KM The second mystery is why, for some of us including myself, changing the brightness can make eye strain symptoms worse or better. Blue light modes and tinted glasses don't really help here. There must be something else other than the widely acknowledged "blue light" that can trigger eye strain.
I don't know, but I suspect that it is a dithering issue.
FRC is a form of temporal dithering which cycles between different color shades with each new frame to simulate an intermediate shade. This can create a potentially noticeable 30 Hz flicker. FRC tends to be most noticeable in darker tones, while dithering appears to make the individual pixels of the LCD visible.
Anyone else notice what looks like color movement in the pixels of a display for a white background? It can be subtle, but when eye strain hits, it almost screams out. Put your face up close to the display and look at the pixels. You should definitely see it that way. It may appear as a sparkly static light texture at first.
For dithering, I think that maybe what is happening is that our eyes are becoming fatigued from being overcome by the colors of the pixels and perhaps some flashing of pixels to create colors between RGB. I don't know really. Just a suspicion.
Add dithering to too much blue light, too bright screens, pwm, and how one thing changes with another, and maybe each adds a bit to the disaster of LED.
How does 6-bit...10-bit, FRC, dithering, all work any way? Are the individual colors of pixels being flashed to create non-red/green/blue colors? I assume that this has to be the case, as the intensity of each individual pixel is not being precisely controlled, so it has be that each pixel is being switched on and off. So what we are experiencing is millions of simultaneous flashings of pixels. Maybe it is that LED backlighting only adds to, or intensifies eye strain in combination with LCD pixels switching on/off.
This also very much reminds me of the audio world. You could think of your old CRT displays as being sine waves or a continuous signal, and your LCD displays as being square waves or discrete signals, where in the audio world, square waves tend to sound harsh to the ears.
Maybe also, as in the audio world, some people are more sensitive to continuous vs. discrete.