Screen Flicker and PWM

Turn a laptop down to its lowest brightness, point a phone camera at it, and you may see flicker on the phone that you cannot see on the laptop itself. What the camera is picking up is the backlight switching on and off. That switching has a name, pulse-width modulation, and it is how a great many screens dim. This guide explains how it works; the Eye Strain guide covers the other screen settings that affect comfort.

How PWM dimming works

A backlight can be made dimmer in two ways: give it less current, or switch it off for part of the time. PWM does the second. BenQ describes monitors without flicker-free technology as driven by PWM, “which turns the backlight on and off rapidly to adjust the screen brightness”, and ViewSonic adds that it “works at a fixed frequency.”

The part of each cycle when the light is on is called the duty cycle. BenQ’s eye-care white paper explains what happens as you turn the brightness down:

"As a user lowers the brightness setting, the duty cycle typically becomes progressively shorter, resulting in a reduced luminance. As a result, the lower the brightness setting, the longer the 'off' periods are, and the more pronounced any flicker may become." — BenQ, Eye-care Technology White Paper

The same paper notes the other end: “At the maximum brightness setting (100%) this technique is not needed and the backlight is illuminated continuously.” ASUS puts it in one line: “Flicker is more noticeable when the display is set to low brightness settings.”

How fast is it?

Makers rarely say, and no source quoted here gives a “typical” figure for screens. Two numbers give a sense of the range. BenQ says some monitor screens “can flicker up to 250 times per second”; EIZO says some of its models use “high PWM method (over 10,000Hz)”.

Higher is generally harder to notice, but not impossible. The U.S. Department of Energy, writing about LED lighting, says that when the eye, the head or the light is moving, modulated light “can be visible even at high frequencies of 1000, 2000, 6000 Hz, or even higher.”

DC dimming and “flicker-free”

The alternative to switching the light off is to give it less current. EIZO sets the two side by side:

ViewSonic describes DC modulation as producing “a constant stream of light”. BenQ’s white paper agrees on the catch: DC control “can be more complicated to implement”, and because of difficulty controlling color in darker images, “DC backlight control is less common.”

Checking your own screen

There is a simple check that needs only a phone. ViewSonic describes it:

1. 1

   Set the screen to full brightness and show something plain and light, such as a white page.

2. 2

   Point your phone's camera at the screen, in ordinary photo or video mode, without taking a picture.

3. 3

   Lower the brightness to about half, then toward zero, watching the phone's screen. In ViewSonic's words, "any flicker will become increasingly noticeable if it's a non-flicker free monitor."

What you are looking for is flicker, stripes or pulsing on the phone’s screen that appears or grows as the brightness goes down. A steady picture at every brightness suggests the screen is not using low-frequency PWM; flicker that grows suggests it is. It is not a measurement, and different phones show it differently.

BenQ’s white paper is frank about why a check like this is useful: “Most screens will not list whether PWM is used for backlight dimming and in many cases the manufacturer may not even know.”

If flicker bothers you

This guide makes no health claim, and neither does it say that PWM is harmful. What the sources do support are practical steps:

• Keep the brightness setting higher and dim the picture another way. Since PWM flicker is most pronounced at low brightness settings, a backlit screen run nearer full brightness has shorter “off” gaps. Showing darker content, for example a dark theme, dims what you see without lowering the backlight setting.
• Match the room. Eye-care bodies note that a screen much brighter than the room around it can itself cause eye strain, flicker or not. The Eye Strain guide covers the settings the eye-care bodies recommend.
• Check before you buy. Look for what the maker says its flicker-free feature actually does, and try the phone check in the shop if you can.

On the question of health effects, the U.S. Department of Energy’s summary is the honest one: whether the people who see flicker most easily are the same people susceptible to migraines, “there is no conclusive answer from the neurology community as yet.” If screens give you headaches or discomfort, that is a question for a doctor.

A note on the standard

The document most often cited on flicker is IEEE 1789-2015, “IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers.” Its scope, as the IEEE describes it, is LED lighting and dimming, not computer screens. The IEEE now lists it as an “Inactive-Reserved Standard”, inactivated on 26 March 2026 through the administrative process for standards “that have not undergone a revision process within 10 years.”

For a steady, warm light of your own, the Amber Screen fills the display with one unchanging color: nothing on the page itself pulses or changes, though the screen’s backlight still dims however that screen dims.

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Vivek Kumar runs ScreenTail from New Delhi. He has worked with computers for about 25 years and studied Computer Science. He is not a display engineer or a color scientist, and each guide says where its facts come from so you can check them.