Why is the brightness of the TV screen always changing?

Customer may have enabled Light Sensor or Advanced Contrast Enhancer

Light Sensor
When the Light Sensor has been enabled, the brightness will automatically adjust to accommodate changes in ambient light intensity, or when the light sensor is being obstructed. It is suggested for this setting to be disabled, please refer to the below procedures:

1. Press [ACTION MENU] button on remote
2. Select Picture
3. Select Light Sensor and set to Off

Advanced Contrast Enhancer
If the above setting has not been enabled, whilst there’s an absence in variance of ambient light intensity or obstruction of the light sensor is absent, please confirm if white subtitles are displayed on the screen or the scene has great variance in brightness.
Please note the settings would allow the picture contrast to enhance automatically in these scenarios, and result in a sense of “changing screen brightness”. It is suggested for this setting to be disabled, please refer to the below procedures:

1. Press [ACTION MENU] button on remote
2. Select Picture
3. Select Advanced Settings
4. Select Brightness
5. Select Advanced Contrast Enhancer (Or Adv. