Monitor Flicker Test (PWM): Check Your Screen for Flicker
Monitor Flicker Test (PWM). Click Start Test to run this monitor flicker test and check your screen for PWM flicker with a pencil or your phone camera. Run the monitor ghosting test online to check your display for this issue directly in your browser — no download required.
Interactive Tool
The moving-line mode sweeps one bright bar across the grey field; nothing flashes. A flashing full-screen flicker pattern is deliberately not offered, because flashing between about 3 and 30 times a second can trigger seizures in people with photosensitive epilepsy.
The Monitor Flicker Test can't detect PWM flicker directly — no browser can read your monitor's backlight-dimming hardware — so instead it walks you through the two methods that actually work: the pencil test and the phone-camera test. Click Start Test to fill your screen with a mid-grey field, lower your monitor's brightness below where you'd normally run it, then wave a pencil in front of the field or record it with your phone's camera and watch for banding or flicker in what you see. PWM usually only shows up below a certain brightness threshold, so if nothing turns up at 50% brightness, keep dropping it before you rule your display out.
If your eyes ache after an evening at a dimmed screen that looks perfectly steady, the Monitor Flicker PWM Test is a monitor flicker test that helps you check whether PWM dimming is pulsing the backlight faster than you can consciously see. Switch to fullscreen, test your monitor at full and low brightness, and you get a clear answer on whether PWM backlight flicker is a likely cause of your eye strain, without buying a light meter.
What the Monitor Flicker PWM Test Checks
Monitor flicker is rarely an obvious blink. It is a rapid brightness fluctuation in the light source behind the image, usually caused by pulse width modulation, and it often counts as invisible flicker: your eyes register it even when your brain doesn't. This page puts a moving pattern on the display that turns that hidden pulsing into something you can see or film. The free screen uniformity test runs fullscreen so you can inspect your panel edge-to-edge under real viewing conditions.
PWM Dimming vs DC Dimming on the Backlight
Pulse-width modulation controls brightness by switching the LED backlight on and off hundreds of times a second. Brightness control comes from the duty cycle, the share of each cycle the light is on, so the light output follows a square wave rather than a steady level. DC dimming lowers the current instead, so the light stays on continuously. A flicker-free panel uses DC dimming or a very fast PWM, and many monitors only switch to PWM below maximum brightness.
Why Flicker Is Not a Low Refresh Rate Problem
Refresh rate is how often the image is redrawn, while backlight flicker is how steadily the light is emitted. A 60 Hz office screen can have rock-steady DC dimming, and a 144 Hz panel can still pulse its backlight at a few hundred cycles per second when dimmed. A higher refresh figure tells you nothing about dimming; only a high frequency PWM or no PWM at all does.
How to Use the PWM Flicker Test
The PWM flicker test online gives you two complementary checks: a moving line you judge by eye and a flicker pattern you film. Run both if you want to test monitor flicker properly; a quick PWM test with only one method can miss it, and repeat each at two brightness settings. The free ips glow test runs fullscreen so you can inspect your panel edge-to-edge under real viewing conditions.
Prepare the Screen Before Testing
- Set the display to its native resolution and a fixed refresh rate, such as 165 Hz or 240 Hz, rather than a variable mode.
- Let the panel warm up for 15 minutes; a cold LCD can behave differently.
- Open a white page or light gray background behind the test and note each brightness level you try.
Moving Line Mode at Hardware Brightness
Change brightness with the monitor's own buttons, not a software slider, because only hardware brightness changes how the backlight is driven. At 100%, a white line sweeping across the screen should look like one smooth line with a soft motion trail. Drop to minimum brightness and look again. If the backlight is pulsing, the same moving line breaks into multiple lines, separated copies with dark gaps between them. TestUFO popularised this blur-trail method, and it needs no camera at all.
Flicker Mode and the Reference Pattern
Flicker mode alternates the screen between light and dark so a camera can lock onto the pulsing. A web page can only change colour once per refresh, so the visual flicker rate it can show is capped at half your refresh rate: 82.5 Hz on a 165-hertz panel. That flicker rate is a reference pattern for filming, not a measurement of your monitor's real PWM.
Running a Phone Camera Test for Backlight Flicker
A phone camera reads its sensor one row at a time, a rolling shutter, so a pulsing backlight shows up as horizontal stripes across the live preview. Point the camera at a bright fullscreen area at your usual brightness, then record a short slow-motion video if your phone supports it, because slow motion makes faint pulses easier to spot. Run the monitor convergence test online to check your display for this issue directly in your browser — no download required.
