Monitor dims when we turn lights off in room.
You need to give a more detailed description of the problem.
The LG 47LH90 is an LED backlit TV with 'local dimming' technology. This means that there are lots of white LEDs behind the screen and these can be controlled in groups to allow localised areas of the screen to be dimmed to produce better contrast between light and dark. It is very likely that when the room is dark, this local dimming is more noticeable. The actual individual LEDs cannot be controlled and so the area that is dimmed will generally be greater than the area that is required to be dark 'blooming effect'. In addition, LED backlit displays only maintain their contrast through a tight viewing angle. If you sit too farto the right or left, then light from non-dimmed LEDs will leak through and the screen will look lighter.
In short, there may not be any fault with your TV.