How to determine if a screen has backlight controlled by PWM? - Page 1
Forum section: electronics / beginners

author: jancelot
author_rank: Regular Contributor
timestamp: October 07, 2016, 09:09:31 pm
content: Just asking the question, how to detect if a screen or monitor (LED backlighting probably) has Pulse Width Modulation to control brightness? This is known to cause eye fatigue.

author: Kjelt
author_rank: Super Contributor
timestamp: October 07, 2016, 09:16:33 pm
content: You need a fast lightsensor to see the pwm signal in the light, perhaps an ldr might work often some usb or video cameras detect the interference with their own refreshrate.

author: electr_peter
author_rank: Unknown
timestamp: October 08, 2016, 11:29:54 am
content: There is no need for expensive tools in such case. Eye fatigue is caused by relatively low frequency PWM which is easily detectable with human eye.To test if display brightness control uses PWM, do this:set display to the lowest brightness settingtake a pencil, ruler or some long narrow object and wave it quickly in front of the screen. You can use your finger or a hand.Take a careful look at a wave pattern - if it is smooth all the way, display uses very high frequency PWM or current control method -GOOD. If a pattern is light-dark-light patches, it is low frequency PWM -NOT GOOD.For this test use lowest brightness on display (likely PWM with low duty cycle). Highest brightness setting may not use PWM (PWM with close to 100% duty cycle).

author: Kaptein QK
author_rank: Regular Contributor
timestamp: October 08, 2016, 08:35:13 pm
content: If you have glasses, just shake them with your hand and look for multiple/repeated patterns

author: jancelot
author_rank: Regular Contributor
timestamp: October 08, 2016, 08:42:39 pm
content: Quote from: Kaptein QK on October 08, 2016, 08:35:13 pmYou must be jokingI think there could be two ways, one measuring some way the motherboard, and the other using high speed cameras that allow to adjust the shutter speed from very low to very high values, and capture until you notice something (if).

author: jancelot
author_rank: Regular Contributor
timestamp: October 20, 2016, 04:56:55 pm
content: After some google search I've found a great blog that talks a lot of PWM used in monitors or screens, and performs a lot of tests. Impressive stuff from that guy:Retro-PC-ManiaNow, in order to make some personal tests in a more professional way, I've bought from eBay the two following photodiodes, which I'll test after being received:OSRAM BPW 21VISHAY BPW 34You are right too that moving rapidly a pen over the dimmed screen it results kind of robotic movement if it has PWM. Also, using a smartphone camera with manual shutter it can be noticed too as you increase the shutter speed.

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2016, 05:02:05 pm
content: White LED phosphors have enough persistence to prevent the PWM from being visible unless it is slow which is certainly a possibility.

author: dannyf
author_rank: Super Contributor
timestamp: October 21, 2016, 09:09:20 pm
content: Use a fast light sensor like photodiode or a photo transistor. A slow sensor like a photo resistor shall not apply.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 21, 2016, 09:17:39 pm
content: photodiode + oscilloscope will let you see whether or not it's PWM and its frequency very easily. Something like this works perfectly:https://www.sparkfun.com/products/9541I used one of those to measure my laptop's screen not too long ago because I was curious about the same thing. It turns out that it used both PWM and linear depending on brightness. The dimmest two brightness settings were 220 Hz PWM, the other ~8 brightness settings were linear. That's on a 3rd gen Lenovo X1 Carbon.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 21, 2016, 09:37:48 pm
content: Quote from: jancelot on October 07, 2016, 09:09:31 pmWith a rolling shutter camera. Like in your phone.Look at the horizontal lines in the attached picture.They disappear at 100% brightness.Install this anyway.https://justgetflux.com/

author: tooki
author_rank: Super Contributor
timestamp: October 22, 2016, 12:16:22 pm
content: Quote from: David Hess on October 21, 2016, 05:02:05 pmWhat's "slow" to you? I readily notice PWM flicker at 1KHz, and really am not happy until 5KHz, where it really no longer appears when saccading.LED phosphors may have some persistence, but given how frequently I notice PWM flicker in LED lighting installations, I don't think it'smuchpersistence.

author: David Hess
author_rank: Super Contributor
timestamp: October 22, 2016, 06:08:06 pm
content: Quote from: tooki on October 22, 2016, 12:16:22 pmSlow to me is 100s of Hz. I wonder if something else is going on like aliasing between the backlight PWM frequency and the LCD refresh.

author: bktemp
author_rank: Super Contributor
timestamp: October 22, 2016, 08:16:31 pm
content: Quote from: David Hess on October 22, 2016, 06:08:06 pmThe decay of the phosphors used in white LEDs is quite fast, typically in the lower µs range. That do not help with flicker.There are two common methods used for PWM dimming: Either using a high frequency (>1kHz), or using a low frequency locked to the framerate, typically 3x framerate (~180Hz). Both methods avoid beat frequencies in the visible range.

author: Kjelt
author_rank: Super Contributor
timestamp: October 24, 2016, 12:06:37 pm
content: And you have the random pwm as i name it, the offset to the pwm start changes randomly or the pwm on wave is split in multiple smaller parts.

author: bktemp
author_rank: Super Contributor
timestamp: October 24, 2016, 12:19:44 pm
content: I have never heard of random pwm, but your description sounds a bit like delta-sigma modulation. There is a patent about random pwm dimming, but they use multiple led strings each with a different pwm frequency.

author: jancelot
author_rank: Regular Contributor
timestamp: November 22, 2016, 10:10:07 pm
content: Hamamatsu sells optical sensors too:http://www.hamamatsu.com/eu/en/index.html

author: MK14
author_rank: Super Contributor
timestamp: November 22, 2016, 10:30:54 pm
content: Quote from: jancelot on October 07, 2016, 09:09:31 pmAlthough you can use the various methods already in this thread.Another way, is to use Google + the exact model number of the model. E.g. Written on the back of it.There are some very good websites, which tell you (reviews), what if any PWM is used in the backlight. Failing that then the photodiode (maybe phototransistor, but LDR's are probably too slow to react, but might work) + a scope is probably good, as mentioned in earlier posts.You can get monitors, such as some of the Benq ones, which don't use PWM (if I remember correctly).