Many laptops have their screens dimmed after returning from “suspend” and cannot get back to their original brightness. The bug hasn’t been fixed for 3 years. A fix is provided in the bug report by putting something like the following in
brt=`cat /sys/devices/virtual/backlight/acpi_video0/brightness` abrt=`cat /sys/devices/virtual/backlight/acpi_video0/actual_brightness` if (( $brt != $abrt )) ; then echo $abrt > /sys/devices/virtual/backlight/acpi_video0/brightness fi
Use the following
find /sys/ -iname '*bright*'
to see if you need to change the exact path to the files.