Camera Settings That Reveal Rolling Bands
- Use the standard photo mode and turn off auto-exposure, or tap and hold to lock it.
- On Android, Pro mode lets you set a manual exposure; a fast shutter speed such as 1/1000 s makes rolling bands sharper.
- Keep the phone still and compare the preview at 100%, 50% and your normal brightness.
Banding that appears only when you dim the screen points to PWM. Pulsing that changes with the game's frame rate points somewhere else.
What a Clean Result Cannot Prove
No bands does not mean no flicker. Shallow modulation, very fast PWM or a mismatch with your camera's readout can all hide it. A lab-grade check uses a photodiode with an oscilloscope or a high speed camera. Treat the phone as a warning light, then confirm with the moving line and a longer comfort session.
Reading Your Screen Flicker Results by PWM Frequency
The gaps in the moving line let you estimate the PWM frequency without any equipment. Each copy of the line is one backlight pulse, so the spacing between copies equals how far the line travels during one PWM cycle:
$$\text{gap (px)} = \frac{\text{line speed (px/s)}}{f_{\text{PWM}}\ (\text{Hz})}$$
Counting the gaps is also a quick way to compare two settings when testing monitor flicker across brightness steps.
Worked Example: Line Spacing and Duty Cycle
Say you test for PWM on a 165-hertz panel with the line moving at 2,880 px/s, and at 40% brightness you count copies about 8 px apart. Solving the formula gives \(f = 2880 \div 8 = 360\) Hz. The line moves 17.5 px per refresh, so you see roughly 2 to 3 copies per frame. One cycle lasts \(1 \div 360 = 2.78\) ms; at a 40% duty cycle the light is on for 1.11 ms and off for 1.67 ms. Because the light switches fully off, its modulation is 100%, far above the 28.8% that IEEE 1789 treats as low-risk at that frequency \((0.08 \times 360)\). The same moving line test on a 2,880-hertz panel would put the copies 1 px apart, too close to see.
Why 1000Hz Is the Common Comfort Line
Most sensitive users stop noticing PWM above about 1000Hz, and IEEE 1789 rates full on-off flicker as low-risk only above 1,250 Hz. People with migraines, photosensitivity or visual stress tend to react at lower frequencies first. A PWM-free panel removes the question entirely.
Tracking Down PWM Dimming on a New 180 Hz Editing Monitor
A video editor who gets migraines has eleven days left on the return window for a 27-inch, 180 Hz panel. Daytime work is fine; evening sessions at 32% brightness end with a pressure headache. The question is simple: keep it with a workaround, or send it back. The answer comes out as roughly 480 Hz PWM below 61% brightness, which makes it a keep, but only at a fixed hardware setting.
I start with the moving line at 1,920 px/s and the hardware brightness at 100%: one clean line. At 32% it splits into a comb of copies about 4 px apart. The formula gives \(1920 \div 4 = 480\) Hz. A 4 px gap read by eye could easily be 3 or 5 px, which spans 384-640 Hz, so I rerun at 3,840 px/s. The copies now sit about 8 px apart, and a one-pixel misread only moves the answer between 427 and 549 Hz. Both speeds agree on 480 Hz, so I trust it.
| Line speed | Gap read | Estimate | Range if off by 1 px |
|---|---|---|---|
| 1,920 px/s | 4 px | 480 Hz | 384-640 Hz |
| 3,840 px/s | 8 px | 480 Hz | 427-549 Hz |
Next I walk the brightness up one step at a time. Copies are still there at 58% and gone at 61%, so this monitor's PWM cut-out sits between those two steps. At 32% each 2.08 ms cycle is 0.67 ms of light and 1.42 ms of darkness; IEEE 1789 would accept at most 38.4% modulation at 480 Hz, and switching fully off is 100%.
The decision: lock the hardware at 62%, then bring evening light down with the monitor's contrast and a warmer picture mode, plus a desk lamp behind the screen. The phone camera agrees: banding at 32%, none at 62%.
The caveat I'd flag is that the cut-out point is model-specific and can move after a firmware update, so I'd rerun the ladder after any update. And the number to watch is not the 480 Hz itself; it's the brightness where PWM switches on, because that decides whether a workaround is even possible. If the copies had appeared at 90%, this would have been a return.
