I had the dim screen problem with my Aspire 9420. Could find no help until I googled my way to this website. Saw all the posts praising the FN key plus right arrow solution and thought at first that the cursor arrow was meant. Eventually saw post from Bill (April 2008) explaining that the arrow key in question is the one level with the space bar at the bottom of the keyboard. And then: the light shone bright once more. Thanks Bill!
ValleyBoy, September 2010