The Shower Test - can you spot PWM?

You can see the archived poll results on the Wayback Machine:

https://web.archive.org/web/20221220093741/https://budgetlightforum.com/t/-/72837

I often hear people suggest the shower test as a way to detect whether a light uses PWM or if it delivers constant current. The test is: Shine the light at fast-moving water, like a shower head. Then look for dotted lines. If it has dots or spots, it uses PWM. If the streaks are smooth, it’s constant current.

But I find the test unreliable and error-prone. So I took some pictures of how this test looks in real life, and I want to know if people can determine the correct answer.

Here’s the pic. The poll choices refer to this picture:

Both lights were running at 30 lm, and both pictures used the same camera settings. Clicking the picture provides a larger version.

Edit: The answer is here.

Neat! I voted CC for the left and PWM on the right.

Chris

I voted for both, as they have dotted lines that look very artificial to me.

did this a long time ago, like 1m below shower head its a lot better to see

with the right shutter speed you can even get very close to the PWM frequency with calculation of drops/shutter speed

If the only light source on rhe shower has PWM you should see it in all water beams, not just in some. So in both pictures the shower is illuminated with a non-PWM light source, or too fast to even notice this way.

I guess…

Nice pic Lexel :slight_smile:

I have trouble telling from that photo. I use a bicycle hanging in the garage; spin the front wheel and shine the light at the spokes. The bike is always there unless it is being ridden. If it’s being ridden I probably have little interest in checking PWM right then. :smiley:

For the poll would “PWM” be any at all, or PWM at a frequency slow enough for a human eye to see it?

Any PWM at all.

But, specifically, the kind which pulses on and off. Not the kind which stays on but does a square wave oscillation between two different levels.

I voted left=constant, right=pwm. But I’m really unsure.

Norman Bates would know.

I think TK is playing a trick on us and one or both are not using PWM. I would normally say right and left is both PWM, but I think the trick is left is no PWM and right is PWM. Possibly no PWM at all.

I can't tell. I'd use a control picture as reference, and compare, if this were my test.

To test for visible PWM, I use a fan. After that, I use a camera with very short exposure time ;)

Streaks and dots in both pics to me :question:

This…

It is.

I use the manual control on my camera/phone camera to check for any “flickering”, normally between 1/1000 to 1/2000 exposure the “flickering” will show clearly. I’m awfully sensitive to mid to low frequency “flickering”, even those that claims by some as “high-enough” or not PWM in its strict sense. For me, it doesn’t matter whether it’s On-Off or “Stay-On but does square wave”, as long as it’s not constant current, it’s being put to “flickering” category for me. :smiley:

yup both have PWM but the right pic is worst :slight_smile:

…and what about the streaks?

It’s like there’s 2 light sources. I get that the water droplets can move at different speeds which isn’t accurately represented with a still picture. But as pointed out by TK it seems like a pretty flawed test.

oh I didnt see that :stuck_out_tongue:

I shine a light at my spinning usb fan to test for pwm. Works well

Is it just me, or is there a face in the water on the right side? Creepy! :open_mouth: