LED Backlight flicker - common & dangerous?

[jericho64]

I became aware of the 'problem' of LED monitors flickering thanks to this YouTube video.

This video put me off buying said monitor, and I recently purchased a ViewSonic VX2370Smh-LED 23" for a decent price.

After doing a bit of further reading I see the 'problem' is caused by lowering the brightness on an LED LCD monitor (and how the tech works to do this) and isn't specific to this Dell.

I've used my phone camera to look at my monitor and I see flickering similar to the YouTube video on the monitor I have bought.

Now, I don't seem to obviously be able to detect the flickering with my eyes directly, but I've done some reading and some people say this can cause eye strain, etc.

My eyes do seem to feel more tired when using the new monitor but I believe this could be A) Because I've read about the issue and the effects it can have B) I've gone from a 19" to 23" screen with brightness higher than I usually use (as I believe this causes less flicker from my reading).

I'd be really interested to get any opinions as I am half tempted to return the monitor, and surely it has implications for many people who use these monitors if its all true.

[ik9000]

Reading the comments some folk say the newer revisions are supposed to have fixed the problem. To find out do what the guy has done - get a camera and film it. See what the playback looks like.

[jericho64]

Reading the comments some folk say the newer revisions are supposed to have fixed the problem. To find out do what the guy has done - get a camera and film it. See what the playback looks like.

To clarify I don't have the monitor that is on the video, but I have filmed my monitor (Viewsonic) through my phone camera and see the same lines that are on that video.

Interestingly though if I turn the camera the lines go the other way. E.g. film with the phone held horizontally and lines go down the screen, if I hold the camera sideways the lines go sideways and if I hold it diagonally the lines, yep, go diagonally.

Would be interesting if a few other people can try this out and see what happens..

[ik9000]

I pinged the guys at TFTCentral.co.uk an email and they gave the following reply:

Thanks for the email. We have a full article about the situation which you can read here: http://www.tftcentral.co.uk/articles...modulation.htm

and we have tested for backlight flickering caused by PWM ever since in all our reviews See the “panel” section of the reviews since Feb 12

so you might find your monitor is reviewed and their findings re this behaviour.

And with the lines changing with orientation of your camera - I wonder if you're getting an effect of the capture rate + screen refresh of the camera itself. Others more in the know will be able to assist further I'm sure <bows out and wishes he knew more about these things>

[jericho64]

I pinged the guys at TFTCentral.co.uk an email and they gave the following reply:



so you might find your monitor is reviewed and their findings re this behaviour.

And with the lines changing with orientation of your camera - I wonder if you're getting an effect of the capture rate + screen refresh of the camera itself. Others more in the know will be able to assist further I'm sure <bows out and wishes he knew more about these things>

Thanks for this.

I've fired them a quick email as they don't have a review of my monitor, hopefully they don't mind.

I've requested a return which has been accepted by the seller.

I'm just not sure what to do. Its a shame as it seems a decent monitor. The only way I'd know if it caused any problems to me would be to use it for a while which may cause problems should I want to return it.

TBH was expecting a few people to tell me theres no danger when I posted this but it doesn't' seem very active in here!

[Lee H]

Do you notice any flicker when not looking through a camera or a phone?

If not, its nothing to worry about - all LED flickers, just so fast your eyes can't detect the changes.

[ik9000]

Do you notice any flicker when not looking through a camera or a phone?

If not, its nothing to worry about - all LED flickers, just so fast your eyes can't detect the changes.

I would agree with this - if you can use it for a long period and your eyes don't hurt reading text documents and watching playback then it's probably all good.

[Pob255]

There's one thing that article doesn't cover is that this has been deliberately used by a few manufactures to increase the response times.

[kalniel]

There's one thing that article doesn't cover is that this has been deliberately used by a few manufactures to increase the response times.

Or decrease perceived response times Like black frame insertion.

[ik9000]

Or decrease perceived response times Like black frame insertion.

sounds painful! Like one of those embarrassing medical tales my Doctor friends tell me about....

