My guess is this model has an ambient light sensor. Most models with that have a way to turn such off.
I just bought a Panasonic 55" plasma TV. I have a lamp to the side of the TV. When I move the lamp further away from the TV, The screens gets darker and is not as good of a picture. When I move the lamp closer, it gets lighter and has a great picture again. Is this normal? I thought plasma would be better with less light near it.