2009)

I believe MrGman combines a home made integrating sphere with a luxmeter to calculate lumen values?


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## pwangdel (Oct 7, 2009)

Thank you all for your additional comments and this useful link. This homebrew box should be good enough for me.



Bullzeyebill said:


> Here is a link to Quickbeams lightbox setup for measuring approximate lumen values. http://flashlightreviews.com/features/lightbox_output.htm
> 
> Other members use a similar setup, selfbuilt being one. He also co-relates his lightbox figures with bounce with lightmeter. Do a search for selfbuilt and his reviews
> 
> Bill


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## PhantomPhoton (Oct 8, 2009)

csshih said:


> I believe MrGman combines a home made integrating sphere with a luxmeter to calculate lumen values?



No I believe he generously donates his time to use a properly calibrated IS in a lab at his place of work.


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## Bullzeyebill (Oct 8, 2009)

PhantomPhoton said:


> No I believe he generously donates his time to use a properly calibrated IS in a lab at his place of work.



MrGman's home made IS resides with another one of our members now. See his IS thread. The first post in that thread shows IS results from the lab, and the third post shows results from his personal IS.

Bill


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## asdalton (Oct 8, 2009)

LuxLuthor said:


> Also recognize that in the beginning of the linked source (upon which a homemade light box was constructed), almost all of his introductory statements are completely wrong.



I have to disagree with several of your "false" judgments (4, 5, 6). Care to offer an explanation of why you think that they are false?


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## joema (Oct 8, 2009)

pwangdel said:


> ....Supposedly you bought a flashlight claiming 200 lumens. Is there a way to verify this claim approximately using a lux meter?...


Most flashlight ads which _mention_ lumens don't claim the light produces that number of out-the-front lumens.

Even if you could accurately measure out-the-front lumens, you'd be complaining about something most mfgs never claimed in the first place.

It's similar to automotive ads which mention horsepower. It's almost always crankshaft horsepower, not measured at the wheels. If you dyno a car and it doesn't produce the "claimed" hp, the mfg would say they never claimed it would, you misunderstood the number.

Another issue: hp or lumens are typically not a guarantee, but a spec -- unless otherwise stated. It's generally an average number which the entire production population as a group will meet, not a pass/fail criteria for every unit off the assembly line.

A very few flashlight mfgs individually calibrate and test each unit to meet an out-the-front lumen number (e.g, HDS/Ra). You cannot assume all other mfgs do that.


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## LuxLuthor (Oct 8, 2009)

asdalton said:


> I have to disagree with several of your "false" judgments (4, 5, 6). Care to offer an explanation of why you think that they are false?



Follow my link in post #2 of this thread. There are actual specific definitions of these terms and how to measure them.


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## asdalton (Oct 8, 2009)

Well, you have to distinguish between the formal definition and the truth. Something can be _not the strict definition_ without being _false_. I got the impression that Quickbeam was giving an informal description, rather than claiming to give the definitions.


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## LuxLuthor (Oct 8, 2009)

asdalton said:


> Well, you have to distinguish between the formal definition and the truth. Something can be _not the strict definition_ without being _false_. I got the impression that Quickbeam was giving an informal description, rather than claiming to give the definitions.



I'm not sure what you mean by distinguishing between the formal definition and the truth. Either you use a term as it is intended or you don't. Just to be clear, I am actually saying that those Quickbeam statements are formally, factually, conceptionally, and simplistically false. If you are seeing his statements as "not the strict definition," or somehow "true," then you need to re-read his statements and my linked post (which contains further, linked, authoritative sources).

The main purpose of my post was to point out that very few people who use the various terms to describe light actually understand what the terms mean and how they are defined. They will then "bend" one or more of the terms to meet what they want to say, and just like you, people will be left with an incorrect understanding from reading a site like that. Those repeated inaccurate uses of the various terms eventually creates signficant mischief and misrepresentation and claims...for example on how a particular flashlight performs.

The last two statements I quoted from Quickbeam's site are apparently trying to use/discredit/qualify Lumens/Lux with regards to their validity in evaluating a light's advertised lumen rating vs. a peak hotspot of a flashlight. There is nothing remotely correct about distinguishing between lumens and lux even for that simplified concept. Those terms have nothing to do with the issue of a hotspot, since lumen refers to a point light source of one candela falling on a one square foot area, and lux is one lumen falling on one square meter area. Period.

Quickbeam's representation of Lux & Lumen in those statements is not remotely accurate....not even in a simplistic explanation. It may be the case that he actually does know how to use the terms correctly, and fully understands the necessary concepts....but such an understanding is not represented with those statements, and a number of other statements on his website. 

His site is then seen as an authoritative source, and when people like you read it, they feel they accurately understand about light measurement terms and use his statements as a foundational basis. That is what I am challenging.


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## asdalton (Oct 8, 2009)

I'm sorry, but I do think that you are being pedantic at the expense of understanding reality. And the fact that you are being so snarky and insulting about it is making me _less_ likely to accept your assertions.

