I have a couple of different LED flashlights. One of them has three different "modes" of brightness, and the way it controls it is via pulse width modulation (PWM). Here is a picture that illustrates how it works:
I know that this particular flashlight's PWM circuit operates at about 120Hz. What this means is that when you move something very quickly under it, when running a low duty cycle, it produces a strobe-light effect, where you'll see many "copies" of the moving object. It reminds me of the way video-games are rendered, and it actually creates a really neat effect because you only "see" very small time-slices. Watching running water this way is absolutely fascinating.
From empirical observation I determine that on the lowest setting it seems to be on about a 2-5% duty cycle. The ghost-images produced are remarkably sharp. On Medium, I reckon it's about 25 to 30%.
Something I've noticed very recently is that when I use this flashlight under dark-adjusted conditions, the brightness that is perceived definitely doesn't seem to scale linearly. I have no scientific light intensity equipment to perform measurements with, but I am more or less convinced that a 1000 lumen light on a 2% square-wave duty cycle appears brighter than a 20 lumen light, all else being equal (which would include incident light energy).
I think there may be some biological explanation for this. Is this an effect that has been observed by others?
Best Answer
Your eyes are very bad at estimating absolute brightness, especially a dark adapted eye looking at a bright light.
One way to measure the 'average' brightness is to try and read the same printed text under the equivalent illumination eg. 20lumens * 100% and 1000lumens * 2%
Remember though that LED flashlight makers are notorious liars when it comes to lumen figures.