why do screens flicker

The Deep Dive

Screen flickering is fundamentally linked to how displays generate and maintain an image. In modern LCD and OLED screens, the image isn't static; it's constantly being redrawn. This redraw rate is known as the refresh rate, measured in Hertz (Hz), indicating how many times per second the screen can update its image. A 60Hz screen, for instance, updates 60 times a second. If this rate is too low, the gap between refreshes can become perceptible, especially in peripheral vision. However, a more common cause of noticeable flicker in contemporary displays, particularly at lower brightness settings, is Pulse Width Modulation (PWM). PWM is a technique used to control screen brightness by rapidly cycling the backlight or individual pixels on and off. Instead of reducing the voltage to dim the light, which can affect color accuracy, PWM maintains full power but varies the duration the light is 'on' within each cycle. For example, to achieve 50% brightness, the light might be on for half the time and off for half the time, thousands of times per second. While this cycle is often too fast for the eye to consciously register, it can still cause strain or be perceived as flicker by sensitive individuals. Older Cathode Ray Tube (CRT) monitors flickered due to the electron beam rapidly scanning lines across the screen, which was a different mechanism but produced a similar visual effect.

Why It Matters

Understanding screen flicker is crucial for eye health and comfort, as persistent exposure to flickering displays can lead to digital eye strain, fatigue, headaches, and even migraines for some individuals. For those with visual sensitivities or certain neurological conditions like photosensitive epilepsy, flicker can trigger more severe symptoms. In professional contexts, such as graphic design, video editing, or gaming, flicker can introduce artifacts, reduce precision, and diminish the overall user experience. Manufacturers are increasingly adopting flicker-free technologies like DC dimming or higher PWM frequencies to mitigate these issues, making informed choices about display technology important for well-being and productivity.

Common Misconceptions

A common misconception is that all screen flickering is a sign of a faulty display. While a broken screen can indeed flicker, many instances of subtle flicker, especially at low brightness, are due to the intentional use of Pulse Width Modulation (PWM) for brightness control, not a defect. Another myth is that if you can't consciously see the flicker, it isn't affecting you. Even imperceptible flicker can cause eye strain, fatigue, and headaches because your brain and eyes are still working harder to process the rapid on-off cycles, leading to cumulative physiological stress over time.

Fun Facts

• Some people can perceive flicker at rates as high as 250 Hz, far exceeding the typical 60 Hz or 120 Hz refresh rates of many displays.
• Early cinema projectors intentionally used a 'flicker fusion' effect by flashing still frames rapidly to create the illusion of continuous motion, a principle similar to how our brains interpret screen refreshes.