Flicker test
Most monitors dim their backlight by switching it on and off hundreds of times a second rather than actually running it dimmer. You cannot consciously see it, but your eyes respond to it — and it is one of the most common causes of screen headaches that people never trace to the screen.
This test flashes
The screen will flash rapidly. Do not run it if you have photosensitive epilepsy or are sensitive to flashing light.
The test opens full screen. Space — next · Arrow keys — back and forward · Esc — exit
Your browser blocked full screen. The test still works in this window, but a full-screen view is more reliable — try pressing F11.
Did the moving bar break into separate copies at any brightness?
What this test does, and what it cannot do
It cannot give you a frequency. No web page can, and anything that offers you one is inventing it.
What it does is show a uniform grey field with a thin bar sweeping across it very fast, at five brightness levels from 100% down to 8%. Under a steady backlight the bar is a smooth blur. Under a pulsing backlight it breaks into a row of distinct copies, one per pulse.
Press space to step down through the brightness levels. Work down from the top: flicker gets worse at lower levels, and the level at which it first appears is useful information in itself.
Set your monitor own brightness low before you start. The field levels in the test simulate a darker image, but PWM responds to the monitor backlight setting, not to what is being displayed. A monitor at 100% brightness may show nothing at all here even if it flickers badly at 20%.
Three ways to check, in order of reliability
- Slow-motion phone video. Record the screen at 240 fps with the monitor dimmed. Rolling dark bands or a pulsing image means PWM. This is the closest thing to proof available at home.
- The pencil test. Wave a pencil in front of a bright uniform area. Multiple sharp pencils means pulsing; one smooth blur means steady.
- This test. Fastest to run, and enough to tell you whether the other two are worth doing.
All three rely on the same principle: a pulsing light samples motion in discrete slices, so anything moving fast enough resolves into separate images.
What to do if your monitor flickers
Raise the backlight brightness and lower the contrast instead. This is the single most effective change. Running the backlight at 80% and reducing perceived brightness through the monitor contrast or a software gamma adjustment keeps the PWM duty cycle high, where modulation is shallowest.
Look for a flicker-free or DC dimming mode. Many monitors from the last few years have one. It may be called Flicker-Free, DC Dimming, Flicker-Less or Anti-Flicker in the on-screen menu.
Check whether it is only the low brightness range. Some monitors use DC dimming above a threshold and switch to PWM below it. Staying above that threshold solves the problem entirely.
On OLED, expect to find flicker. Most OLED panels, including phones and OLED monitors, dim by PWM at frequencies in the low hundreds of Hz, and it is often worse at low brightness than on LCDs. Some newer panels offer a high-frequency dimming mode — it is worth looking for if you are sensitive.
Windows 11
- Content adaptive brightness control changes the backlight as the picture changes, which will make this test inconsistent. Settings → System → Display → Brightness, and turn it off.
- Laptop battery saver dims the panel, often into the PWM range. If your laptop only gives you headaches on battery, this is why.
macOS
- Turn off automatically adjust brightness in System Settings → Displays before testing, or the ambient sensor will change the backlight underneath you.
- MacBook displays use their own dimming approach and are generally reported as low-flicker, but external monitors connected to a Mac behave exactly as they do on any other machine — the monitor is doing the dimming, not the computer.
Common questions
What is PWM flicker?
Pulse-width modulation is how most backlights control brightness. Instead of running the LEDs at lower power, the display switches them fully on and fully off very rapidly, and the proportion of on-time sets the perceived brightness. Lower brightness means a shorter on-period and a deeper flicker. Some displays do this at a few hundred Hz, which many people are sensitive to; others use several tens of kHz, which almost nobody notices.
Can a browser actually measure PWM?
No, and any page that claims a frequency in Hz is guessing. The strobing happens in the backlight driver, below anything JavaScript can observe — a browser cannot see the light coming off the panel. What a browser can do is make the strobing visible to you, which is what this test does: a bar moving fast across a uniform field breaks into separate copies when the light is pulsing, and stays a smooth blur when it is not.
What is the pencil test?
The oldest way to check, and it needs no software at all. Hold a pencil loosely between finger and thumb in front of a bright, uniform screen area and wave it quickly back and forth. Under steady light you see a continuous blur. Under a pulsing backlight you see a row of separate, sharp pencils, because each pulse freezes the pencil in a different position. It works because your eye integrates light over time and the pulses sample the motion.
How do I confirm it with a phone camera?
Record the screen in slow motion — 240 fps if your phone offers it — with the monitor set to low brightness. If the backlight is pulsing, the footage will show dark bands rolling through the frame, or the whole image pulsing between light and dark. Steady backlights record as an even image. This is the most reliable confirmation available without instruments.
Why does flicker only appear at low brightness?
Because that is when the off-periods are longest. At full brightness the backlight is on almost continuously and there is little to see. As you dim it, the on-periods get shorter and the gaps get longer, so the modulation depth increases. This is the cruel part of the problem: turning the brightness down to reduce eye strain is exactly what makes the flicker worse.
Is flicker actually harmful?
It is not damaging in any lasting sense, but it is well established as a trigger for eye strain, headaches and migraine in people who are sensitive to it. Sensitivity varies enormously between individuals — most people notice nothing at all, and a minority cannot use a flickering display for an hour. If you get headaches from one screen and not another at the same brightness, this is a prime suspect.