LED backlight flicker

[nahero]

Posted this in Samsung SA850 thread, but I'd like to see some comparison with other LED backlit monitors, so here are the pics of my Dell U2412M at various brightness settings - you can clearly see the on and off states of LEDs, which at 35% looks like the monitor has a refresh rate of around 60Hz.
Pics were taken with shutter speed of 1/25 sec.

And the original picture used for the test - just a 2px white line on a black background:
http://dl.dropbox.com/u/2564500/Pics/flicker_test.jpg

The method is simple - just display the picture full screen, focus with your camera and then take the picture while moving the camera from left to right relatively fast. Change the brightness level and repeat.

 

[10e]

You can also put up a white picture and wave your open hand in front of it. I agree the Dell U2412M has back-light flicker but it's nowhere near 60hz.

The Samsung SA850s pass this test I listed above at almost every brightness level that I tried above 20.

 

[Mr.Pixel]

You can also put up a white picture and wave your open hand in front of it.

The problem with this is that the ability to see flicker varies a lot from person to person. I know some people who can't see LED lights flickering at 120Hz even when trying, while I find the same extremely distracting. Using the camera gives a more consistent way of measuring it.

 

[John721]

Not to derail things, but, is flicker a concern with most LED backlit LCD displays (monitors and televisions both)? The possibility of increased eye strain is a definite turn off for me with the LED backlighting technology.

Edit: Thanks for the info, everyone.

 

[tbNB]

I'm very sensitive to CRT 60hz flicker, so much so that I would go around work and bump the refresh rates to 85hz if their video card allowed, whether they noticed it or not!

I turned my U2412M down to zero and do not notice that same flicker when viewing the monitor with peripheral vision where it was most noticeable/detectable with CRTs. Now if I wave my hand or dart my eyes around, yes, I see the strobe effect which is not terribly bothersome at all. But I'm not playing a lot of games yet either but using it currently for photography, web and videos.

 

[nahero]

@10e could you post photos as well, just for the fun of it?
Though I must say I'm convinced about SA850 when it's coming from you.

@John721 - LED flickering is not distracting when watching movies or playing games, but when reading text or working with graphics it can be very distracting. Again, some are very sensitive to this, some are not bothered at all. With these photos you can see for a fact that flickering is there, but you should try it for yourself.

 

[10e]

The problem with this is that the ability to see flicker varies a lot from person to person. I know some people who can't see LED lights flickering at 120Hz even when trying, while I find the same extremely distracting. Using the camera gives a more consistent way of measuring it.

I agree. I'm not that sensitive to it, but the test I do is the pencil or finger test where I wave one in front of a white screen at different brightnesses. If I see separate distinct edges I know there is flicker.

I'm very sensitive to CRT 60hz flicker, so much so that I would go around work and bump the refresh rates to 85hz if their video card allowed, whether they noticed it or not!

I turned my U2412M down to zero and do not notice that same flicker when viewing the monitor with peripheral vision where it was most noticeable/detectable with CRTs. Now if I wave my hand or dart my eyes around, yes, I see the strobe effect which is not terribly bothersome at all. But I'm not playing a lot of games yet either but using it currently for photography, web and videos.

Same here. I was notorious for telling people that they had flickering CRTs back in the 90's at work and fixing it. I also never noticed a flicker or eyestrain due to the U2412M.

@10e could you post photos as well, just for the fun of it?
Though I must say I'm convinced about SA850 when it's coming from you.

I an take photos if you'd like. But I'm lazy so they will be at zero brightness

 

[poindexter]

@all: I'd like to suggest we post the shutter speed used in the photos, so we can get an approximation of the Hz the PWM is operating at.

@10e, I'd also really appreciate some photos of the SA850 displays

Two displays I have tested with the waving test and with a camera that do not appear to use PWM at all are the 1366x768 (cheapest) display on the Thinkpad T520 and the display on the 27" iMac (model # iMac12,2). My camera has no shutter speed control, so the images are terrible, but I can't see any evidence of PWM.

 

[nahero]

@poindexter - the photos were taken with 1/25 shutter. Edited the post.

 

[ToastyX]

Not to derail things, but, is flicker a concern with most LED backlit LCD displays (monitors and televisions both)?

