Is it possible to verify the claimed lumens using a lux meter?
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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.
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.
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?
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?
Marduke
Flashaholic
There is no direct correlation between lux and lumens, as they measure two seperate things.
HKJ
Flashaholic
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).
gswitter
Flashlight Enthusiast
You can't correlate lux meter samples with lumens using this method.
Re-read what LuxLuthor wrote and referred to above.
kramer5150
Flashaholic
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.
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.
qwertyydude
Flashlight Enthusiast
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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.
Bullzeyebill
Flashaholic
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.
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Bullzeyebill
Flashaholic
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
csshih
Flashlight Enthusiast
I believe MrGman combines a home made integrating sphere with a luxmeter to calculate lumen values?
Thank you all for your additional comments and this useful link. This homebrew box should be good enough for me.
PhantomPhoton
Flashlight Enthusiast
No I believe he generously donates his time to use a properly calibrated IS in a lab at his place of work.
Bullzeyebill
Flashaholic
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
asdalton
Flashlight Enthusiast
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?
joema
Flashlight Enthusiast
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.
Follow my link in post #2 of this thread. There are actual specific definitions of these terms and how to measure them.
asdalton
Flashlight Enthusiast
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.
