Eco Sensor and Screen Brightness
Eco Sensor measures the light in your room and optimizes the brightness of the TV automatically to reduce power consumption. This causes the TV to brighten and dim automatically. If you want to turn this off, go to MENU > System > Eco Solution > Eco Sensor.
Do not block the sensor with any object. It can decrease picture brightness.
Preventing burn-in.
Avoid keeping a still picture or a picture with static elements (black bars, black borders, logos, etc.) on your TV for more than two hours at a time. If that is unavoidable, change the picture or change to another channel for a minute or two every couple of hours. Reducing the brightness and contrast of the screen when it is displaying static elements will also help.