Avoid PWM for screen dimming

[jonpolak]

Why/User Benefit/User Problem

Pulse Width Modulation PWM is often used to control the screen brightness by reducing the duty cycle of the LED backlight. This is problematic because it is actually flashing very quickly instead of uniform light. This gives many users headaches.

Description of the feature

Use DC current regulator to control the brightness of the backlight

[mmstick]

The brightness of a diode is not controlled by the amount of current given to it, hence why LED light bulbs in a dimmable switch are either completely off, or completely on. Brightness of a LED can only be simulated using PWM, and dimmable LED bulbs use a "smart chip" to convert current differences into PWM. The frequency of the PWM shifts is so frequent that your eyes can't tell the difference. So if you're getting headaches, it's because the frequency of the shifting is far too low.

[jonpolak]

Hi Mike -- LED brightness can and is often controlled with analog (current driven) modes. Here's a 2011 application note from Aimtec

Here's just the chart from Digikey's 2016 article on how to build a PWM controller to simulate the Brightness (luminous flux) dependency on current

. FULL ARTICLE HERE

PWM became the standard in cheap displays because it's the easiest way to maintain chromatic stability while reducing brightness. This article from NOTEBOOK CHECK explains why PWM characteristics of displays (like switching frequency) cause headaches. They have a full list of every machine they reviewed PWM characteristics. You'll note that high end devices (like the 2019 Macbook pro) has a zero switching (DC current controlled brightness) or very high fequency. System76 uses Clevo displays (I believe) and you'll find a couple of those in the list cited above. Interestingly they have PWM switching frequency of zero.