How to translate lux to lumen?

[Flymo]

I own a Gossen Multisix photo-lightmeter, which can measure all kinds of light for general photography, including Lux readouts.
But how can I translate Lux into lumen for measurements with my flashlights?
:thinking:
Anybody?

Thanks !

 

[Reptilezs]

lux and lumens are not convertable. lux measures spot intensity and lumens measure total output

 

[nerdgineer]

+1 on Reptilezs' answer.

Technically, you would need to measure the lux output at 1 meter over all 4 pi stearadian of solid angle around the flashlight and integrate it.

If you assume radial symmetry of the light beam, you could just measure the lux at all elevation angles relative to the light axis (with the axis of the beam being 90 degrees, the plane normal to the axis being 0 degrees, and the back of the light being - 90 degrees) and sum up that "slice" of measurements in very small increments with each measurement scaled by the cosine of the elevation angle as defined above.

Then you'd have to normalize the result against a similar measurement taken with a calibrated light source to convert your answer into lumens.

Most people who seek lumens estimates have some way of mixing all the light coming out of the box (central beam and spill) into a single light meter measurement which they then compare with the measurement of some known light to estimate lumens output based relative scaling.

The usual source for sort of standardized comparative measurements of flashlight output is the FLR website, which includes directions on how the lightbox used there is made. FLR took care of all this work for years; but sadly, the author Quickbeam has retired from this so no more new reviews will be made.

 

[winny]

If you know the beam angle where the intensity have dropped of to half of the center reading, you can take a "lux reading" (as people often misuse on this forum) at one meter (at the center) and enter 1000 times that value and the beam-angle you came up with on this web-page:
http://led.linear1.org/lumen.wiz

For large beam-angles, it's fairly precise for most lamps.

 

[waiklau]

This is the formula I use:

Lux to Lumen Conversion = Overall Output Lux x 0.0139 = Lumens

example:

Fenix P1D-CE Lux Max 2700 overall 7850

7850 x 0.0139 = 109 Lumens

 

[winny]

Lux to Lumen Conversion = Overall Output Lux x 0.0139 = Lumens

This is totally wrong unless you use the exact same integrating shoe-box as flashlightreviews.com/Quickbeam does, which you don't.

 

[Quickbeam]

Winny is correct. Each person's lightbox would need to be individually calibrated. Mine was calibrated using test lights which were then tested for actual lumen output by a testing lab. The conversion factor was then calculated. Even so, it is not laboratory-accurate and is subject to error and should only be considered an approximation.

 

[OhMyGosh]

Lumens is the total output of a light
Lux is the brightness of the surface it is shining on.

 

[lowatts]

One way to think of it is Lux is like the force of water coming out of a nozzle; Lumens is like the total amount of water coming out of a nozzle.

A tinny nozzle can be putting out a powerful jet of water (such as in a water jet cutter) and not be putting out much total volumn. A larger water nozzle, because of it size, can put out a lot more water but still not have the intensity like the water jet.