Monitor Flicker Test: Check Any Screen for PWM and Other Flicker

Test 1: The 60-second comfort check (no equipment)

Before you film anything, find out whether your own symptoms track the backlight. This costs you a minute and it decides how seriously to take the rest of the page.

1. Work at your usual brightness for a minute, in a dim room. Read normal text. Note how your eyes and forehead feel.
2. Push brightness to maximum and wait another minute. Add a lamp behind the screen so the brightness is not painful. PWM dimming is deepest at low brightness, so raising brightness is the closest thing to switching the pulsing off.
3. Compare. If the pressure behind your eyes eases at high brightness and returns when you dim, the backlight is the prime suspect. If nothing changes, the cause is more likely glare, focus fatigue, dry eyes, or color temperature.

This is a subjective screen, not evidence. It just tells you whether to keep going. The brightness relationship is the same one behind the brightness and eye strain trap, where turning the screen down to feel gentler makes the flicker worse.

Test 2: The slow-motion camera method (the reliable one)

A camera's sensor scans line by line, much faster than your eye refreshes. When a backlight pulses on and off, the camera captures the on and off moments as bright and dark stripes. Your eye smooths those out. The camera does not. That is why this is the definitive home test.

1. Drop the brightness to about 20%. PWM is deepest at low brightness because the backlight is dark for more of each cycle. Testing dim gives you the best chance of catching it.
2. Open slow-motion, not normal video. Switch your camera to its slow-motion mode and choose the highest frame rate available. Faster capture catches faster flicker.
3. Fill the frame with white. Open a blank white document or page on the screen under test, then hold the phone 20 to 30 cm away so the white fills the view.
4. Record a few seconds, then play it back slowly. Watch for horizontal dark bands rolling up or down the image. Bands that move through the frame are the signature of a pulsing backlight.
5. Climb the brightness ladder. Repeat at 50% and 100%. If the bands are strong at 20%, weaker at 50%, and gone at 100%, that is textbook PWM whose duty cycle rises with brightness.

Per-phone camera guidance

Any recent phone works. Higher frame rates catch higher-frequency flicker, so use the fastest mode you have.

What the stripes mean

Reading the result is straightforward once you know what you are looking at:

Judge the clip at the brightness you actually work at, not just at 20%. A panel that bands badly at 20% but sits clean at the 60% you really use is a different problem from one that bands at every setting.

How bad is 240 Hz? Frequency is only half the answer

"Is 240 Hz PWM bad" is the question people arrive with, and the honest answer is that the number alone does not settle it. Two things decide how a pulsing backlight feels:

• Frequency. How many times per second the backlight cycles. Lower is worse, because each dark gap lasts longer and your visual system has more chance to resolve it.
• Modulation depth. How far the light drops on each cycle. A backlight that dips from 100% to 80% is a very different experience from one that snaps fully off, even at the same frequency.

The NotebookCheck PWM ranking treats frequencies above roughly 500 Hz as fine for most people and notes that reported problems cluster below about 250 Hz, which puts 240 Hz right on the uncomfortable edge. IEEE 1789-2015, the recommended practice for LED flicker, works on the same logic: the lower the frequency, the shallower the modulation has to be before it counts as low risk, and above roughly 1.25 kHz it stops restricting flicker at all.

You can read depth off your own clip without any instruments. Pause on a banded frame and compare the dark bands to the bright ones. Bands that go near black mean deep modulation. Bands that are only a slightly darker gray mean shallow modulation, which most people tolerate even at low frequency. Deep bands plus a low frequency is the combination that drives symptoms.

Test 3: The pencil test (quick check)

No slow-motion mode handy? Use a pen. In a dark room, turn the screen to a bright white image, hold a pen a few inches in front of it, and wave it quickly side to side. A steady display blurs the pen into one smooth streak. A pulsing display strobes it into several separate, ghostly copies, one for each flash of the backlight. It is not a measurement, but it is a fast yes-or-no hint before you reach for the camera.

Test 4: On-screen motion pattern (with a caveat)

The pattern below moves a bright bar across a dark strip. In a dim room, stare at one edge and let your eyes flick across to follow the bar. On a steady display it blurs into a smooth trail. On a pulsing display it can break into separate copies, the same stroboscopic effect as the pencil test.

Test 5: The pixel inversion pattern (LCD only)

Backlight pulsing is not the only thing that makes a screen shimmer. Every LCD flips the voltage polarity across its liquid crystal on a schedule, because leaving a steady DC charge on the crystal would damage it. If the two polarities do not land on exactly the same brightness, fine one-pixel patterns start to crawl or shimmer. That is pixel inversion, and it has nothing to do with your backlight.

