What is PWM dimming?
PWM stands for pulse-width modulation. Instead of making the backlight dimmer, the screen switches it fully on and off very quickly and changes how long each flash lasts. At 100% brightness the light may stay on. At 30%, it’s off 70% of the time. The lower you set the brightness, the longer the dark gaps between flashes.
Rates vary widely between models, from a couple of hundred flashes a second to many thousands. Many LED-backlit monitors, laptops and OLED phones use PWM, at least at lower brightness settings.
Can you see PWM flicker?
Not when you hold your gaze still. But your eyes jump several times a second, and during each jump a flickering light smears into a trail of separate images called a phantom array. In a lab study, people could tell flickering light from steady light this way at an average of about 2,000 flashes a second.1 Your retina also responds electrically to flicker well above the rate you can see.2
PWM sensitivity symptoms
People who are sensitive to PWM commonly describe:
- Headaches or migraine attacks after screen use
- Eye strain, eye pain or a burning feeling
- Nausea or dizziness
- Trouble focusing, or a wired, tense feeling
- Symptoms that are worse at low brightness or at night
PWM sensitivity isn’t a formal diagnosis, and research hasn’t caught up with what people report. But flicker’s links to headaches and eye strain are well established: in a double-blind office trial, replacing flickering lights with nearly steady ones cut headaches and eye strain by more than half.3
Why does it get worse when you dim the screen?
Because dimming with PWM means longer dark gaps. Many screens are flicker-free at full brightness and only start pulsing below a certain level, so the settings you choose at night, when your eyes are tired, are often the ones with the most flicker.
How DC dimming is different
DC dimming lowers the steady current to the light source instead of switching it on and off. The light gets softer without pulsing. The Firelight goes a step further: its incandescent bulbs run on smooth DC power, so there’s none of the 120-times-a-second ripple an ordinary bulb picks up from AC wall power. Try the brightness slider to see the difference.
How to check a screen for PWM
- The pencil test. Turn the brightness down and wave a pencil quickly in front of a white area. A smooth blur is a good sign; a row of separate pencils suggests PWM.
- Your phone’s camera. Dark bands rolling across the screen in the camera view can point to PWM, though some cameras hide it or show bands for other reasons.
- Reviews that measure flicker. Some review sites measure each screen’s flicker with a light sensor and list the PWM frequency.