How to convert Selfbuilt's Lightbox values to Lumens


Newly Enlightened
Jul 11, 2018
Everyone always asks me, when will I get myself a properly calibrated Integrating Sphere (IS) for lumen measures? The short answer is when someone pays for one for me.

But the real question is, can the relative output values of my home-made milk carton lightbox be converted to lumen estimates? First off, a few points about my lightbox.

I generally modeled my lightbox design on Quickbeam's (aka Doug Pribis) site. Sadly, this excellent resource is no longer active (and the domain has been taken over by someone who has pirated Doug's original treasure-trove and added other unconfirmed and pirated material ). But at least the background info still seems to be there, untouched.

My lightbox differs in one important way - I've reversed the light and sensor placement, to facilitate runtimes. In my case, the flashlight enters the flat bottom of the milk carton, and the sensor is located on the side of the carton near the base. The bezel of the flashlight thus serves as its own baffle, preventing any light from shining directly on the sensor.

As Doug noted, this is hardly a perfect integrating sphere. But it doesn’t have to be – as long as you realize the results are simply relative output values, you can still draw meaningful comparisons between lights. But can you convert my milk-carton output values to estimated lumens by some sort of conversion factor?

To begin to answer that question, you would need calibrated IS data for the lights tested in the lightbox. Well, if I had a calibrated IS, I would be using it. But there are a number of other people – and manufacturers – who do have them. Although not as good as actually testing my specific samples in their ISs, can we tell something meaningful from lights we have in common?

Below is a graph showing how lights in my lightbox correlate to the reported IS values by three members here (MrGman, ti-force, and bigchelis) and three manufacturers (Fenix, 4Sevens and Novatac). Each data point represents one output mode of a given light we have in common. I have matched the reported batteries and time post-activation for the lights in question (if multiple time points were available, I picked the last one we both had in common). There are about 150 unique data points in the total set, representing over 40 lights.


Staff member
Feb 21, 2003
Do not see your graph. Check one of Selfbuilts reviews. He has info on the lightbox.