Due to various transfer changes during encoding processes and the fact that many modern HDTVs contain "power saving" auto systems, you generally should use the menu screen on any DVD or Blu-Ray to adjust the brightness on your television for that specific film.
The brightness control on HDTV - due to many factors - is far less standard than on traditional analog TVs.
You'll have a particular problem with HDTVs if you have not set their internal clock correctly because many of them make the picture darker if they think it is daylight and brighter if they think it is night, because they assume you'll need it to be brighter if your room has more light in it.
Some of them don't do this by timer but by a light sensor so if you have a lamp near your HDTV it can cause it to broadcast at the wrong brightness level unless you go into the menu and adjust it.