Disclaimer: this article is rather old, and does not reflect linux' current state of brightness on the Thinkpad E135
first, it was fixed on maximum. no tool could change it,
/sys/class/backlight/acpi_video0/brightness could be written to and changed, but the backlight didn't. Tried the solutions in1, excluding those which would just echo to above /sys/... file, until I found out Kernel parameter
acpi_backlight=vendor did the trick.
/sys/class/backlight no longer contained an
acpi_video0 directory, but a
thinkpad_screen directory, that has the same brightness files, which change on using the Fn keys for brightness change.
If I write to them their content changes, but not the backlight, but at least the Fn key combos work.