The Shower Test - can you spot PWM?

There’s zero chance someone will be able to see 10kHz or faster PWM doing that.

Fast movement of light source - the best PWM test.

maybe but

a. at that frequency, isn;t it imperceptible?
b. would the shower test work for that, assuming PCM/no PCM matters at that frequency?

Ha ha. It was a trick question. She’s not even using a flashlight.

She’s screwing with us… “PWM” are the initials of the person whose face is on the right side :wink:

Lol……Selene is playing a game for sure… :smiley:

I just read her response. Sounds like both are PWM however, just at different frequencies or types…. :stuck_out_tongue:

… (this space intentionally blank, to put the answer at the top of page 2 instead of the bottom of page 1) …

Okay, the poll has been running for a day now. That provides enough results… so here’s the answer.

First, as of this comment, the poll results are:

  • A. 65% (39 votes) : Left=PWM, Right=PWM
  • B. 10% (6 votes) : Left=PWM, Right=Constant
  • C. 17% (10 votes) : Left=Constant, Right=PWM
  • D. 8% (5 votes) : Left=Constant, Right=Constant

Total votes: 60

The correct answer is B. The light on the left uses PWM (FW3A), while the light on the right delivers constant current (L3 L10). Both were running at 30 lumens.

There was enough information in the pictures to determine which was which but, just as in real life, it required looking very closely. Click for the zoomed-in picture and look at the narrowest streaks in the middle area of each image. That’s where the visual difference is most apparent.

Of the 60 people who answered, 90% picked a wrong answer:

  • 82% had a false positive (thought the right image used PWM)
  • 25% had a false negative (thought the left image used constant current)
  • 17% got both wrong
  • 10% chose the correct answer

The big, bright dotted lines are not PWM. Most of the smaller dotted lines aren’t PWM either. Those are just oscillations in the shape of the water droplets. The water vibrates, spins, or jiggles rapidly as it falls, and this changes how it reflects light. So even a constant current light looks like it has PWM unless you look really closely and are good at interpreting the data.

This is why I don’t recommend the shower test. It’s very error-prone, even among flashlight enthusiasts.

The difference is shown in this little detail section which I cropped out of the full-size image:

Also, in the left image, this part should be a dead give-away for anyone who knows what they’re looking at. It shows a single water droplet oscillating at two different frequencies simultaneously, but… falling water droplets don’t really do that:

As one of the people who was able to correctly tell which was PWM, I think there’s a few things to note - some of them are general remarks, some are comments about the quality of the test or the poll’s evaluation of the test’s quality. In no particular order,

The pwm here is of the faster sort that is not commonly a big problem. Yet close examination does reveal its presence, so it’s at least shown viable even for high frequencies of pwm which may be hard to notice.

The stream of water is one which we could not see with our own eyes, nor in motion or from multiple perspectives, and so we must guess at the speed and character of the water flow. This is true to an extent even in person, since most showerheads do not produce variation-free patterns of water.

It would have made me more sure of what I was seeing if I had a better idea of the specific camera settings and attributes, rather than just knowing the exposures were equal. Not exactly necessary, but while I may reasonably suppose that the iso was higher than the camera is able to be noise-free at, the aperture was opened wide enough that on whatever format this is, at whatever distance this is, the focal plane was a certain depth, and I may assume the exposure time was within a certain range, at least for me knowing as much as possible about the camera’s configuration helps me to subtract out that influence. We did know it was 30 lumens, which was useful to know. All this is partly because you can choose odd settings and get odd results, or choose settings that don’t make things very clear.

The picture quality is deceptive; you don’t realize until you zoom in that there is any more detail to be had - while I’m for the above reasons unable to be certain to the degree I would prefer, I would point to color noise, noise reduction size (likely basic built-in NR rather than purposefully applied) and large areas out of focus as reasons why someone not looking closely wouldn’t realize that within the full-size image are some smaller areas which are in good enough detail to see the fast pwm we’re looking for.

While 50/60Hz or even 100/120Hz flicker (not PWM) of some mains powered lighting allows things to travel fairly large distances between each cycle, the kilohertz scale pwm of a decent flashlight requires you to consider how far something can travel in a fraction of a millisecond, and then zooming in to look at that scale. The larger droplets smooth out the fast pwm so that it can’t be seen, we see only the effect of their changing shape. It is, however, conceivable to me that one might see similar patterns and correctly guess that there was a slower flicker or pwm going on. That gets into where this test isn’t very good; without great confidence in your analysis, you can come up with multiple plausible explanations for what you see, and be unable to choose.

So what we really need is a shower of something less random, like a box of cereal being poured or something.

Hourglass?

yes, same here…

Yeah, that would help. :slight_smile:

Slow PWM is easy to detect with almost any method. Fast PWM is tricky though… Whatever moving object is used, it needs to be going very fast. I find that a spinning fan is usually not fast enough to show fast PWM well, and falling water typically isn’t either. The camera could see the individual frames when doing a close-up shot and zooming into the image, but it’s hard to see by eye.

For seeing it with low-tech tools, I’d recommend something long and very thin which can be waved through the beam. Thin to reduce the speed necessary to put gaps between frames, and long to help reach higher velocities.

I don’t really need to do it this way any more, since work gave me a really nice DMM with a frequency function. But before that, I found that something like a long thin plastic card worked well. I mostly used a plastic ruler which came with V’s glasses. Here’s how that looks with slow PWM and faster PWM:

My el-cheapo DMM (ZT102) has frequency and duty cycle functions and they seem to do pretty good. I was pretty impressed. Kinda handy when doing driver dev and testing.

Oh, nice. My cheap DMM doesn’t do that… maybe I should get one of those instead. :slight_smile:

I didn’t realize DMMs can do that. In fact, I think mine can!

Honestly, I didn’t really realize either until about a month ago. “I wonder what this one is for…? Ohh!” :person_facepalming:

Yeah, it’s pretty easy to measure PWM speed with a DMM if it has a frequency function. Take the head of a light, like a Convoy S2+ or similar, with a normal LED and reflector inside, but no driver. Run wires from LED+ and LED- out the back end. Connect a DMM to it, and you now have a light-sensing device.

After that, basically just turn it on and point another flashlight at it.

This works because LEDs don’t just convert electrons into photons. They can also run in reverse, converting photons into electrons. They’re not very efficient in reverse, but it’s good enough for measuring PWM.

I think I have a light sensor from an old Arduino kit somewhere, but this sounds more fun.