Refresh Rate Test
Check the refresh rate your screen is running at, in Hz, and see how smooth that rate looks.
Screen Flicker Test
Check your screen for PWM flicker by eye: fill it with a gray or white field at low brightness, then wave a pencil in front of it or look through a phone camera. The page helps you see flicker; it cannot measure it.
Flashing light warning
The alternating patterns flash the whole screen between black and white. Do not use them if you are sensitive to flashing lights or have a history of photosensitive seizures.
Showing Gray 20%
Set your display to low brightness, then wave a pencil quickly in front of this dark gray field. Several crisp copies of the pencil suggest flicker; a smooth blur suggests none. A phone camera pointed at the screen may show rolling dark bands.
Tick the box above to unlock the alternating patterns.
Human vision cannot see most PWM flicker. Many backlights switch hundreds or thousands of times per second, far faster than the eye can follow, so seeing nothing here does not mean your display does not flicker. Only a light sensor connected to an oscilloscope can measure it; these patterns are a visual aid. The rates shown for the alternating patterns are estimated from how often the browser draws frames, which can be lower than the display's refresh rate.
Runs entirely in your browser. Test results, audio and video never leave your device.
How to check a screen for PWM flicker
- Lower your screen's brightness setting. PWM flicker is usually strongest at low brightness, and the gray fields change only the pixel level, not that setting.
- Pick Gray 20%, Gray 50%, Gray 80% or White, then press Start test or click the preview to fill the screen. The arrow keys, Space or a tap change the field, and Esc exits.
- Wave a pencil or your fingers quickly in front of the screen. Several crisp, separate copies suggest flicker; a smooth blur suggests none.
- Point a phone camera at the screen. Dark bands rolling through the preview are another sign; slow-motion video or a faster shutter can make them clearer.
- Raise the brightness a step at a time and repeat. Flicker that fades at higher settings points to PWM dimming.
- For the flashing patterns, tick "I understand and want to see the flashing patterns" in the Flashing light warning. They flash only while the test fills the screen, and once they are unlocked, Next and the arrow keys step through them too.
What is PWM flicker?
PWM (pulse-width modulation) flicker is the rapid on-and-off pulsing some screens use to dim: instead of lowering the light steadily, the backlight or OLED pixels switch on and off many times a second, with longer off periods at lower brightness. It is usually too fast to notice on a still image, but it can show as separate copies of a moving object or as bands in a camera preview.
PWM vs DC dimming
Screens dim either by pulsing the light or by keeping it steady at a lower level. Some do both, and switch to pulses only at low brightness. This is how each looks in the pencil test.
| Dimming | How the light changes | What the pencil test shows |
|---|---|---|
| PWM | Pulses on and off; the off time grows as brightness goes down | Separate, crisp copies, most visible at low brightness |
| High-frequency PWM | The same pulsing, many times faster | Copies packed close together, or a blur that looks almost smooth |
| DC dimming | Less current, so the light stays steady with no pulses | A smooth blur at every brightness |
Why is my screen flickering?
- Only at low brightness: most likely PWM dimming. Some phones and monitors have a setting that reduces it; the name varies by maker.
- On and off, or with black flashes: often a loose or faulty cable, or a port, adapter or dock at its limit. Reseat or swap the cable and connect directly.
- In games, as the frame rate changes: variable refresh (FreeSync or G-Sync) can make brightness pulse, especially on OLED and VA panels. Compare with it turned off.
- After changing the refresh rate: choose a rate the monitor lists, then confirm it with the Refresh Rate Test.
- With a motion blur reduction mode on: backlight strobing pulses the light once per refresh on purpose. Turn it off if it bothers you; the Ghosting Test shows the blur it targets.
- On Windows, open Task Manager. If it flickers along with the rest of the screen, the display driver is the likely cause; if it stays steady, an app probably is.
- For step-by-step fixes for each cause, see why is my monitor flickering?
What this test can detect
- Separate copies of a pencil or finger waved in front of a gray or white field, a sign of pulsing light.
- Dark bands rolling through a phone camera's preview of the screen.
- Flicker that fades as you raise the brightness setting, which points to PWM dimming.
- How noticeable black-and-white flashing is to you at a half, a quarter, an eighth and a sixteenth of your refresh rate.
How it works
The static fields are solid Gray 20% (#333333), Gray 50% (#808080), Gray 80% (#cccccc) and White (#ffffff). They set the pixel level only; the brightness setting, which controls PWM dimming, stays where you put it.
The alternating patterns switch the whole screen between black and white every 1, 2, 4 or 8 frames, only while the test fills the screen. Each time it fills the screen, the page times 30 frame intervals after a 10-frame warm-up, takes the median and snaps it to a common refresh rate within 2.5% to label each flash rate: at 60 Hz, 30, 15, 7.5 and 3.75 Hz.
Limitations
- A browser cannot measure PWM frequency, duty cycle or flicker depth, and cannot certify a screen as flicker-free. That takes a light sensor, usually read with an oscilloscope.
- Seen steadily, flicker faster than roughly 60 to 90 Hz blends into constant light, so a still screen can look fine while it pulses. The pencil test works through motion, which is why it can reveal much faster PWM.
- The flash rates are estimated from how often the browser draws, which can be lower than your refresh rate. A browser can flash no faster than half the refresh rate.
- A camera's shutter speed can hide bands, and room lights or the screen's own refresh can add bands that have nothing to do with PWM. Treat camera results as a hint and compare with a screen you know.
- The alternating patterns flash the whole screen. Do not use them if you are sensitive to flashing lights or have a history of photosensitive seizures; a reduced-motion setting does not slow them down. This page gives no medical advice.
Frequently asked questions
How can I find my screen's PWM frequency?
Look for a review that tested it. Reviews get PWM frequency and flicker depth from a light sensor and an oscilloscope, often at several brightness levels. At home, the pencil and camera checks show whether flicker is there, not how fast it is.
Do OLED screens flicker?
Many OLED phones do, most of all at low brightness, because they dim with PWM; some offer a higher-frequency or DC-like dimming option. OLED monitors and TVs vary by model. Try the pencil test with the brightness low, and check reviews for measured results.
How do I check my phone for PWM flicker?
Open this page on the phone, lower its brightness, choose Gray 20% or Gray 50% and press Start test, then wave a pencil in front of it. For the camera check, film the phone with a second device, since a phone cannot film its own screen.
Is the pencil test reliable?
Yes, as a hint. Crisp, evenly spaced copies at low brightness are a clear sign of pulsing light, but very fast PWM can still look like a smooth blur. Do it in a dim room: some LED lamps flicker too, and their light on the pencil can add copies that have nothing to do with the screen.