This is nothing new. Any monitor that uses PWM for brightness regulation will flicker at a lower brightness, and pretty much every LCD monitor uses PWM, including CCFL-backlit LCD monitors. The question is how bad is the flicker, and that depends on the particular implementation, so different panels can have different results.

 

[Mr.Pixel]

Not to derail things, but, is flicker a concern with most LED backlit LCD displays (monitors and televisions both)?

The average flicker among displays may be getting worse in some ways. As brighter backlights are introduced and LCDs transmit more light it is necessary to use a smaller duty cycle (smaller fraction of the time on during each cycle) to maintain the same brightness level.

 

[poindexter]

@poindexter - the photos were taken with 1/25 shutter. Edited the post.

Thanks. In your original post you said that it looks like 60Hz at 35%? Do you mean the PWM frequency or the regular refresh rate?

For example, I count roughly 18 on/off cycles in each of your pictures (except 100%). I think this would be 18/(1/25) = 450Hz for the PWM frequency for all brightness levels under 100%

 

[nahero]

Yes, it "looks" like 60Hz to me, but I wrote that to simply illustrate what I mean. Yes, the LEDs operate at a fixed frequency of 450Hz (or 430Hz I think), but what you actually see in the photos are ON (white lines) and OFF intervals generated by the PWM algorithm. The lower the brightness, the shorter the ON interval, thus the white lines are narrower. It's how the human eye and brain percieve that kind of strobe effect.

 

[tk-don]

This is nothing new. Any monitor that uses PWM for brightness regulation will flicker at a lower brightness, and pretty much every LCD monitor uses PWM, including CCFL-backlit LCD monitors. The question is how bad is the flicker, and that depends on the particular implementation, so different panels can have different results.

CCFLs show much less flicker due to the "memory effect" of the phosphors used to produce the red green an blue light spectras. Energy saving bulbs / ccfl lamps still emit light about one minute after they have been turned off. No matter what kind of PWM duty cycle one uses, a LED turns off nearly instantly - about as fast as a transistor can switch.

One method would be to drive each LED individually, so that none are in phase with each other. I.e. at 50% duty cycle, every other LED should be driven 180 degrees out of phase. At a 25% duty cycle, each should be 90 deg out of phase compared to the "neightbor" LED, etc... But it's much much easier to just apply the same PWN signal to all LEDs

 

[10e]

As requested, S27A850 @ 0% brightness, 1/25 camera speed.

 

[poindexter]

Thanks, 10e. It looks flicker free even at 0% to me.

Hopefully prices for this monitor will fall eventually. $800+ is a bit rich for my blood.

 

[nahero]

Totally flicker free, thanks 10e!

 

[Comixbooks]

CCFLs would make me sick to no end.....couldn't use them for 9 years untill led lcds came out.
Used a projector for games for two years or so then CRT for everything else.

 

[Mr.Pixel]

Thanks for the pic 10e. [strike=on]Looks like it is indeed flicker free![/s]
Edit: seems it does flicker (see post by charles_k).

 

[Mr.Pixel]

In case anyone finds it useful there is a new article about PWM and backlight flicker at TFTCentral: http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm.

Disclaimer: I wrote a good chunk of the article. That said, I did it with the intention of helping people to understand more about the technology, and it's available for anyone to read.

 

[charles_k]

I just registered to let you see what I get on the s27a850d

So I don't think we can call this flicker free

And I get eye strain by watching this display so I'm returning it

 

[Mr.Pixel]

Too bad, I was just about to buy one. Looks like about 180Hz flicker. Thanks for the info Charles_k.

I'm off to buy a catleap and modify it.

 

[charles_k]

That's what I was thinking to do, actually I already bought a shimian and resold it because of the eye strain (160hz pwm), but just after that I wondered if I can make a standalone current regulated led driver and use it instead of the original pwm led driver.
But i not really good with electronics, can I ask what you plan to do and especially how ?

 

[Mr.Pixel]

charles_k: Let me know if that works out for you. I don't have the necessary skills (yet).

I was going to buy a large neutral-density filter that would cut 1-1.5 stops and place it between the diffuser and the LCD itself. That should bring the max 350cd/m2 brightness down to 175-123 cd/m2, close enough to just calibrate out the remainder. Don't know if there are any long-term heat or bleaching effects though.

