The screen on my Acer Laptop goes dull

[countrykid]

Every so often, and seemingly for no reason, the screen on my 2 year old Acer Aspire 5633W LMi suddenly dims. It's as if it has gone into eco mode if such a thing exists. It happens regardless of being on mains or battery, and stays that way for ages. Shutting down and rebooting does not solve the problem. I tried the laptop without the battery installed, hence mains only, and it remained dim.
Any ideas please. Just before I began typing this, the screen suddenly returned to normal brightness, and is fine, but I'd like to find the cause and know how to deal with it, as I am sure it will dim again before long. Thanks