Comparing flashlights

[jenskh]

Hello,
I am quite new here, but has already got hooked.
I have bought several flashlights, and also a lux meter to be able to compare them. I have also made a spreadsheet to be able to integrate my lux measurements into lumens. I had here planned to post an example of my work, but I am not able to post a picture. How do you do it?

Best regards from
Jens

 

[WadeF]

How did you convert lux into lumens? Did you make a bounce box?


and as far as posting pictures, from The Welcome Mat topic:

http://www.candlepowerforums.com/vb/showthread.php?t=171978

 

[jenskh]

Thanks for the description on how to post images.

Here is how I do the calculation of lumens based on my lux readings:
I fix the light at a position (usually 1 m above floor) and pointing towards the floor. I then measure lux along the floor from the senter of the beam and as a function of the distance from the center. All readings are taken normal to the floor.
To get lumens, I just integrate lux times the delta area. I then get curves like shown below. In that diagram I have also measured with 2 m distance just to check that I get the same lumens calculated. And I think it is as close as can be expected. This light has very step gradients around the spot, so a small deviation in coordinates means a lot for the lux readings.
What do you think?

 

[magic_elf]

Its gonna be quite tough since the lux values will vary at each point on your floor. I think this would only work with lights which have a defined hot spot and spill beam. Otherwise it's gonna be really hard for those which have soft hot spots which fade into the spill.

To make sure that this system works, you should try using this system of calculation with varying positions of your lights.
For example compare the resulting lumens you calculated with the light position 50cm (smaller area, higher intensity) and 100cm (larger area, lower intensity) from the floor. They should result in more or less similar numbers.

 

[jenskh]

To make sure that this system works, you should try using this system of calculation with varying positions of your lights.

That is exactly what I have done in the figure above, where I have used 1 m and 2 m for the same light.

In my spreadsheet for integrating lux into lumens, I assume a linear variation of the lux between my readings. A smooth transition is therefore actually easier than more sudden variations in the flux.

 

[jenskh]

To make sure that this system works, you should try using this system of calculation with varying positions of your lights.

That is exactly what I have done in the figure above, where I have used 1 m and 2 m for the same light.

In my spreadsheet for integrating lux into lumens, I assume a linear variation of the lux between my readings. A smooth transition is therefore actually easier than more sudden variations in the flux.

 

[nerdgineer]

Nicely done, and creative... Welcome to CPF!

 

[TORCH_BOY]

Welcome, Great graph well done