PWM = Pulse Width Modulation
It is a method to control voltage level. Here, specifically, Lukasz is referring to how the backlight brightness in LCD screens is controlled.
One level method is vary the voltage statically using a voltage controller. So, if you ask for 50% brightness on a screen fed with 5V power, the line would deliver 2.5V.
PWM uses temporal modulation. It generated a square waveform and modify the ratio of full voltage and 0V to create the expected value. For the same example, PWM would generate a waveform with 50% on at 5V and 50% OFF at 0V, resulting in an average of 2.5V and 50% brightness.
The problem comes when the frequency of the PWM is not high enough, then some people can see the flicker due to those fast ON/OFF cycles.
In summary, PWM is a brightness control type for LCD and some implementations are slow enough that people can see flickering, resulting in eye strain.
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