Easy to Understand Lumens Vs Lux Explanation
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dc38
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Lumens are very difficult to measure directly. Typically, an integrating sphere, as mentioned, is used. This is just a specially shaped container, made to take the lumens emitted, and spread them out very evenly, and then measure one spot with a device that measures lux.
If you know the surface area inside the container, and the entire surface is covered with the same intensity of light, because the light is spread evenly...if you know what the intensity is at one spot, you know what it is at all the other spots.
As lux = lumens per square meter, you can then calculate the lumens from the lux.
A LOT of people on this forum for example have made their own versions of this, using foam balls, cardboard boxes, and even just shining the lights at the ceiling and measuring what bounced down at a lux meter on the table, etc.
None of these will be as accurate as a real integrating sphere, and all of them have strengths and weaknesses, but, we like them nonetheless, as they do give a third party feel for a light's output, even if flawed.
So lux is easier to measure, and, is also used to CALCULATE the lumens.
Well said Teej. I agree homemade light boxes and spheres that I and others own do have their limitations. You do have to take these measured numbers with a grain of salt. How accurate they are is effected by the testing methods and the design itself. But mostly by the initial calibrating. That's the really difficult part.
Some have had a light actually tested in a sphere and try to use that as a baseline. Not a bad way to go. The possible flaw with that is your specific light box might not read consistent across the entire light size, beam pattern style, and mainly lumen level spectrum. And it counting on that said sphere and light being exact.
Others, like selfbuilt & myself included, use as many ANSI rated lights as possible to try to create a good baseline and multiplier number. The flaw with this design is the lack of consistency between manufacturers published numbers. Over the years I have discovered certain brands always tend to read high, others low, and some right in the middle. For example, Eagletacs always appear to read higher than their published numbers. Zebralights tend to run lower than their published numbers. 4sevens always tend to read very close to published numbers. And etc. So the problem is - which manufacturer is correct? I'm sure all 3 of these companies use true integrated spheres. So that means that there might actually be variables between true spheres, or their testing methods vary and thus their results vary, who knows. Now I rarely find numbers drastically off, but inconsistencies are there. They are usually within about 10% of their ANSI ratings.
So all we can do is find the best median from testing as many ANSI rated lights as possible. As long as your testing methods and equipment stays consistent, this will at least give consistent results from lights you test. Granted your numbers could actually be 10% high or 10% low - but at least it SHOULD give consistent numbers for comparison purposes.
I feel very confident in my lightboxes design that when I say one light tests at 900 lumens and the next light tests at 1000 lumens - rest assured that second light truly has about 10% more lumens. Now its possible beam types could affect these results, but I personally have never seen any noticeable variances in readings between beam types. In my lightbox anyway. My lightbox has absolutely no edges, corners, or flat spots. Nothing for light to "catch" on. So an intense spot should not be able reflect back on the sensor any easier than a powerful floody light.
Anyway, almost any lightbox and testing method is going to be better than purely guessing. For a ANSI rated light purchase - testing said light might not make sense. After all, odds are the light will likely be very close to the manufactures published numbers.
When these lightboxes truly come in handy is for testing modified lights, or of course non ANSI rated lights. Its nice to see what type of results you get when you swap in 3ea XML2 U2's on copper in an older model TK75 or whatever favorite light you own. Then you see the gains of your work. A lightbox and lux meter itself, make excellent tools for modders or light builders. They allow you to see what gains (or losses) you get when you try a certain LED, reflector, dedoming, lens, spring mod, or even different batteries. You see instant results.
All this aside, the best thing about using lightboxes and meters is that they add another fun and interesting factor into this wonderful hobby of ours.
Some have had a light actually tested in a sphere and try to use that as a baseline. Not a bad way to go. The possible flaw with that is your specific light box might not read consistent across the entire light size, beam pattern style, and mainly lumen level spectrum. And it counting on that said sphere and light being exact.
Others, like selfbuilt & myself included, use as many ANSI rated lights as possible to try to create a good baseline and multiplier number. The flaw with this design is the lack of consistency between manufacturers published numbers. Over the years I have discovered certain brands always tend to read high, others low, and some right in the middle. For example, Eagletacs always appear to read higher than their published numbers. Zebralights tend to run lower than their published numbers. 4sevens always tend to read very close to published numbers. And etc. So the problem is - which manufacturer is correct? I'm sure all 3 of these companies use true integrated spheres. So that means that there might actually be variables between true spheres, or their testing methods vary and thus their results vary, who knows. Now I rarely find numbers drastically off, but inconsistencies are there. They are usually within about 10% of their ANSI ratings.
So all we can do is find the best median from testing as many ANSI rated lights as possible. As long as your testing methods and equipment stays consistent, this will at least give consistent results from lights you test. Granted your numbers could actually be 10% high or 10% low - but at least it SHOULD give consistent numbers for comparison purposes.
