To use Energy Saving feature
HOME Setting icon All Settings icon Picture Energy Saving
Reduces power consumption by adjusting peak screen brightness.

Auto
  TV sensor detects the ambient lighting and automatically adjusts the screen brightness.
  This feature is available on certain models only.
Off
  Turns off the Energy Saving mode.
Minimum / Medium / Maximum
  Applies the pre-set Energy Saving mode.
Screen Off
  Screen is turned off and only sound is played.
  Press any button except the power and the volume buttons to turn the screen back on.

If you use Energy Saving function, brightness of your TV will be affected.