Monitor Flicker

11th April 2018 Colour Accurate Monitors

Back in the dark ages of CRT monitors, flicker was a constant problem. CRT monitors, put simply, worked by using a ray of electrons to excite excitable phosphors to display colours. The electron beam would sweep, in lines, back and forth across the display from top to bottom. This refresh cycle would occur at various speeds depending on the quality of monitor you had and the resolution you were running on it. A lot of monitors refreshed at 60hz, meaning flicker was constant, especially to those of us with sensitive eyes.

Better CRTs offered 85Hz, 100Hz, even 120Hz refresh cycles. These were a lot more stable and comfortable to use, but if you looked ‘off axis’ – i.e. with the side of your eyes, you could still easily see the flicker. (If you recall old video footage of monitors with nasty looking flicker – this is because of the correspondence between the video frame rate and the monitor flicker rate. That is, say you’re recording at 30 frames per second, and the monitor is refreshing at 85hz, some of your 30 frames on the monitor will be lit, some won’t, some only half lit etc, so you get the weird half lit horrible flicker issue you see on a lot of old documentaries!).

Now – LCD monitors, those with fluorescent lights at least, pretty much solved this issue. LCD monitors are, in essence, a bunch of tiny colour filters in front of a light source – the backlight as it is termed. The backlighting used in these was effectively steady, so no apparent flicker. Bliss, and much less eye fatigue! But in truth they DID have their own flicker issue, just well masked by an aspect of the behaviour of fluorescent lights.

Here’s how:

With LCD monitors, brightness is mostly controlled with a cheap, cheerful and generally effective technique known as PWM.

Pulse Width Modulation works by essentially powering the panel on and off very quickly – so quickly the eye can’t really see it (i.e. up to hundreds of times per second), and the lower brightness you want the longer the off cycles versus the on cycles. Again, this is happening many times per second, and causes the apparent brightness of the screen to drop (which we all know is a good thing especially for print work, but also for eye fatigue in general). Now, with fluorescent backlighting, the fluorescent lights glow in-between cycles, so you don’t really get any flicker from this method of brightness control. The panel remains lit to the eye the whole time.

Now, monitor makes are busy swapping fluorescent backlights for LED backlights. These have advantages – they use a lot less power, can be controlled individually so can help with uniformity, can have advantages for white point control and so on. So in most ways, these are a very good thing. But, LED lights exhibit nearly instant on/off behaviour. So if you modulate brightness using the PWM technique, then as you head towards lower brightness, the screen will unfortunately exhibit flicker again as the longer off periods become visible to the eye. And unfortunately, so far at least, PWM circuitry has come along for the ride with LED backed monitors. Most likely because it’s known to work, and very very cheap to implement.

PWM Behaviour

Eizo EV2736W with anti-flicker circuirty
Eizo EV2736W with anti-flicker circuirty

Different people have very different sensitivities to this – indeed for maybe 95% of people it’s really no issue at all, or only an issue if they want to do something unusual like run their screen at very, very low brightness (sub 60 cd/m2). But, there are a small percentage of people who are deeply bothered – either consciously by seeing the flicker, or more commonly sub-consciously through eye fatigue and headache issues. Turns out this type of flicker can be a real trigger for migraines. Which are never good!

Anti-flicker and low blue light technology was originally invented by BenQ, and all the BenQ monitors we carry (the SW and PV lines) have these technologies.  Combined with their great value and excellent colour reproduction, we think these monitors are an ideal choice for eye sensitive imaging enthusiasts.

If you're after more of a 'business graphics' level of monitor, rather than a wide gamut imaging monitor, we offer the Eizo EV range that has anti-flicker circuity (effective down to about 60 to 70 cd/m2 – go lower than this and there will still be flicker but very few people run their screens that low anyway). We have sold these to several customers specifically sensitive to these issues - with excellent feedback. So we’re confident these are also a great choice in this area.

We have a page that highlights our current choices for monitors good for eye fatigue:

Here’s an easy way to assess the flicker of your current screen: Take your mobile phone and put it in camera mode. Hold it to your screen, and then run the brightness down from 100 to 0 percent. You will almost certainly see flicker via the camera display at some point, even those specifically claiming to be flicker free, at very low brightness. Of course this just tells you if there is flicker, not whether or not you’re sensitive to it!

Another perspective and more detail on PWM is here, along with some useful tips to reducing it.