Currently I tend to believe all monitors use PWM and that it depends on the frequency if it is detectable or not. I bought the BenQ because it was the very first device that was publicly advertised as being "flicker-free". I recently tried the Dell U2515H, which is supposed to be "flicker-free" (tested, but not advertised), too, and compared it to my BenQ EW2440L.
The Dell almost immediately causes eye strain, although I lowered its brightness and contrast to match the BenQ's low brightness (doing so pretty much destroyed the picture quality).
It's hard to see why there's eye strain:
- Both monitors are supposed to have a true 8 bit panel with (probably) no FRC.
- Both of them were tested to be "flicker-free" (that is, no detectable PWM by using the photo diode + oscilloscope test)
There could be numerous reasons why the Dell, for me, causes eye strain:
- It uses FRC after all
- The PWM frequency is not high enough
- The backlight does not use enough yellow phosphor
- The backlight is too bright at minimum brightness (35 cd/m² vs 19 cd/m²)
I believe it is try and error until we have more information about PWM, the backlight and panels.
What I do not yet understand is why I have to lower the brightness of all my LED devices to a minimum level. In bright sunlight I do not have any problems all day. But looking at an average, much less bright LED screen for some minutes hurts.
I also have an iPad 4 which Is supposed to be flicker-free and which can use at 0 brightness (but not higher). It supposedly has a true 8-bit IPS panel with an A-TW polarizer (the Dell does not have this).
Same goes for my iPhone 4, which is OK at higher brightness settings if I do not use it too much during the day but I had to buy and test several iPhone 4 and 4s just to find a device with a usable screen. Not sure if the reason is different display charges or the OS version. The iPhone 4 I got is still on iOS 6.0.1. All other device were either on iOS 6.1.3 or even iOS 7.