is there a way to measure the Lumen output of an LED with a footcandle meter

ptruslow

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Joined
Mar 2, 2012
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1
i have access to a footcandle meter, that is all i have, and i want to know the brightness of my xm-l t6 flashlight. it is a Kaidomain C8. the reflector can be removed. i was thinking that without the reflector installed, i could measure the brightness at one foot in the middle of the beam, at a set distance, say one foot because we are using footcandles. and somehow use the beam angle, which is 125 degrees to find the total lumen output. i am interested to know how this could be done.
 

yifu

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Joined
Oct 15, 2011
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713
Location
Australia
Unfortunately not. A bare LED like an XM-L has differing spectral distribution as the viewing angle changes. A certain lux figure measured at a specific part of the beam will change if the meter is moved a few centimetres. Even outside the 125 degrees, there is spill. The only way to accurately measure lumen output is using a calibrated integrating sphere. There are quite a few CPFers who have one of those and are willing to measure lumen output if you send your light to them.

If you just want an approximation, then setting up a ceiling bounce test is an easy way to estimate lumen output. Set your meter on the floor and point your light at the ceiling, then point another light with a known (and accurate lumen output) and calculate the lumen output using the lux difference measured. This is however, not 100% accurate, as the figures follow a more logarithmic plot than a linear plot.
 

Bigmac_79

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Jun 3, 2011
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1,511
Location
Kansas
You can't get a super-accurate measurement without an integrating sphere, but if you want to invest a small amount of time in it, you can get much more accurate than a ceiling bounce test by making your own light box. You can search here on CPF to find some threads on how other people have made their light boxes, or if you light I can give you details on how I made mine.

Once you have a light box made, you can sort of "calibrate" it. First you test various lights in it that you can find reliable measurements of the lumen output from other people's integrating spheres, then you can compare the measurements of the real lumen output to the readings from your light box, and find a conversion equation. The more lights that you have that you can get reliable measurements of real lumen output, the better your equation will be, and the better you'll be able to estimate the output of the lights that haven't been measured in integrating spheres.

If (like me) you don't already own a ton of lights that other people have measured, you can still use a light box for more precise measurements (than a ceiling bounce) to compare the overal output of the lights you do have, you just won't be able to convert that to lumens.
 
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