Why does my TV screen dim or brighten by itself?

The Ambient Light Detection function (also called ECO Sensor on some older models) 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

The method for turning this function on or off will differ depending on your model

2016 models or older: System > Eco Solution > Eco Sensor > select On or Off
2017 models or newer: Settings > General > Ambient Light Detection > use the slider to turn on or off