I feel very confident in my lightboxes design that when I say one light tests at 900 lumens and the next light tests at 1000 lumens - rest assured that second light truly has about 10% more lumens. Now its possible beam types could affect these results, but I personally have never seen any noticeable variances in readings between beam types. In my lightbox anyway. My lightbox has absolutely no edges, corners, or flat spots. Nothing for light to "catch" on. So an intense spot should not be able reflect back on the sensor any easier than a powerful floody light.
Anyway, almost any lightbox and testing method is going to be better than purely guessing. For a ANSI rated light purchase - testing said light might not make sense. After all, odds are the light will likely be very close to the manufactures published numbers.
When these lightboxes truly come in handy is for testing modified lights, or of course non ANSI rated lights. Its nice to see what type of results you get when you swap in 3ea XML2 U2's on copper in an older model TK75 or whatever favorite light you own. Then you see the gains of your work. A lightbox and lux meter itself, make excellent tools for modders or light builders. They allow you to see what gains (or losses) you get when you try a certain LED, reflector, dedoming, lens, spring mod, or even different batteries. You see instant results.
All this aside, the best thing about using lightboxes and meters is that they add another fun and interesting factor into this wonderful hobby of ours.
If I see one trend emerge in HOW the non-ANSI IS work, is that most of them seem to under represent flooders and over represent throwers....and as many of the baseline models ARE tighter beamed, the calibration tends to be closer to the thrower calibration and further from the flooders...and, sure, the spectrum response is going to be all over the place.
Most of the weakness of a DIYS IS is that the light MUST be evenly distributed to have a single point measurement be representative.
The simplest version of that is the ceiling bounce with the meter next to the light on the table, etc. MOST light from a tight hot spot will come straight down from the reflections off the ceiling over the table. If the (floody) light puts say, 60% of its spot OUTSIDE the table's diameter, most of the output is not measured with the same weighting given to the light coming down on the meter, on the table, etc.
So, given the high-tech coatings and engineering that goes into a real IS, a DIYS IS is going to be off, and, it will tend to be off the same way across the board, for example being "calibrated" more for throwers than for flooders, or to the emissions from one LED at a particular amperage/drive level vs another, etc.
It can get complicated, as variables include corona and spill proportions to hot spot proportions, and a lot of other variables that impact the difficulty of getting an even and representative distribution to fall on your lux meter's sensor.
The other issue you can't control is the consistency from light to light in actual output. A batch to batch variance in reflectors, LED, etc, can mean that if you measure 3 lights, you could have them 15% apart in output, and, the factory output spec, based upon a 3 light AVERAGE, means that YOUR light might be within 15% of that, high or low.
Add that variable set to your DIYS IS's built in variable sets, your lux meter's built in variables, the IS variables, etc, and, when you add it all up, you COULD see dramatically different differences in perceived measurements, which, after crunching the stats, could be all the same.
Example: If the dogma is that SureFire is accurately measuring the lumens of their lights (Which can be correct, given gov contracts, etc...) - and, assuming that SF lumens are good for calibration...whatever the beam characteristics of the SF you use to calibrate are, become the reference you use as a baseline to measure the SF.
So SF lumens will, in that scenario, tend to be "right", because they were used to calibrate themselves....and lights that use different LEDs, beam angles, and so forth, will be "off", as they were not used to baseline themselves, but compared to another light's baseline.
If, for example, Zebralights's were used to establish the "baseline", then only ZL's would be found to be accurate, and all the throwy/different beam distribution and spectrum lights would be "off".
So, a real IS is the correct answer, and DIYS versions can ball park things, but, you would really need to do a lot of work to account for all the variables, and not just be satisfied to say, well, "its right for the known lights".
If your DIYS IS is actually as accurate as a real IS, and you could go to court and use it to prove that X light is over stating or understating its lumens, then, you could make a lot of them, and sell them to all the suckers paying many thousands of dollars for a real one, instead of buying, or just making your DIYS version. (You is the "royal you", not a poster or anyone in particular...)
That said, we ALL use our lux meters to bench line with...as we don't have an IS, and probably never will. Hence, we do take our DIYS measurements with the proverbial salt.
Most of the weakness of a DIYS IS is that the light MUST be evenly distributed to have a single point measurement be representative.
The simplest version of that is the ceiling bounce with the meter next to the light on the table, etc. MOST light from a tight hot spot will come straight down from the reflections off the ceiling over the table. If the (floody) light puts say, 60% of its spot OUTSIDE the table's diameter, most of the output is not measured with the same weighting given to the light coming down on the meter, on the table, etc.
So, given the high-tech coatings and engineering that goes into a real IS, a DIYS IS is going to be off, and, it will tend to be off the same way across the board, for example being "calibrated" more for throwers than for flooders, or to the emissions from one LED at a particular amperage/drive level vs another, etc.