A Comfort Test for Display Flicker at Real Brightness
Display flicker often shows up as discomfort after an hour rather than anything visible, so a real-use session matters as much as the pattern. Keep room lighting, viewing distance and content the same as your daily setup.
| Session | What to do | What it tells you |
|---|---|---|
| High brightness | 90-100% brightness for 25 minutes | If comfort is fine here, dimming is the suspect |
| Your usual brightness | Normal level for 50 minutes on the same work | Symptoms here point to PWM at that level |
| Second screen | Same content on a laptop or other monitor | Symptoms that vanish implicate the first panel |
| Bias light added | Lamp behind the monitor, same brightness | Improvement suggests glare and contrast, not flicker |
A Brightness Ladder for Low Brightness Testing
Step down instead of jumping straight to low brightness, because PWM often kicks in only below a set point:
- 100% for 12 minutes on a bright page.
- 70% for 12 minutes.
- 45% for 12 minutes.
- Your night brightness for 40 minutes of real work or gaming.
Run the moving line at every rung of the brightness ladder; the rung where the copies first appear is where the monitor switches to PWM.
Symptoms to Log During Eye Strain Sessions
Write symptoms down as they happen instead of trusting memory: eye pressure, headaches or a dull headache behind the eyes, eye fatigue, blurred vision, dry eyes and general fatigue. Long screen sessions cause digital eye strain on their own, so note when each symptom started and at which brightness.
Ruling Out Other Causes of Flickering
Before you blame the backlight and test a monitor for PWM flicker again, rule out the settings that create flickering on their own.
Backlight Strobing, Variable Refresh Rate and Overdrive
Backlight strobing (sold as motion blur reduction or blur reduction) flashes the backlight on purpose to cut motion blur. Variable refresh rate modes such as VRR, adaptive-sync and G-Sync can flicker when gaming frame rates swing, and HDR mode switching or aggressive overdrive can add flashes too. Turn each off, then retest.
Pixel Inversion on LCD Panels
LCD panels flip the voltage on their pixels every frame, and on some panels and colour patterns that pixel inversion produces a faint shimmer or crosstalk between areas of the screen. It looks like flicker but is not backlight-driven, so it doesn't change with brightness. OLED screens skip pixel inversion, though many dim with low-frequency PWM of their own light.
Cables and the Graphics Driver
Sparkles, blanking or random flashes usually come from a marginal cable, a loose port or the graphics driver. Swap the cable, try another port, and update the driver before judging the backlight.
Choosing a Flicker-Free Monitor After a PWM Flicker Check
If your results show PWM below 1000Hz, look for a flicker-free monitor: a VESA ClearMR or TÜV flicker-free certification, a review that measured the PWM, and an anti-glare finish. "Eye care" labels alone are marketing. The same applies to ultrawide and portable monitors, which often dim differently on battery or single-cable power.
Keep, Adjust or Return Your Gaming Monitor
- Keep it if the line stays smooth at the brightness you use and long sessions are comfortable.
- Adjust it if copies appear only at low settings: run the hardware at 100% and lower brightness with the room light or picture mode instead.
- Return it if the gaming monitor pulses at your normal brightness and the symptoms repeat across cables and apps.
Frequently Asked Questions
- Which mode should I start with, Flicker or Moving Line?
- Start with Moving Line. It needs no camera: if the sweeping bar breaks into several separated copies instead of one smooth blur, your backlight is pulsing. Use Flicker mode afterwards if you want to film the screen with a phone and look for rolling bands.
- Does the flicker rate I pick show my monitor's real PWM frequency?
- No. A browser can only change the screen once per refresh, so the on-screen flicker tops out at half your refresh rate. It is a reference pattern for filming. To estimate the real PWM frequency, divide the moving line's speed by the gap between its copies, or use a photodiode and oscilloscope.
- What phone camera settings make PWM banding easier to see?
- Use the normal photo or video mode, lock the exposure (tap and hold on iPhone, Pro mode on Android) and choose a fast shutter speed if you can. Slow-motion video also helps. Rolling bands that appear only when you dim the screen point to PWM.
- Why does my monitor only flicker at low brightness?
- Many monitors run the backlight steadily at full brightness and switch to PWM below a set point. Test at your usual brightness and at minimum brightness, not only at 100%, or you can miss the flicker completely.
- Does a clean result prove my monitor is flicker-free?
- Not completely. Very fast or shallow flicker can escape a phone camera and your eyes. Treat a clean result as a good sign, and confirm it with a long session at your normal brightness and a review that measured the monitor's PWM.
- Is PWM flicker harmful?
- For most people it is not dangerous, but people prone to migraines, photosensitivity or eye strain often feel discomfort from PWM below about 1,000 Hz, especially in long sessions. A flicker-free or high-frequency PWM monitor avoids the problem.
- What can I do if my monitor uses PWM?
- Set the monitor's own brightness above the point where flicker stops and lower the image with contrast or a darker picture mode instead, turn on DC dimming if the monitor has it, or replace it with a certified flicker-free display.