PWM is used to regulate the brightness of LCD and LED displays. How does that concern us? The pulsations can be bad for your health and more specifically for the users eyesight. They can lead to migraines, headaches or other forms of discomfort after long-term usage. In most cases those pulsations have a frequency somewhere around 200Hz. They cause multiple contractions and expansions of the pupils, which bears down negatively on your health. They are especially harmful in models with LED backlight because the colors of the emitted light are not as inert as the colors of the CCFL backlight and so they reach high amplitudes and rapid luminance changes. Basically, the problem comes from the flickering light, not from the crystals of the LCD monitors.
Default PWM curve:
- On 100% brightness it shows a rather flat curve with 60hz, this is tolerable even for flicker sensitive people.
- On 75% brightness the PWM sequence changes to 240hz with high amplitude alternating, this can be already noticable for flicker insensitive people.
- Below 50% you get the typical stroboscopic effect which can have a negative impact on your eyesight.
Adjusted PWM curve:
- The lowest possible PWM sequence with a 60hz refresh cycle will be used on all brightness levels.
Here are two videos to show the difference in motion:
- Recorded with S7 slow motion camera mode; Starting with 100% brightness -> 60% -> 20% -> 5%.
- The first video shows the default Samsung kernel.
- The second video shows the pwmfree kernel.