How to run the monitor flicker test
A web page can't measure flicker by itself, but it can show the patterns that make flicker easy to see. Three checks cover almost every case:
- Phone camera check (PWM): open the 50% gray pattern, set the screen to about 30% brightness and look at it through your phone's camera. Rolling or still dark bands mean the screen dims with PWM. Raise the brightness and the bands often disappear. If your camera has a manual or "pro" mode, a fast shutter (1/1000 s or faster) makes the bands clearer.
- Pencil test: on the white pattern, wave a pencil quickly from side to side. A smooth blur is fine. Several separate pencils, like a strobe, mean the light is pulsing.
- Inversion check (LCD): look closely at the pixel checkerboard and fine lines without moving. Shimmering, crawling or moving bands mean the panel's inversion is visible, which some LCDs show at certain refresh rates or overdrive settings.
Then tick what you noticed above. The cause finder ranks the most likely reasons and tells you what to try first. If you want to confirm the refresh rate your screen actually runs at, use the refresh rate test.
Why does my monitor flicker?
Monitor flicker almost always comes from one of six places. The quickest way to narrow it down is to record the screen: flicker that shows up in a screen recording is created by software, while flicker that doesn't is happening in the cable or the screen itself.
| Cause | Typical sign | Try first |
|---|---|---|
| Cable or port | Black-outs, sparkles, changes when the cable moves | Another cable and port, no adapter or dock |
| Refresh rate or resolution | Started after changing display settings | Native resolution and a supported refresh rate |
| Driver or app | Visible in a screen recording, only in one app | Update the graphics driver, turn off hardware acceleration |
| PWM dimming | Worse at low brightness, bands on camera | Higher brightness, a flicker-free screen |
| Variable refresh rate | Dark game scenes with changing frame rates | Cap the frame rate or turn off G-Sync/FreeSync for that game |
| Panel or backlight | Flickers before the operating system starts | Warranty or repair |
A second monitor that flickers only on one input, or only with one computer, is almost never broken: swap the cable and the input before anything else.
PWM flicker: why some screens bother your eyes
Many LCD backlights and most OLED screens dim by switching the light on and off very quickly, called pulse-width modulation. At full brightness the light is on almost all the time; at low brightness it is off for most of each cycle, which is why PWM flicker is strongest when the screen is dim. Most people never notice it, but some get eye strain or headaches, especially from low PWM frequencies. Reviews that measure displays, such as Notebookcheck's, list the PWM frequency, and monitors sold as "flicker-free" dim without PWM. The IEEE 1789 recommendation describes which combinations of frequency and modulation are considered low risk.
This page is not a medical test. If flicker gives you headaches or eye strain that doesn't go away, talk to an eye doctor.
Laptop screen flickering: Lenovo, Surface, MacBook and others
On laptops, the cause is more often a power-saving feature than a broken screen. Work through these in order:
- Power saving: panel self refresh (PSR) on Intel graphics and adaptive or content-based brightness can cause flicker, especially on battery. Turn them off and test again.
- Driver and BIOS: Lenovo screen flicker and similar reports on other brands are often fixed by the manufacturer's latest display driver and BIOS update, installed from the maker's support site rather than through Windows Update.
- Hinge cable: if the flicker changes as you open or close the lid, the display cable inside the hinge is worn. That is a repair job.
- Surface Pro 4 screen flicker is a known hardware fault of that model, and Microsoft ran a replacement program for affected units. Software settings rarely cure it, so contact Microsoft Support or a repair shop.
- MacBook Pro screen flicker: update macOS, turn off automatic brightness and True Tone to test, and check whether it also happens in safe mode. Flicker that remains in safe mode and with an external display connected points to the hardware.
How we tested this tool
We read back the pixels of every pattern: the gray, white and dark gray screens have the exact values we set (128, 255 and 38), and in the checkerboard and line patterns neighbouring pixels alternate exactly one pixel apart with no blur. The patterns are drawn at the screen's device pixel ratio; we checked this at a ratio of 1. Tap, the arrow keys and Esc move through and close the patterns. In the cause finder we ticked each of the 11 observations on its own and in combinations and checked that each points to the right causes and that the cause with the most matching signs comes first. We tested in a desktop browser and at a 375 px phone width. The page cannot measure flicker or PWM frequency itself, and we have not measured any particular monitor with it.
Frequently asked questions
Can a website detect screen flicker by itself?
No. A web page can't measure how the backlight or OLED pixels pulse. This test shows still patterns that make flicker easy to see with your eyes or a phone camera, and helps you work out the cause.
Why does my monitor flicker?
The most common causes are a loose or poor cable, a wrong refresh rate or resolution, a graphics driver or app problem, PWM dimming at low brightness, variable refresh rate in games, and less often a failing panel or backlight. If the flicker shows in a screen recording, it is software; if it shows before the operating system starts, it is hardware.
What is PWM flicker?
Many screens dim by switching the light on and off very fast, called pulse-width modulation. Most people don't see it, but a phone camera shows it as dark bands, and some people get eye strain or headaches from low PWM frequencies, usually at low brightness.
Is screen flicker bad for your eyes?
Visible flicker and some low-frequency PWM can cause eye strain and headaches in sensitive people. This page is not a medical test; if symptoms continue, talk to an eye doctor.
Why is my laptop screen flickering?
On laptops, flicker is often caused by power-saving features such as panel self refresh or adaptive brightness, an outdated display driver or BIOS, or a worn display cable in the hinge. Flicker that changes when you move the lid points to the cable.
Does this test flash or blink?
No. Every pattern is a still image; the page never flashes. Any flicker you see comes from your screen, which is the point of the test.