How to measure lumens?

[BlahHead]

Hi torch-bearers,

Without any professional equipment, how do you measure measure a flashlight's lumen reading?
I heard something about ceiling bounce test, and what does it relate to the lumen count?

Thanks! Happy shining! 😀

 

[PeaceOfMind]

You can't accurately measure an actual number of lumens without some calibrated equipment and an integration sphere.

Bouncing the light off of a ceiling and observing the general brightness of the room is a good way to get an estimate of which light is brighter without beam pattern getting in the way. And obviously which light is brighter will correspond with which has more lumens. However you can't measure an actual number of lumens this way (i.e. you can say light A is putting out more lumens than light B, but without a calibrated integration sphere you can't say that light A is putting out 112 lumens)

 

[BlahHead]

Thanks for the reply.

How efficient would you say is a reflector? Say, if an LED spews out 900 lumens, how much would it be its output? (I know there's a whole wide range of qualities, but would... 500 lumens be considered efficient?)

 

[PeaceOfMind]

Between the reflector and a glass lens you probably lose something like 20-30% of the light that is actually coming out of the LED.

 

[BlahHead]

Hmm.. could you possibly reduce the loss by having a smooth reflector, not an orange peel?

What's the advantages of an orange peel reflector?

 

[Robin24k]

The loss is due to the fact that the light gets reflected or passes through a medium, so reflector type isn't going to change anything. You will get better efficiency with an optic though.

The orange peel reflector smoothes out the beam pattern for more spill.

 

[videoman]

If you want to measure lux as a rough but in the ballpark figure, you will need a camera that displays shutter speeds and aperture (most do) on auto or program mode and a white wall in a dark room. The wall should be mat rather than shiny. Place the light at exactly 3 feet from the wall and turn it on. Take the camera and set the ASA to 100 and get a reading on aperture and shutter speed. Use this chart http://www.999-wed.com/pg/pg9-infrared-conversion-tables.htm to get the EV value based on the 100 ASA and cross reference the shutter speed and the aperture and you will get an EV value. This EV value will translate it directly to the equivalent LUX value. Let's say you got 1/30th of a second at F 5.6 aperture, then on the chart the EV value is 10 and hence the lux value on the above chart is 2600. Remember that 3 ft. distance as the norm. Every doubling of light to wall distance will DECREASE the light by a factor of 4. Hope this helps. Now converting lumens to lux, hmm. perhaps there is a chart for that direct conversion somewhere but I think that lumens are a little LESS bright than LUX, so if you have 2600 LUX, then lumens would be in the order of perhaps 20 % more. I may be mistaken on this part, please correct me if I am wrong.

 

[videoman]

Forgot to mention, make sure the camera sees only the white area in the viewfinder. It helps to Zoom in on that. Camera to wall distance is not critical as the camera will see the same level of illumination regardless of distance.

 

[njet212]

Check this thread below, started by run4jc..

http://www.candlepowerforums.com/vb/showthread.php?263723-3-21-Update-Home-Made-Pseudo-Integrating-Sphere-photos-and-readings!