Flicker & PWM Test.
Steady grey fields to run the pencil-wave and phone-camera flicker tests, weighted to the low brightness where PWM dimming bites hardest.
How to test for screen flicker
A web page can't read your backlight, but two simple physical tests reveal flicker your eyes smooth over. Both need a steady, solid field to work against — the fields here are weighted dark, because PWM flicker is deepest at low brightness. Set your monitor brightness low first, or the effect may hide.
- 1. Lower the brightness. Turn your monitor's brightness well down — this widens the dark gaps in PWM and makes flicker visible.
- 2. Open a dark grey field. Go full screen on the 5–10% grey so the whole panel is one steady tone.
- 3. Run the pencil test. Wave a pencil quickly in front of the screen: a smooth blur is flicker-free; several sharp strobe copies mean PWM.
- 4. Or film it. A phone camera shows rolling dark bands over a flickering backlight and a clean image over a steady one.
PWM frequency & comfort
How dimming is done decides how easy a screen is on the eyes. Higher PWM frequency — or no PWM at all (DC dimming) — means less strain.
| Dimming | Frequency | Comfort |
|---|---|---|
| Low-freq PWM | < 250 Hz | Visible flicker; eye strain and headaches for sensitive users |
| Mid PWM | 250 – 1000 Hz | Flicker on the pencil/camera test; some fatigue over long sessions |
| High-freq PWM | > 1000 Hz | Rarely perceptible; usually comfortable |
| DC dimming / Flicker-Free | No flicker | Backlight held steady, brightness set by voltage — easiest on the eyes |
If the tests show flicker and you get eye strain at your screen, raise the brightness (dim the room instead), or look for a monitor with DC dimming or a Flicker-Free certification. Not everyone is sensitive — if the camera shows flicker but your eyes are fine, there's nothing to fix.
What your results mean
What the pencil wave or camera preview shows against a grey field tells you whether your backlight is PWM-dimmed and how to make it easier on your eyes.
| What you see | What it means | What to do |
|---|---|---|
| A single smooth blur when you wave the pencil | Flicker-free / DC dimming — no PWM | Nothing to do. |
| Several distinct, strobe-like pencil copies | PWM dimming — the backlight is switching fully off and on | Raise the monitor brightness (dim the room instead) to shorten the PWM off-time. |
| Rolling dark bands across the phone camera preview | The backlight is flickering at a rate the camera catches | Same fix: raise brightness, or check the monitor menu for a DC-dimming / flicker-free mode. |
| Strobing fades away on the brighter grey and white fields | Low-frequency PWM that's worst at low brightness | Keep working brightness higher; dim the room rather than the screen for low-light use. |
| Strobing or camera bands persist even on the white field | Low-frequency PWM across the whole brightness range | Look for an anti-flicker / DC-dimming mode in the monitor's menu, or consider a Flicker-Free certified display. |
Frequently asked questions
What is monitor flicker and PWM?
Many screens dim their backlight by switching it fully on and off very fast — pulse-width modulation, or PWM. The lower you set the brightness, the longer the "off" part of each cycle, so the flicker gets deeper. If the switching frequency is low, your eyes and brain can perceive it, even if not consciously, and it is a common cause of eye strain, fatigue and headaches. DC-dimmed or "flicker-free" displays hold the backlight steady instead.
How do I test my monitor for flicker?
Two reliable manual tests, because a browser cannot sample the backlight directly. The pencil test: set your monitor brightness low, open a solid grey field full screen, and wave a pencil or your fingers quickly in front of the screen. On a flicker-free display you see a smooth blur; on a PWM display you see several distinct, strobe-like copies of the pencil. The camera test: point your phone camera at the screen — rolling dark bands across the preview mean the backlight is flickering.
Why is flicker worse at low brightness?
With PWM dimming, brightness is controlled by how much of each cycle the backlight stays on. At full brightness it is on almost the whole time, so there is little flicker; as you lower brightness the on-time shrinks and the dark gaps grow, deepening the flicker. That is why PWM-related eye strain often shows up when people turn the brightness down in a dark room — exactly when they expect it to be more comfortable.
How do I stop flicker hurting my eyes?
Raise the monitor brightness (and reduce it by dimming the room instead), which shortens the dark gaps in PWM. Better, use a monitor with DC dimming or a "Flicker-Free" certification, or enable any anti-flicker / DC-dimming mode in your monitor menu. On OLED phones and laptops, look for a "high-frequency PWM" or "flicker reduction" setting. If you are flicker-sensitive, check reviews for the PWM frequency before buying.
My screen looks fine to me — do I still have flicker?
Possibly. PWM flicker is often invisible to conscious sight yet still causes fatigue over a long session, which is why the pencil and camera tests exist — they reveal flicker your eyes smooth over. If those tests show strobing or camera bands but you feel fine, you are simply less sensitive to it; no action needed. If you get unexplained eye strain or headaches at the screen, flicker is worth ruling out.