RunTheTests
/

Laptop Screen Flicker Test

Check for PWM backlight flicker — and turn the brightness down first, which is the part people miss.

Flashing images warning

This test displays rapidly changing patterns which may trigger seizures in people with photosensitive epilepsy. It will not start until you choose to begin, and it stops automatically.

How this test works

A white bar sweeps quickly across a black field. If the backlight is lit continuously the bar blurs smoothly; if it is pulsing, the pulses strobe the moving bar into separate copies, and the gaps between them are the gaps in the light. It is the pencil test — waving a pencil in front of the screen — made reproducible without a pencil. The instruction that matters most is to lower the brightness first, and it is counter-intuitive: pulse-width modulation dims by shortening how long the backlight is on rather than by reducing its current, so the dark gaps get longer as the screen gets dimmer. Flicker invisible at full brightness is often obvious at twenty per cent.

What the results mean

Smooth blur
The backlight is lit continuously at this brightness. No PWM visible.
Separate copies
The light is switching on and off. Each copy is one pulse.
Lower brightness first
The condition that matters. PWM shortens the on period as it dims, so the effect grows.
Frequency
Not measurable. A page has no light sensor and your eye is the detector.

Common problems and fixes

I see nothing at full brightness
Expected, and it is why the brightness instruction comes first. Turn it down to around 20% and look again.
I see copies — can I fix it in software?
No. Dimming behaviour is in the display. The practical options are running brighter, which lengthens the on period, or a monitor that dims by current rather than pulsing.
Does flicker cause headaches?
For a minority of people, yes, and particularly at lower frequencies. If you get eye strain at a screen and not away from one, this is worth ruling in or out before changing anything else.

Frequently asked questions

Why is flicker worse at low brightness?

Because PWM dims by shortening how long the backlight is on rather than by reducing its current. Lower brightness means a shorter on period and longer dark gaps, so the effect grows exactly as you would not expect.

Can a website measure flicker frequency?

No. It would need a light sensor. What a page can do is move something quickly enough that a pulsing backlight strobes it into separate copies, which makes your eye the detector.

What is flicker-free or DC dimming?

A display that dims by reducing the current to the backlight instead of pulsing it. There are no dark gaps, so there is nothing to strobe a moving object.

More in Repair