It can get complicated, as variables include corona and spill proportions to hot spot proportions, and a lot of other variables that impact the difficulty of getting an even and representative distribution to fall on your lux meter's sensor.
The other issue you can't control is the consistency from light to light in actual output. A batch to batch variance in reflectors, LED, etc, can mean that if you measure 3 lights, you could have them 15% apart in output, and, the factory output spec, based upon a 3 light AVERAGE, means that YOUR light might be within 15% of that, high or low.
Add that variable set to your DIYS IS's built in variable sets, your lux meter's built in variables, the IS variables, etc, and, when you add it all up, you COULD see dramatically different differences in perceived measurements, which, after crunching the stats, could be all the same.
Example: If the dogma is that SureFire is accurately measuring the lumens of their lights (Which can be correct, given gov contracts, etc...) - and, assuming that SF lumens are good for calibration...whatever the beam characteristics of the SF you use to calibrate are, become the reference you use as a baseline to measure the SF.
So SF lumens will, in that scenario, tend to be "right", because they were used to calibrate themselves....and lights that use different LEDs, beam angles, and so forth, will be "off", as they were not used to baseline themselves, but compared to another light's baseline.
If, for example, Zebralights's were used to establish the "baseline", then only ZL's would be found to be accurate, and all the throwy/different beam distribution and spectrum lights would be "off".
So, a real IS is the correct answer, and DIYS versions can ball park things, but, you would really need to do a lot of work to account for all the variables, and not just be satisfied to say, well, "its right for the known lights".
If your DIYS IS is actually as accurate as a real IS, and you could go to court and use it to prove that X light is over stating or understating its lumens, then, you could make a lot of them, and sell them to all the suckers paying many thousands of dollars for a real one, instead of buying, or just making your DIYS version. (You is the "royal you", not a poster or anyone in particular...)
That said, we ALL use our lux meters to bench line with...as we don't have an IS, and probably never will. Hence, we do take our DIYS measurements with the proverbial salt.
Last edited:
lightliker
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- Mar 18, 2011
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Nicely explained!
neutralwhite
Flashlight Enthusiast
+1 - this was the explanation I needed all this time. there was me buying more Lumens when I actually wanted more Lux.
thanks Teej!.
thanks Teej!.
JohnColter
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Thanx, this is super helpful. Really love the analogy!
John
John
Afroderp
Newly Enlightened
If my retarded self could understand that explanation, anyone can!
Good job on summing that one up and putting it into laymans terms.
Good job on summing that one up and putting it into laymans terms.
Thanks for the positive feedback!
Sea Urchin
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Thanks for an analogy that helped me get my head together over Lux + Lumen. Brilliant!
Gloh
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Great analogy. Thanks for writing this!
Just a question though!
My torch has an 800 and 1020 Lumen setting and yet they look identical in terms of lux when I compare the two beams.
Why is it that there is no discernible difference to the naked eye?
Just a question though!
My torch has an 800 and 1020 Lumen setting and yet they look identical in terms of lux when I compare the two beams.
Why is it that there is no discernible difference to the naked eye?
Probably because the beam is focused tightly enough that the 800 lumen output has already created enough glare that your "eye's light meter" has already pegged its needle, and you simply don't have the dynamic range in optical lux sensing to see further brightness.
The way you WOULD see the difference (Assuming your light actually goes from 800 to 1020 L...), is to pick a DISTANT target, one far away that you can barely see it with the high setting...then switch to the lower setting, and the object will disappear (Or at least get visibly dimmer). Typically, you need to get the lux on target down to a range that is within your eye's (terrible) dynamic range...to be able to resolve differences between lux output.
IE: You will typically be OK at seeing OBJECTS, but, NOT OK at judging brightness in of itself.
If you shine at a wall, all you'll see is a bright spot on the wall, and, unless the spot gets larger or smaller, typically, most people can't tell the difference in output well at all between two hot bright spots.
If you shine the light out into a field, at a far away target, etc...you might see more, or less, target though, with different output.
The OTHER option is if your light's output is not changing, due to a circuit issue, the cells not having the juice to support a higher output, etc.
For perspective, the lux is the lumens per square meter...so, the drop from 1,020 to 800 lumens will produce a proportional drop in lux. (A drop of ~ 22%)
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Gloh
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Right I see.
So you're saying that there should be a difference since 1050 is 'deeper' in terms of your analogy and is emitting more packets of light.
So whilst I don't see the difference close up, if I use it, as you said, in a field, technically the 1050 lumen setting would comparably be of higher lux than the 800 lumen setting and make more objects visible in the dark?
So you're saying that there should be a difference since 1050 is 'deeper' in terms of your analogy and is emitting more packets of light.
So whilst I don't see the difference close up, if I use it, as you said, in a field, technically the 1050 lumen setting would comparably be of higher lux than the 800 lumen setting and make more objects visible in the dark?
