Screen Flicker (PWM) Test

How to check a screen for flicker

Many screens dim by switching their backlight on and off very quickly, a method called pulse-width modulation (PWM). The "off" gaps are longest at low brightness, which is when any flicker is most likely to show. Your eyes usually cannot see it; a phone camera often can.

1. 1

   Open the white field full screen and set the screen's own brightness to its maximum.

2. 2

   Point your phone's camera at the screen, in ordinary photo or video mode. You do not need to take a picture.

3. 3

   Lower the brightness in steps, to about half and then toward the minimum, watching the phone's screen as you go.

4. 4

   Look for flicker, stripes or dark bands on the phone's screen that appear or grow as the brightness falls. Repeat with the gray fields for a second look.

Reading what you see

A steady picture on the phone at every brightness suggests the screen is not using low-frequency PWM. Stripes or bands that grow as you dim it suggest it is. That is all this check can tell you: it does not measure a frequency, different phones show it differently, and a screen can still use PWM at a speed the camera does not catch.

This page does the showing and your camera does the detecting. The fields never change by themselves: you move between them with the buttons, the arrow keys, or Space in fullscreen.

Why nothing flashes here

A flicker check does not need a flashing page, only a steady one, so this page never flashes. The only tool on ScreenTail that does is the Stuck Pixel Fixer, which stays behind a warning; Photosensitivity & Safety explains how it is limited.

Screen Flicker and PWM explains how PWM and DC dimming work and what "flicker-free" means from maker to maker, and Eye Strain: Settings That Help covers the other screen settings that affect comfort.