For example, lumens are a measure of the total amount of light (radiant energy weighted by the sensitivity of the human eye) emitted by an object. That's the _whole point_ of having such a unit. To say that such a description is "false" is, well, false.


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## Marduke (Oct 8, 2009)

But he is correct....

While Lux is not _technically_ the MAXIMUM point of a beam (it can be measured anywhere), CPF as a whole, and the industry, uses it to measure the center hotspot of a beam as a means of describing the throw.

Lumens are a measure of overall light output (calibrated to the human visual response). That's the entire purpose of an integrating sphere.

Two lights can be the same overall brightness (lumens) by have vastly different beam characteristics, giving it different lux numbers for the center hotspot.

Lux (when given typically as a single value of the hotspot) cannot tell you the overall brightness of a beam. A 10 lumen and 1000 lumen light can both have identical lux values at the center of the beam given the right optical conditions.


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## LuxLuthor (Oct 9, 2009)

asdalton said:


> I'm sorry, but I do think that you are being pedantic at the expense of understanding reality. And the fact that you are being so snarky and insulting about it is making me _less_ likely to accept your assertions.
> 
> For example, lumens are a measure of the total amount of light (radiant energy weighted by the sensitivity of the human eye) emitted by an object. That's the _whole point_ of having such a unit. To say that such a description is "false" is, well, false.



Seeing a factual discussion as being pedantic, snarky, or insulting is in the eye of the beholder. No such inferences were made or intended, but knock yourself out.

Quickbeam's statement: "*[FONT=Arial, Helvetica, sans-serif]Lumens is the measurement of the overall output." [/FONT]*[FONT=Arial, Helvetica, sans-serif]is false because he does not qualify it as you did in terms of lumens ONLY applying to the subset of the overall visible output (radiant energy) weighted to the sensitivity of the human eye which is why the lumen is based upon the candela. 

With his definition, the entire "overall output" of the sun would have to fit into his description of lumens, despite the fact that there are vast amounts of solar light radiation falling outside of the human eye's visible range. 

This is a relevant issue because of the spectral distribution of various light sources, and specific photometric sensors required to accurately test for them (i.e. LED vs Incand vs laser) vary widely.[/FONT] Extreme examples are <370nm UV emitting LED's being expressed in Lumens as a frequency invisible to the human eye. Quickbeam used a self described "cheap" light meter that is not likely calibrated nor the proper sensor/profile setup to be used for LED.

Likewise, his statement "*[FONT=Arial, Helvetica, sans-serif]Lumens is all of the light measured no matter where it goes." [/FONT]*[FONT=Arial, Helvetica, sans-serif]again ignores the fact of light existing on a wide electromagnetic spectrum. "Lumen" references a selected portion of that spectrum. The specific frequencies (colors) of light are then given relative fractional values as compared to the most visible green color (555nm) to the human eye. "Lumen" is not all of the light measured.

Furthermore, it is not valid to say "no matter where it goes," since this would include reflected, refracted, focussed, and other changes to the output angle that is specified (steradian) in what the actual definition of a lumen (based upon a candela) really means.
[/FONT]



Marduke said:


> But he is correct....
> 
> While Lux is not _technically_ the MAXIMUM point of a beam (it can be measured anywhere), CPF as a whole, and the industry, uses it to measure the center hotspot of a beam as a means of describing the throw.



Lux as a unit is actually defined. As you say, LUX MEASUREMENTS can be taken at various distances, beam locations, and sensor angles incident to the light source. There are many warnings given in reputable photometric sources (see my link in post #2, but here is another resource) to not take measurement without controlling or eliminating the variables introduced by using reflectors, refraction from lenses, ambient light, and other interfering items. 

The readings taken on Quickbeam's site _(which he no longer controls since selling that domain)_ use lights with all kinds of variable differences. He does give numerous warnings of how limited his results are, and how various testing methods & calculations changed mid-stream, and the importance of looking at beamshots.

Except in those regulated lights, I'm not even sure if he controlled for maximum supplied (battery) voltage/amperage in all other cases. He has posted about scale revisions, and other appropriate ballpark estimate 'disclaimers' on this page (to his credit)....but people still use his tests like it is real & objective data.

There is no standardization of a specification that I have seen to evaluate a flashlight's "throw." Nor have I seen any objective source describing reliable parameters for evaluating a hotspot (created by a reflector, lens, and/or light source shape, etc.). It would need to include at least the placement of a calibrated photometric sensor at some distance & angle from a light source, size of sensor relative to spot, and location of the beam pattern that should be used -- as an industry standard. 

Readings would still vary wildly depending on duration of measurement, reflector size/shape/surface texture/focal length adjustment, initial light startup vs. stable, regulated light source output, etc...Hence the mischief on unrealistic output claims of many lights. 

I am aware that some people at CPF take readings of what they believe is "the hotspot" and try to relate that to throw. As a relative comparison, if done objectively and with same conditions from light to light, it can be at best--interesting. I have said previously in this thread that there is nothing wrong with trying to get some idea of how lights compare on a relative basis. My issue is when those attempts move into an absolute, quoted value, or when ballpark testing concepts are represented as factual and/or repeated over and over without anyone questioning the lack of fundamental controls and standards.



