Most screens dim their brightness not by reducing voltage, but by turning on and off very rapidly —
called PWM (pulse-width modulation). At 60Hz or higher, your eyes blend it into steady light.
But your brain still processes the flicker. Point a camera at any screen around you and let's
measure how fast it's pulsing.
PWM flicker is linked to eye strain, headaches, and migraines — even when you feel fine.
Manufacturers rarely disclose PWM frequencies. Budget screens often flicker at 240Hz or even 60Hz.
Premium OLED screens use DC dimming (no flicker) at mid-to-high brightness, but switch to PWM at
low brightness — typically below 50% — where it's hardest to notice. EEG studies show measurable
changes in brain alpha-wave activity from screens flickering at these rates.
How to use this: Point your phone camera at another screen — a laptop,
TV, monitor, or tablet. Lower its brightness to 20–40% where PWM is most aggressive. Hold the
camera steady, about 20–30cm away. Press start.
—
Hz · detected flicker frequency
waiting for camera
—camera fps
—flicker depth %
0frames sampled
brightness over time
frequency spectrum
screen type guess—
flicker depth—
safety verdict—
known matches—
▶ how does PWM work, and why does it matter?
LEDs can only be "on" or "off" — they can't partially dim the way an incandescent can.
To create the appearance of lower brightness, manufacturers switch them on and off at high
frequency: at 20% brightness, the LED is on 20% of the time and off 80% of the time, cycling
hundreds of times per second. This is PWM — pulse-width modulation.
The problem: at low frequencies (below ~1000Hz, especially below 250Hz), the visual cortex
detects the flicker even if you're not consciously aware of it. Studies show increased eye
muscle fatigue, reduced contrast sensitivity, and cortical loading from low-frequency PWM.
For people with photosensitive conditions, even 3000Hz can cause issues.
DC dimming (no flicker) is the alternative — it varies voltage instead. Most OLED phones
use DC dimming above ~40% brightness, but switch to PWM at low brightness because DC dimming
at very low levels causes color shifts and backlight uniformity problems.