46GW945 Dim picture

I have had the tv for about 3 years and since then we have noticed that the picture has gotten dimmer. It is at the point now that if a movie or tv show has a lot of black scene in it we can't watch it as we can't make out the picture. (We are pretty much limited to the Disney channel or Telemundo because of all the bright colors. You have no idea the toll that Spanish tv takes on a relationship.) What can I do to fix the dim picture problem?