Soreeyes That's very interesting then. I thought it was essentially all LED light sources I have a problem with, but maybe not! The list is short, but so far, I don't seem to have problems with the Nintendo Switch LED backlight or the overhead LED lights they installed at work to replace the old fluorescent tubes.
Now I'm wondering what I'm really being bothered by. I bought a flicker-free LED BenQ monitor back in 2013 thinking all my problems would be behind me, only to notice the same type of sharp eye/head pain, but that it just took a bit longer to be brought on.
Are there other types of flicker besides PWM that could be a factor? If there is, why are these other forms of flicker not as problematic on a CCFL-backlit monitor? I can also get similar sharp eye pain / head pain if I increase the brightness too high on my known-good Dell U2410, but it takes longer to set in than any LED-backlit monitor gives me.