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Thread: Lumens & Lux

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    Question Lumens & Lux

    What is the difference between Lux & Lumens ? are they measurements of the same thing & if so how do you convert one to the other ?
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    Dave Abraham's Avatar
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    Lumens are used to measure the output from a light source
    Lux is a measurement of light hitting a surface

    Table below from Wikipedia



    SI photometry unitsQuantitySymbolSI unitAbbr.Notes
    Luminous energyQvlumen secondlm·sunits are sometimes called talbots
    Luminous fluxFlumen (= cd·sr)lmalso called luminous power
    Luminous intensityIvcandela (= lm/sr)cdan
    SI base unitLuminanceLvcandela per square metrecd/m2units are sometimes called nitsIlluminanceEv
    lux (= lm/m2)lxUsed for light incident on a surface
    Luminous emittanceMvlux (= lm/m2)lxUsed for light emitted from a surface
    Luminous efficacy lumen per wattlm/Wratio of luminous flux to radiant flux; maximum possible is 683.002 lm/W
    Last edited by Dave Abraham; 16-05-08 at 08:02 PM.
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    OK
    So torch 1 has 25000Lux
    torch 2 has 2500 lumens
    Which is better/brighter ?
    Last edited by colinicky; 20-05-08 at 08:02 AM.
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    It depends. I might be wrong but as I understand it lumens is the total light output from a source, whereas lux is the light intensity at a particular point on a surface so is dependent upon beam angle ie a narrower beam angle would produce a higher lux reading at a given point as opposed to a wide angle.

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    Quote Originally Posted by colinicky View Post
    OK
    So torch 1 has 25000Lux
    toch 2 has 2500 lumens
    Which is better/brighter ?
    Your comparing apples and oranges, you can't do that.

    FFS, just ring me tomorrow and order a Salvo.


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    Phil
    Maybe just maybe I will ring after I hopefully meet Rob at Wraysbury next week to look at the Salvo 50 & if you do another super offer again
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    As said, lux is the intensity hitting a surface. It's used a lot in building design because it specifies what the illumination on a surface should be at a certain distance from the light (like desks in an office) rather than specifying a light's output which is a bit irrelevant.

    Are you sure the other torch's units are lux? I always thought lux levels were dependant on distance of the target from the source and various other environmental factors.
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    Quote Originally Posted by NotDeadYet View Post
    Are you sure the other torch's units are lux? I always thought lux levels were dependant on distance of the target from the source and various other environmental factors.
    Yep listed in LUX. It is the Tillytec range of torchs & all are listed with "LUX"
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    Quote Originally Posted by colinicky View Post
    Yep listed in LUX. It is the Tillytec range of torchs & all are listed with "LUX"
    Doesn't seem right, just re-read the original post and 25,000lux... Just for a bit of perspective, the recommended light level for an office is 500 lux at desk height. The spec for the torch is 50 times that level.
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    Hmmm. A lux is the light that falls onto a surface placed at 1m from a single candle, so to measure a torch in lux one would have to specify the conditions or else it is meaningless.

    However, equally, to specify in lumen on would need to know the cone geometry.

    For any comparison 1000 lumens, concentrated so that it shone onto exactly 1 m2 would light up that square metre with an intensity of 1000 lux.

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