Screen flicker, PWM dimming and why some phones give you a headache

Some people find that a particular phone gives them sore eyes or a headache after an evening of reading, and that another phone with an apparently similar screen does not. The usual explanation offered is blue light. The more likely one is flicker: the method the display uses to dim itself.

Two ways to make a screen darker

A display can reduce brightness by reducing the current through the emitters, or by switching them fully on and off very rapidly and varying the proportion of time they spend on. The second method is pulse-width modulation, and it is widely used on emissive panels because driving the emitters at low current shifts their colour — a panel dimmed that way develops a visible tint, worst at the lowest settings.

Switching avoids the colour shift by keeping the emitters at a consistent drive level whenever they are on. The cost is that the light is no longer continuous. It is a rapid train of pulses, and the lower the brightness, the smaller the proportion of each cycle the screen is lit.

Why it matters at night

The effect is strongest at low brightness, which is exactly when people read in bed. A phone that is comfortable during the day can be the source of the problem at night, which makes the cause hard to identify — the phone is the same, the setting is not.

What determines whether you notice

The frequency of the switching. A low frequency means long dark intervals, which the visual system can detect indirectly even when nothing appears to flicker. Higher frequencies are easier on the eye, and some manufacturers now advertise a high-frequency dimming mode for exactly this reason.

The depth of the modulation also matters: a panel that dims between fully on and fully off is harsher than one that modulates between two non-zero levels. Two phones quoting the same frequency can therefore differ in how they feel.

Individual sensitivity varies enormously, and it is not a matter of imagination. Some people are entirely unaffected; others get reliable symptoms within minutes from a specific panel. There is no point arguing about it — the only useful test is whether a given phone bothers a given person.

How to test a phone before buying

• The camera test. Point another phone's camera at the screen, set the first phone to low brightness, and look at the preview. Rolling bands moving across the frame indicate switching. Faster, fainter bands suggest a higher frequency; broad, dark, slow ones suggest a low one. It is crude but it separates panels quickly.
• The pencil test. Wave a light-coloured object quickly in front of a dimmed screen in an otherwise dark room. Continuous light produces a smooth blur; pulsed light produces a series of discrete images.
• Read on it. Half an hour of reading at low brightness in a dim room is the test that matters. If you have a headache afterwards, no specification will talk you out of it.

What helps if you are sensitive

Look for a high-frequency dimming setting and turn it on; several makers include one, often buried in the display or accessibility menus. Raise the screen brightness and lower the contrast of what you are reading instead — a dark theme at moderate brightness is often more comfortable than a bright theme at minimum. Use a warm colour filter, which does not address flicker but does reduce overall intensity. And do not read in a completely dark room: ambient light lets you run the screen higher, which puts the panel in the part of its range where modulation is shallowest.

If none of that helps, the panel technology itself is the variable. Panels that dim by varying current are steadier by nature, and are common on the lower-cost end of the market for reasons that have nothing to do with eye comfort. That is one of the few cases where a cheaper phone is genuinely the better choice for a particular person, and it is worth knowing before spending more rather than after. Comfort specifications of this kind sit alongside the ones covered in what a brightness figure means — both describe the panel in conditions the headline number ignores.