Start the pattern below, then sit at your normal viewing distance and let your eyes rest on it. Check the corners and edges as well as the middle, since inversion artifacts are often strongest away from center.

A clean panel renders the pattern as a flat, still gray. A panel with inversion artifacts shows a faint shimmer, a slow crawl, or blotchy patches that shift when you move your head. Rows and columns catch different inversion schemes, so try all three.

Not PWM? What else makes a screen flicker

Here is the split that saves people the most time: PWM flicker is invisible to the naked eye, constant, and worse when you dim the screen. If you can actually see your screen flickering, flashing, dropping black frames, or stuttering in and out, that is almost never PWM. Chase these first, because most of them are free to fix.

Windows: rule out the settings before you blame the panel

Most reported "my screen is flickering" cases on Windows come down to a setting rather than the hardware. Open Settings, then System, then Display, then Advanced display, and confirm the refresh rate matches what your monitor is rated for. Turn off variable refresh rate there and in the NVIDIA or AMD control panel while you test. Then swap the cable and update the graphics driver. Only once the visible flicker is gone does the camera test tell you anything useful about PWM. If you are also chasing evening eye comfort on the same machine, our Windows blue light filter setup covers what Night Light does and does not handle.

Mac: check the refresh rate and the external link

On macOS, open System Settings, then Displays, and confirm the resolution and refresh rate for each screen. Flicker on an external monitor over USB-C or a dock is usually a bandwidth or cable problem, so test the panel plugged straight into the machine before you conclude anything about the display itself.

Laptop panels and OLED screens each have their own quirks. If you are testing a laptop, our laptop PWM flicker guide covers what you can change when the panel is not replaceable. For OLED, where dimming is usually done with deep low-frequency pulsing, see why some OLED displays cause eye strain.

Confirm with measured data

Two independent labs measure display flicker with photodiodes and publish the numbers, so you can look up your exact model instead of guessing:

• RTINGS monitor image flicker test measures flicker frequency at different backlight levels and flags whether a monitor is flicker-free.
• NotebookCheck PWM database ranks hundreds of laptops, phones, and tablets by measured PWM frequency. It notes that frequencies above roughly 500 Hz are generally fine for most people, while problems cluster below about 250 Hz.

If you cannot find your model, the camera test plus the brightness ladder is a solid substitute.

The lab method behind those numbers is a photodiode pointed at the panel with its output on an oscilloscope, which traces the actual light waveform and gives you both frequency and modulation depth. That is the only way to get a hard number, and it is why measured databases are worth checking even after your own clip comes back clean. Your phone is a good detector. It is not a meter.

You found flicker. Now what?

If your screen pulses and you are sensitive to it, you have three honest options, ranked by how reliably they work:

1. Switch to flicker-free hardware. A display with DC dimming or measured PWM above 1,000 Hz is the durable fix. See our guide to verifying a flicker-free monitor.
2. Keep hardware brightness high, dim with software. Since PWM is worst at low brightness, keep the backlight near full and use a software dimmer to darken the image instead. This avoids the deep low-brightness pulsing.
3. Raise brightness and soften the room. A free partial fix: nudge brightness up and add gentle ambient light so the screen does not need to blind you.

Not sure whether your panel dims with PWM or DC in the first place? Our guide to PWM monitors and how to spot them covers what the specs do and do not tell you. For the full background on why low-frequency PWM causes symptoms, who is most sensitive, and how OLED and LCD differ, read our PWM flicker and headaches hub. For the broader picture on screen comfort, see the digital eye strain guide.

The whole test, as a checklist

Work down the list in order. Stop when you have your answer.

• Run the 60-second comfort check. Do symptoms ease at full brightness?
• Rule out the visible causes first: cable, refresh rate, VRR, graphics driver.
• Set the screen to about 20% brightness and open a blank white page.
• Film it in slow motion at the highest frame rate your phone offers, from 20 to 30 cm away.
• Play it back and look for rolling dark bands.
• Repeat at 50% and 100%. Note whether the bands weaken as brightness rises.
• Pause on a banded frame and judge the depth. Near black, or just slightly darker gray?
• Run the one-pixel pattern to check for LCD pixel inversion.
• Look your exact model up on RTINGS or NotebookCheck for a measured frequency.
• If it flickers and you are sensitive: raise hardware brightness, dim in software, and plan a flicker-free panel for the next upgrade.