[ltp] Inconsistent screen brightness with a t61

[Micha]

I have a thinkpad t61 with nvidia graphics and I have a strange problem where
the screen doesn't always achieve maximum brightness. Under the same ambient
light conditions and maximum software brightness (fn+home) sometimes the screen
feels almost too bright and sometimes I feel like I have to strain to see the
text.

Could this be a software issue or is this a hardware issue?

Thanks