[peterb]

Most lights powered from an AC power source exhibit flicker to a greater or lesser degree. In the UK it will be flicker at 100Hz. However the eye/brain does not consciously detect flicker above around 25Hz, although fluorescent tubes can sometimes be seen to flicker if they are looked at from the edge of the eye as peripheral vision is more sensitive to movement (which in effect is what flicker is)

In incandescent lighting, the thermal lag of the filament smooths out the pulses of light, and in older CRT televisions which had a flicker 25Hz, longer persistence phosphors achieved the same effect.

If you can visible detect flicker in a monitor, then it should be rejected, but if you can't (and all monitors are switching pixels on and off) then I doubt it is anything to worry about.

[DanceswithUnix]

The bit in that article about using a linear voltage to drive the LEDs is I think fantasy, it would make the monitor less efficient, less reliable and harder to manufacture (hence more expensive) so people wouldn't buy it. Trying to get an even light across all the LEDs around the display would be a nightmare. I suspect that macbook they site as being linear is actually just driving the pwm at something like 10KHz. That would require some care in the driving circuit, at least not to start radiating radio emissions like crazy, but should be relatively easy.

I have worked with cheap consumer displays in the past which were all LED backlit (on a small monochrome display it is easy and cheap). I used error diffusion techniques to drive the backlight, so that camera trick would produce some very funky patterns on one of my displays I did that because at very low brightness levels I didn't like the simple PWM output, at normal levels I just couldn't see it though.

[ik9000]

Most lights powered from an AC power source exhibit flicker to a greater or lesser degree. In the UK it will be flicker at 100Hz. However the eye/brain does not consciously detect flicker above around 25Hz, although fluorescent tubes can sometimes be seen to flicker if they are looked at from the edge of the eye as peripheral vision is more sensitive to movement (which in effect is what flicker is)

In incandescent lighting, the thermal lag of the filament smooths out the pulses of light, and in older CRT televisions which had a flicker 25Hz, longer persistence phosphors achieved the same effect.

If you can visible detect flicker in a monitor, then it should be rejected, but if you can't (and all monitors are switching pixels on and off) then I doubt it is anything to worry about.

The TFTCentral article linked above discusses this 25Hz thing- it's not actually true. We can see much higher frequencies, and not just peripherally. I thought the threshold for non-peripheral was circa 50Hz and peripheral can detect even higher.

[DanceswithUnix]

The TFTCentral article linked above discusses this 25Hz thing- it's not actually true. We can see much higher frequencies, and not just peripherally. I thought the threshold for non-peripheral was circa 50Hz and peripheral can detect even higher.

It is a very personal thing. I think my threshold is slightly lower, as I have always been able to tolerate cheap CRT monitor flicker with ease. Others have told me they couldn't stand CRTs below 75Hz, and I have to believe them.

[Pob255]

On CRT's it wasn't just about consciously perceived flicker it was about the autonomic perceived flicker.
ie you don't "see" the flicker but your eyes and nervous system do pick it up.
In my case a 60hz CRT wouldn't look like it was flickering but would trigger headaches after extended periods, up the rate to 75hz and the headaches wouldn't occur.
CRT was a different effect because it was a complete top to bottom single pass of the electron beam, yes tft screens do a single pass as well but it's a change state pass of the lcd matrix, not a phosphor luminescence trigger of scanning electron beam.

As to flicker generated by AC current, the problem is that while a monitor is powered by AC current the panel is not, it's powered by DC, a monitor has a power board in it (or a separate power brick) to rectify the AC into DC.
By reducing the voltage to the back light LED's you could reduce the brightness of them in a flat manner, but the range of brightnesses is not a great and dynamic control is far more complex to achieve than it is by PWM control.

[DanceswithUnix]

The links here were about the panel being uncomfortable to use rather than actually seeing flicker, so sounds like much the same thing. As I said, I am not that sensitive to flicker (or headlamps at night, possibly related?) hence my comment that this is a personal thing which you can't read off a review unless you have calibrated your own tolerence somehow.

Sadly the money I save on cheap monitors has to go into expensive speakers, even in my 40's I seem to have quite a high frequency range of hearing and can't tolerate sloppy speaker design

My point with voltage controlling LEDs is that each LED will respond differently to a given current, each one will have a slightly different turn on voltage. So each LED would require its own regulator transistor at least, and even then you would probably have to do some sort of binning of the devices to get matched sets of diodes. Nah, just turn them on and off. If it flickers, turn them on and off faster.