Background
Screen flicker and PWM dimming explained
Many screens control brightness by switching their light source on and off very quickly instead of running it at a lower steady level. This is called pulse-width modulation (PWM). The switching is usually too fast to see directly, but some people notice it as eye strain or headaches, and a phone camera can often reveal it as moving dark bands.
How PWM dimming works
With PWM, the light source is always either fully on or off. Lowering brightness makes each on-pulse shorter while the off-gaps get longer. At high brightness, the light may be on almost all the time and there is little or no flicker. At low brightness, the screen spends most of each cycle dark, which is when flicker is most pronounced.
PWM frequencies vary widely between devices, from a few hundred to tens of thousands of times per second. Higher frequencies and a smaller difference between on and off are less likely to be noticed. Many LCD monitors now use DC dimming, which lowers the current instead of pulsing, and are sold as flicker-free. OLED phones and some OLED monitors still use PWM at lower brightness levels.
Why a phone camera can see it
Most phone cameras use a rolling shutter: they read the image line by line from top to bottom. If the screen switches off and on while the sensor is reading, some lines catch the screen lit and others catch it dark. The result is horizontal dark bands that often drift through the image.
A faster shutter speed, set in the camera’s pro or manual mode, makes bands clearer. Slow-motion video can also show the screen pulsing. Bands are a sign of brightness modulation, but the camera’s own readout speed affects how many bands appear, so the number of bands is not a measurement of frequency.
Reducing flicker
- Raise the screen’s brightness setting and lower the content brightness instead, for example with dark mode or a dimmer wallpaper. Higher hardware brightness usually means less PWM.
- On phones, software dimming features such as Reduce White Point on iPhone or Extra dim on Android lower brightness in software. That lets you keep the hardware brightness higher.
- Some Android phones include a high-frequency PWM or anti-flicker option in display settings.
- When buying a monitor, look for flicker-free or DC dimming in the specifications, or independent reviews that measure flicker.
Who it affects
Sensitivity to flicker varies a lot between people. IEEE 1789-2015 is a published recommended practice that relates flicker frequency and modulation depth to the risk of noticeable effects. If you suspect flicker is causing headaches or eye strain, try a flicker-free display for comparison and talk to an eye-care professional about persistent symptoms.
Common questions
- Does a high refresh rate mean no flicker?
- No. Refresh rate is how often the image updates. PWM is how the light source is dimmed. A 240 Hz monitor can still use PWM backlight dimming, and a 60 Hz monitor can be completely flicker-free.
- Why do I see bands on my TV in photos but not with my eyes?
- Your eyes average the light over time, but the camera samples it line by line in a fraction of a second. High-frequency PWM is invisible to most people yet still shows in photos.
- Is backlight strobing the same as PWM?
- Strobing modes for motion clarity also pulse the light, but once per frame and synchronized with the refresh. They reduce motion blur deliberately, and they can cause visible flicker at low refresh rates.