The Shower Test - can you spot PWM?

LED Flashlights – General Info
1

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

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

2

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.

3

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

Chris

4

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

5

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

6

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:

7

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:

8

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

9

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.

10

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

11

Norman Bates would know.

12

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.

13

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 ;)

14

Streaks and dots in both pics to me :question:

15

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:

16

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

17

…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.

18

oh I didnt see that :stuck_out_tongue:

19

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

20

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