It is reported that some people see flicker or experience eye strain or headaches when working with LED backlit monitors. This would be caused by pulse width modulation where the backlighting is switched on and off rapidly at varying on and off times to change the brightness. If this frequency of on and off switching, the pulse width modulation frequency, is below 500Hz or even 1kHz, these problems are reported by some people and not by others. Although the brightness of most CCFL backlit monitors is also changed using pulse width modulation, this is less of an issue because the CCFL tubes don't turn off completely during the off cycle, because they have some after glow, reducing the on/off contrast. Here's my question: have you experienced any of these issues - flicker, eye strain or headaches - when using an LED backlit monitor and if so, which monitor did you use and do you know by any chance the pulse width modulation frequency?