How to calculate lumens

[Fallingwater]

I have a light meter, but its reading is in Lux.
I understand lumen = lux * m^2, but my brain starts to hurt when I try to translate this in a practical procedure to measure them.

 

[Marduke]

You can't calculate lumens from lux, because the light doesn't emit uniformly.

It's like trying to measure the volume of a room by just measuring the length of the biggest wall. Sure, it gives you an idea, but it can't translate.

 

[KeyGrip]

Technically, you need an integrating sphere. Comparing relative intensity is a good way to get a ballpark lumen figure.

 

[Crenshaw]

you cant really calculate lumens from candlepower or lux, or any other conversion between those three i think. You can get a rough idea, but the best way to guage how bright your Dx light is is by comapring it to other lights. For example, my UFC3 is dimmer then my P1DceQ5 on medium(96lm), but not alot dimmer, so im guessimg maybe 50-70 lumens. This can however be inaccurate due to different intensities of hotspots, different spills, etc.

Another way is to measure is by finding out the lux readings of other lights, im sure there must be a list somewhere here. But then, is not the best way either, unless youre looking purely for throw.

In other words..get the light anyway, and keep it as a shelf queen if your present light is already brighter..



Crenshaw

 

[LukeA]

You can get a rough estimate by finding the area and brightness of both the hotspot and spill. [(Lux/m^2)spot]+[(Lux/m^2)spill]=lumens.

 

[Fallingwater]

You can get a rough estimate by finding the area and brightness of both the hotspot and spill. [(Lux/m^2)spot]+[(Lux/m^2)spill]=lumens.

Ow. Brain hurts. *sigh* this is why I didn't take an engineering university course.

 

[meuge]

Ow. Brain hurts. *sigh* this is why I didn't take an engineering university course.

The practical way to estimate overall light output is by measuring indirect illumination (outside the hotspot and spillbeam, preferably at a 90-degree angle to the beam).

This way, you can compare lights to each other, without needing to measure absolute values. Also, if you have a light the output of which has been established, you can easily calculate the approximate output of an unknown light, by using this relative technique.

All you'd need is a solid box, lined with white material (painted white, etc...) and a single hole. Place the lux meter inside, orthogonally to the beam, and you're set to make relative measurements.

 

[Kilovolt]

I suggest you read this topic:

http://www.flashlightreviews.com/features/lux.htm

as well as the related ones under the heading "Q&A".

I think it will give you a sufficiently clear idea without causing any undue smoke from your brain.

 

[Marduke]

I think it will give you a sufficiently clear idea without causing any undue smoke from your brain.

Because we all know, if you let the magic smoke out, it doesn't work anymore

 

[nanoWatt]

Direct all the light into an integrating sphere, and take measurements.