Marduke said:


> Lumens are a measure of overall *[visible]* light output (calibrated to the human visual response). That's the entire purpose of an integrating sphere.



Agreed in general sense with addition of the word, "visible" inserted as I did in red. The I.S. is the gold standard.



Marduke said:


> Two lights can be the same overall brightness (lumens) by have vastly different beam characteristics, giving it different lux numbers for the center hotspot.
> 
> Lux (when given typically as a single value of the hotspot) cannot tell you the overall brightness of a beam. A 10 lumen and 1000 lumen light can both have identical lux values at the center of the beam given the right optical conditions.



While I understand this point, the essence of the problem in all these attempts at objective comparisons is that there is not a standard established to specify and control for all the variables introduced with a typical reflector flashlight. Variety in LED spectral distribution, photometric sensor calibrated to type of light source, and the other items referenced in my other thread ( linked in post #2 above) are other important aspects to consider.

It's fine for people to come up with comparison methods. That's not my issue. Hell I did a version of that with my destructive testing thread in my sig, and despite my being ultra compulsive and repeating tests on separate days, and with multiple bulbs....at best my lux readings are a relative ballpark comparison to each other.


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## Size15's (Oct 9, 2009)

asdalton said:


> For example, lumens are a measure of the total amount of light (radiant energy weighted by the sensitivity of the human eye) emitted by an object. That's the _whole point_ of having such a unit. To say that such a description is "false" is, well, false.





Quickbeam said:


> Lumens is the measurement of the overall output.


Both can not be correct.
Either it's a measure of the total overall output, or a measure of the human response to the type and quantity of light.

1 lumen of red light is a different amount of light than 1 lumen of green light because the human eye responds to red light and green light differently, and also responds differently under different lighting conditions.
The response to LED 'white' light is different to incandescent light.


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## csshih (Oct 9, 2009)

hence the "spectral response filters" and "Cosine and color corrected measurements" ?
these filters/corrections are used so that the meters see as human eyes see it, right?


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## asdalton (Oct 9, 2009)

LuxLuthor said:


> Quickbeam's statement: "*Lumens is the measurement of the overall output." * is false because he does not qualify it as you did in terms of lumens ONLY applying to the subset of the overall visible output (radiant energy) weighted to the sensitivity of the human eye which is why the lumen is based upon the candela.
> 
> With his definition, the entire "overall output" of the sun would have to fit into his description of lumens, despite the fact that there are vast amounts of solar light radiation falling outside of the human eye's visible range.



I thought that it should be obvious from the context (a _flashlight website_) that by the generic word "output" Quickbeam was referring to _visible light_, and not total energy emitted (or for that matter, water or oobleck). If he had written "energy" without qualification, which he didn't, then you would have a point.



LuxLuthor said:


> Furthermore, it is not valid to say "no matter where it goes," since this would include reflected, refracted, focussed, and other changes to the output angle that is specified (steradian) in what the actual definition of a lumen (based upon a candela) really means.



So what? We can experimentally measure the lumen outputs of light sources that do not emit light uniformly. And the measured output does not depend upon the direction in which the light is focused or bent.


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## Marduke (Oct 9, 2009)

I think someone forgot to put on their common sense hat this morning.


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## Swedpat (Oct 9, 2009)

Using a luxmeter and careful ceiling bounce tests of different lights I have came to the conclusion of an approximate value of lumens of a flashlight. In my case I assume that 1 lux of the luxmeter corresponds to 18 lumens.

Regards, Patric


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## Bullzeyebill (Oct 9, 2009)

Patrick, that is fine, as long as you can keep the enviornment pretty much the same for all of your tests. I use a small bathroom without a window, and monitor the enviornment for consistancy. Comparative testing is all I do with my lights, using some of my lights with a known value of lumens for the comparisons. Right now I pretty much use my Surefire P60L, as a baseline, calling the lumen value 80 lumens, or my Malkoff M30 calling it 220 lumens, which is probably pretty close to actual output.

Bill


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## Swedpat (Oct 9, 2009)

Bill,

Yes, I do my tests on the same place in the same room. Also I take in consider different beam widths and make sure a light with wider beam size is placed closer to the ceiling to get equal illuminated size on the ceiling surface, this to ensure a fair measuring. 

Regards, patric


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## LuxLuthor (Oct 9, 2009)

csshih said:


> hence the "spectral response filters" and "Cosine and color corrected measurements" ?
> these filters/corrections are used so that the meters see as human eyes see it, right?



Correct.


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## Sparky's Magic (Apr 22, 2010)

My even simpler method is to use the ceiling of a darkened room with my Quark Turbo on
max and assume the lumens to be the stated 230L; the new light will be either over or
under in brightness and I'm happy with this uncomplicated process.
I assume 230L for the Quark to be accurate as it is visibly brighter than several in the 180-220L range.


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