What PWM is and why it flickers
Most LCD monitors control brightness by adjusting how much power goes to the backlight. There are two ways to do this. The first is DC dimming, which reduces the actual voltage to the LED backlight. The second is PWM (Pulse-Width Modulation), which keeps the backlight at full power but switches it on and off rapidly.
PWM dimming is cheaper to implement and keeps color accuracy consistent across brightness levels. It is used by most budget and mid-range monitors, and by many smartphone screens.
The on/off switching happens at a fixed frequency, typically between 60Hz and 3000Hz. At higher frequencies the flicker is invisible to most people. At lower frequencies, some individuals can consciously or unconsciously perceive the pulsing.
Who is actually affected
Research and user reports consistently show that PWM sensitivity varies significantly between individuals. Most people cannot perceive PWM at any commercially available frequency. A subset of people experience eye strain, headaches or general discomfort after extended use of PWM monitors, particularly at lower frequencies.
People with photosensitive conditions, migraine history, or certain neurological profiles report more sensitivity. There is no test to predict sensitivity before you buy. The only reliable test is whether you experience symptoms.
What frequencies are common
Frequency affects how harmful flicker is for sensitive people:
The frequency does not change with brightness. What changes is the duty cycle. At 100% brightness the backlight is on 100% of the time (no flicker). As you lower brightness to 50%, the backlight is on 50% of each cycle. At 10% brightness, it is on only 10% of each cycle. This is why symptoms get worse at lower brightness settings.
How to test your monitor
The most common method is the camera test. Open your phone camera app in video or photo mode and point it at your monitor screen. Move the camera slightly while watching the screen preview. If you see dark bands scrolling across the preview, your monitor uses PWM.
Cameras have their own frame rate and shutter speed. The dark bands appear when the camera captures during the off phase of the PWM cycle. If the preview looks uniform and you see no bands at any movement speed, either your monitor uses DC dimming or the PWM frequency is much higher than your camera can detect.
Our PWM Test tool provides an additional check using the browser. For a definitive measurement you need a photodiode and oscilloscope, or a purpose-built flicker meter like the Opple Light Master.
Monitors that avoid PWM
Monitors marketed as flicker-free use DC dimming instead of PWM. Most monitor manufacturers from around 2015 onward offer at least some flicker-free models. Look for: flicker-free certification, DC dimming, or TUV Rheinland flicker-free certification.
OLED displays generally do not use PWM backlight dimming because OLED pixels emit their own light. However, some OLED panels use a different form of flicker called high-frequency PWM at the pixel level for low brightness, which some users still find problematic.
Managing symptoms if you have a sensitive monitor
If you cannot replace the monitor immediately:
Long term, the solution is a DC dimming monitor or a high-frequency PWM panel (above 1000Hz).