 

[charles_k]

charles_k: Let me know if that works out for you. I don't have the necessary skills (yet).

actually ...me neither , I'll have to ask one of my acquaintance, and/or on forums.

I was going to buy a large neutral-density filter that would cut 1-1.5 stops and place it between the diffuser and the LCD itself. That should bring the max 350cd/m2 brightness down to 175-123 cd/m2, close enough to just calibrate out the remainder. Don't know if there are any long-term heat or bleaching effects though.

oh that may be a good idea , I didn't know those filters exited, but those are mostly for camera ?, Is it possible to find a filter large enough for a screen ?

 

[Mr.Pixel]

Some makers do produce 24" by 21" thin (flexible?) sheets. The problem is making sure they are thin enough and don't polarize. In the process of looking...

 

[charles_k]

ok, I'll be thankful if you can keep me informed on what you can find

edit : these pics may be helpful : http://www.overclock.net/t/1225919/yamakasi-catleap-monitor-club/1400_40#post_16795542

edit2 : I guess you have the same user name on OCN, I've just seen your post in the "Yamakasi Catleap Q270 (100hz guaranteed)" thread

 

[k9wazere]

I take it that all LED monitors tested thus far have discernible "pencil test" flicker?

I've just tried it with the Benq GW2750 I'm about to return, and sure enough that gives a strobing effect.

And it gives me massive eye strain.

 

[Crake8]

You can also put up a white picture and wave your open hand in front of it. I agree the Dell U2412M has back-light flicker but it's nowhere near 60hz.

This is weird. I have a new U2312HM which should be the same monitor in this respect. I can turn down my brightness to 0 and see no strobe affect while waving a pencil/finger/pen in front of it but if it I wave the same object in front of my 2005 Viewsonic IPS monitor I see strobing effect with anything under about 80-90 brightness and with a BenQ GW2450 that I recently tried, I saw strobing effect under 90 brightness. (Note, I have no camera to check it or can you do it on an Ipad?)

Did Dell change or is that test not really valid?

 

[Crake8]

This is weird. I have a new U2312HM which should be the same monitor in this respect. I can turn down my brightness to 0 and see no strobe affect while waving a pencil/finger/pen in front of it but if it I wave the same object in front of my 2005 Viewsonic IPS monitor I see strobing effect with anything under about 80-90 brightness and with a BenQ GW2450 that I recently tried, I saw strobing effect under 90 brightness. (Note, I have no camera to check it or can you do it on an Ipad?)

Did Dell change or is that test not really valid?

I took photos at 30% brightness and 0% of my U2312HM with an Ipad II's standard camera. Is an Ipad photo valid for this test because mine shows no flicker (and this is consistent with my waving tests - see above), photos below. I would think that if the photo produced that wide of line as I quickly moved the Ipad from right to left as I pushed the shot button, it would have shown pulse-gaps if the monitor's PWM produced them, no?


30% brightness:

0% brightness:

 

[Mr.Pixel]

I take it that all LED monitors tested thus far have discernible "pencil test" flicker?

Most seem to, but not all of them. The 27-inch pre-ThunderBolt Mac cinema display had no flicker that I could see (though I never did a photo test to be sure). I'm almost positive it does not use PWM.

This is weird. I have a new U2312HM which should be the same monitor in this respect.

The U2412M and U2312HM use different panels, and are even different resolutions.

Is an Ipad photo valid for this test because mine shows no flicker (and this is consistent with my waving tests - see above), photos below.

Should be valid, but would need to know exposure time to be sure.

 

[CrabJuice]

I took photos at 30% brightness and 0% of my U2312HM with an Ipad II's standard camera.

Very interesting. Would you mind checking the sticker on the backside of the monitor as where and when the monitor was made?

Easiest proof of concept here would be to test the Ipad camera on another monitor that fails the pencil test.

 

[Crake8]

Very interesting. Would you mind checking the sticker on the backside of the monitor as where and when the monitor was made?

Version: Rev 05 Date: May 2012 Location:I saw no location of manufacture anywhere on the back label.

Easiest proof of concept here would be to test the Ipad camera on another monitor that fails the pencil test.

I had the same idea and I tested. The same iPad camera showed on-off bars in a Viewsonic display that does show strobe effect with the pencil waving test. See my pictures of the Dell and Viewsonic in this thread I started:http://hardforum.com/showthread.php?t=1702487