Samsung TV (UA**KS****): Why does my TV screen dim or brighten by itself?
The Eco Sensor function measures the light in the room and changes the screen brightness automatically to reduce power consumption.
The screen will be brighter in a brightly lit room and darker in a dark room.
You can turn the Eco Sensor on or off by going into:
Screen Menu > System > Eco Solution > Eco Sensor > select On or Off.