Hello. My problem is low range brightness (or fog effect like) in High Sierra 10.13.6 or Sierra 10.12.6. All seems work normal, slider changes brightness level from system prefs, fn keys work too, but display isn't bright like in El Capitan with IntelBacklight.kext.
In El Capitan with IntelBacklight.kext this fog effect was too, I found that in IntelBacklight.kext I have to change PCHL value to 0 instead of -1 to get full brightness range. But in new way PCHL not used
I tried with AppleBacklightFixup and WhateverGreen, same result
Hope we find answer together.
Thanks