Modern monitors use PWM to control the brightness of the screen. This is not problem at maximum brightness, but at low brightness it causes eye pain.
Iris is fixing this by controlling the brightness with the video card and by changing the white point of the monitor. This ensures that your backlight is at high frequency even at low brightness levels.
The best solution to this problem is to actually use E-ink device. Other solution is to buy flicker-free monitor. Using flicker-free monitor or using Iris is basically the same, but nothing can replace using E-ink or not using electronic devices at all.