For a large number of users glare from glossy screens and improperly adjusted screen brightness are significant factors contributing to eyestrain. However, they are not the only or even the primary causes for everyone.
My previous negative experience with Nxtpaper 3 wasn't due to a lack of fiddling with brightness or color settings. It stemmed from hardware-level mechanisms that cause severe discomfort for a significant portion of the population. There are factors beyond what is easily seen, like reflections.
"Light is light. At the same brightness it doesn't matter if a backlight, frontlight or ambient". The human visual system evolved over millennia to process light reflected off objects. Emissive displays, like LCD and OLED, are direct light sources that project photons straight into the retina.
In a 2023 Harvard study researchers found that direct backlit LCDs cause retinal cells to produce "reactive oxidative species" (ROS), a key factor in photo-oxidative damage. The study concluded that one could use an E-Ink display with a frontlight for three times longer before reaching the same level of cellular stress.
https://sid.onlinelibrary.wiley.com/...1002/jsid.1191
In contrast, a reflective mono eink display with its frontlight turned off is optically analogous to paper. It emits no light of its own. Even when its frontlight is on, the light is diffused across the screen's surface before reflecting to the eye - a far less direct and aggressive pathway than a backlight. The Harvard study confirms that even front-lit eink is significantly less stressful on retinal cells than a backlit LCD.
Beyond the nature of the light source, its stability is important. PWM rapidly cycles the backlight between 100% ON and 100% OFF. The ratio of on-time to off-time creates the perception of reduced brightness. While most people cannot consciously perceive flicker above around 90Hz, our neurological system can detect it at far higher frequencies, leading to headaches, migraines, and fatigue.
Even more problematic for some is Temporal Dithering (aka FRC). Using FRC panels rapidly flash adjacent pixels between shades of colours, tricking the brain into perceiving a desired colour otherwise beyond the capability of the panel. This constant, high-frequency pixel-level flashing is a known cause of severe neurological symptoms in many users.
Flicker sensitivity is an extremely common condition estimated to affect the majority of computer users. While the causes are multi-factor, sensitivity to the high-frequency flicker from PWM and FRC is a major contributor. Many who report headaches and fatigue from screens are likely reacting to this flicker without knowing its source.
Sensitivity is not a binary switch, it’s a spectrum. One person may be entirely unaffected by PWM on a device, while for another it is debilitating. Even for those who do not feel immediate symptoms, research shows that the brain subconsciously detects flicker up to 200Hz, which can still cause fatigue and reduce cognitive performance over time.