PWM - What is it, How does it work and how to detect it.

jon_slider

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Compare and contrast LED, Incandescent and Daylight tints, as seen by iPhone 5:

Pic 1. Phillips Hue Tri LED home lighting system (91 CRI Pulse Width Modulated) 3000K
Pic 2. 53 Watt Incandescent (100 CRI House Electric) 3000K
Pic 3. Daylight inside, 12 noon, partly cloudy (100CRI sunlight) ~6000K

IMG_0457.JPG

IMG_1601.JPG

IMG_1602.JPG


And the LED assortment I use at different times of day, photo taken also at 12 noon with overcast sun
XPG2 6000K, N219b 4000K, XPG 3000K (all drivers use NO PWM)
Note the Nichia produces the most realistic Tint match in the color of the Buckskin

IMG_1604.JPG
 
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/steve/

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Hi there!
I think I found a better way of detecting PWM and even beeing able to estimate the dutycycle!
Today everybody has a smartphone. So just open your camera app and turn on your light and point it as close as possible into your camera. (I don't know how bright the light can be to not kill the sensor, mine camera did 1W just fine). Then you see something like a black pic with white ,regularly appearing stripes(you can see a few of them at once at your screen), depending on your frequency slowly moving to one side. You can estimate the dutycycle from the width of the stripes. I saw that the stripes got more shallow on my minimaglite LED when I switched it into 25% mode.Post some pictures of it if it is working with your Phone's Camera.
have fun
PS: I could see these stripes up to somewhere about 800 HZ.
 

jon_slider

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> I could see these stripes up to somewhere about 800 HZ.

I hope you post a photo

I also use my phone
IMG_9900.JPG


this is how I use a piece of printer paper to compare beams. I no longer own any PWM lights, because the interfere with my photos
IMG_3231.JPG
 

jon_slider

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Im bringing these forward to help someone who asked in another thread, "How Can I detect PWM?"

[Eagletac]D25A on 3lms on the left. On the right is a 47s Mini - these are the two fastest PWM lights I've seen - this sweep is as fast as I can move my arm. [middle light is a known NoPWM control]

dca917438da3ea76b37ff93a5461b6d3.jpg


...
the definitive way to tell is a fast sweep across a DSLR on time exposure, side-by-side with a known fast PWM light, and a true CC light (as above). My own eye-ball detection method is catch the spill in a reflection/mirror and move my eyeballs left to right quickly - I can quite easily see the PWM on the 47s Mini, but it is very hard to detect it on the D25A.

A new way to detect PWM, use the spin cycle
see the 4minute 15second mark…

Quiz, why are there dots, instead of a constant streak of light?:)
Screen%252520Shot%2525202015-07-23%252520at%2525201.56.45%252520PM.jpg


-----
Originally Posted by Cataract
wave your light in front of your face. NOW, PWM will be easiest to detect on you light's lowest mode ...
you don't need to send the lights directly into your eyes... just hold it so you can see the light on the side of the reflector as such:
133_146ea8_afd1c9b9_oJ.jpg.thumb

Turn the lights down if necessary, so you can see the light clearly enough and now wave it sideways like this:
This is a picture of a constant current light. Any of your lights on maximum current should look exactly like that. [no dots, just blurry streak]
134_146ea7_da949891_oJ.jpg.thumb


Here's a picture of [slow] PWM:
135_146ea6_198b9f1_oJ.jpg.thumb


Here is what "[fast]" PWM looks like:

137_146ea4_add3c831_oJ.jpg.thumb

----

My iPhone sees the PWM much more obviously than my naked eye. Both lights are on Medium in this next photo:
IMG_9900.JPG


Neither light has PWM on high:
IMG_9897.JPG


When I wave the lights, the PWM is more obvious to my naked eye, than to the camera.
IMG_9898.JPG

Other ways to see the effect of PWM
Shine the light at a stream of water from a faucet, or shower. The PWM light will freeze the drops in midair, kind of entertaining. Similar in Snow.

Shine the light at the fanbelt of a car, the PWM light will seem to "stop" the belt, similar to a timing light. This also works on spinning things, like fan blades.

I find my iPhone is quite good at finding PWM if I fill the screen with the light or shoot close range. Waving also works well, its just harder to photograph.

Video also shows PWM in some exposures or angles
McGizmo Haiku:


An older Maratac:
https://youtu.be/2jL7TiIQ4yM?t=4m39s
 
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Hondo

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Is 4500hz PWM an issue?.
this is in relation to a pflexpro Nichia drop in I'm considering buying ?.
avoid?.
thanks!.

For me, and 99.XX% of people, no.

