This is a revised version of my post from 2013 previously published at http://devbraindom.blogspot.com/2013/03/eliminate-led-screen-flicker-with-intel.html.
Although still popular, that post was full of factual mistakes.
The brightness of LED backlight in TFT displays is sometimes controlled using a technique called PWM or pulse width modulation. If implemented incorrectly, particularly if PWM frequency is too low, it can introduce distracting visual effects and induce eye strain, headaches, and even dizziness in some people.
Many older laptops set PWM frequency for their backlight too low. Luckily, in laptops equipped with Intel i915 GPU the PWM frequency is often controlled by the GPU and thus can be adjusted by a software.