I'm also having this problem with a Gateway MP8709 with Intel 945
chipset.
This bug looks related/duplicate of:
https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/121833
Seems as though there are 8-9 discrete backlight levels listed in
/proc/acpi/video/VGA/LCD/brightness:
levels: 100 37 12 25 37 50 62 75 87 100
current: 0
but whatever is handling the ACPI brightness events is jumping in steps of 5%,
missing 12, 37, 62 and 87.
The other bug referenced mentions that echo'ing an invalid value to
/proc/.../brightness will error/fail but
/sys/class/backlight/XXX/brightness will accept the bad value and
essentially turn the backlight off.
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[gutsy] Screen brightness works only at certain values.
https://bugs.launchpad.net/bugs/149547
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