Can you still tell it from a non-PWM light if you go looking for it? Yes.

Would I buy it? Yes. Once you get over ~1000 Hz, I don't let that influence my decision at all. And I can tolerate using some of my older lights with 100 - 200 Hz PWM, although I see it a lot in use.
 

Lexel

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Is 4500hz PWM an issue?.
this is in relation to a pflexpro Nichia drop in I'm considering buying ?.
avoid?.
thanks!.

I have noticed flicker on my budget flashlighs.
I did a photo in my shower with 1/400s and could spot 11 bright and one fade point, so it is probably 4500Hz PWM.

Now I am questioning how I can see flicker at my stillstanding flshlights, on old TV´s or PC Moniotors flicker stopped at 75Hz, 60Hz was a no go for me.

IMG_8481.jpg
 

jon_slider

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fwiw, It has come to my attention that some lights that technically do not use PWM, because the Pulses do not drop all the way to Zero, are also NOT Constant Current. My goal in posting this is to eliminate the impression that just because a light does not use PWM, it does not mean it necessarily does not use Pulses that can interfere with Photos.

Examples of lights that do not use PWM by the technical definition, but also are not Constant Current lights, include HDS, Zebralight, Eagletac, and Nitecore lights, that use Pulses that dont drop all the way to zero. More details in these threads:
List Lights that use PWM and or Pulses that are NOT True Constant Current Lights
List Lights that use NO PWM and ARE True Constant Current lights.
 

Hondo

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Some of my high voltage Quarks exhibit this behavior too. But not all have it. David called it "circuit warble" .
 

ridwan_neutron

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when im using pwm in flashlight with frequency around 1khz, there is buzzing noise. so i decided to use high frequency pwm around 20 khz and the noise is gone.
 

etc

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Malkoff M361 uses a flicker rate of 310 Hz. I tries really hard to detect it. No luck with water yet, faucet.
Will try the belt in the car next.

I think there is a threshold where you cannot see PWM. 100 Hz I might see, 300, not. The statement I can see PWM is meaningless because it does not qualify the frequency.
 

RI Chevy

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Just for the record, as I have previously stated several times. I have 3 M361N's, all of them display PWM on low, and I have the audible whine on low and medium. My eyes and ears can detect it.
With that said, I really like the drop in. Tint is very nice.
 

jon_slider

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Malkoff M361 uses a flicker rate of 310 Hz. I tries really hard to detect it. No luck with water yet, faucet.
Will try the belt in the car next.

I think there is a threshold where you cannot see PWM. 100 Hz I might see, 300, not. The statement I can see PWM is meaningless because it does not qualify the frequency.

PWM is very difficult for most people to detect. There are also lights that produce "noise" that is technically not PWM, but still causes sensitive people to get migraines, such as Zebralight.

when waving a light does not show PWM, even when we know from Malkoff that it is there, another technique that sometimes works is:
taking an extreme closeup of the LED,
see this post #76 that discusses this photo of a Zebralight:
32489842195_7ae754fc63_c.jpg


"noise" is very difficult to photograph, the camera has to be really close, and at just the right distance.
Video closeups can also show "noise", but again it only works at just the right distance.

imo, it is pointless to argue whether PWM matters or not, if someone does not notice it.
for me, if the light has PWM, or pulses that produce "noise", then it is not Constant Current.
I leave it up to each individual to decide if PWM or "noise" is acceptable.

many, if not most, LED lights use PWM or pulses that produce "noise". Some people get very hostile about being told about it, and make all kinds of excuses why it does not matter to them, and should not matter to me. Im not going to argue about whether PWM matters to me, or someone else, because the conversation becomes hateful and unproductive. Many people Love their Zebras, HDS, Malkoff, etc, and they dont care about "noise" or PWM. So I just report what I find, without expecting to change hearts and minds.
 
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Hondo

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Malkoff M361 uses a flicker rate of 310 Hz. I tries really hard to detect it. No luck with water yet, faucet.
Will try the belt in the car next.

I think there is a threshold where you cannot see PWM. 100 Hz I might see, 300, not. The statement I can see PWM is meaningless because it does not qualify the frequency.

If it does not bother you, just enjoy.

I have no problem finding 2,000 Hz PWM when I go looking for it, and I can show it to others as well. I only use the method of holding the light on about a 45 degree angle so I can see into the reflector from the side, and whipping it up and down looking for the "dotted line" of light (as in many photos above, including mine).

Personally, I can use a 100 Hz light. When I move about, I am aware of the strobe-like effect, but it does not give me a headache or anything. But both my wife and I find the 100 Hz PWM LED car tail lamps irritating while driving at night.
 
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