Lumens & Lux

[colinicky]

What is the difference between Lux & Lumens ? are they measurements of the same thing & if so how do you convert one to the other ?
TIA

[Dave Abraham]

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

[colinicky]

OK
So torch 1 has 25000Lux
torch 2 has 2500 lumens
Which is better/brighter ?

[steve6690]

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.

[divingniknaks]

Originally Posted by colinicky

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.


.

[colinicky]

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

[NotDeadYet]

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.

[colinicky]

Originally Posted by NotDeadYet

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"

[NotDeadYet]

Originally Posted by colinicky

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.

[Lou]

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.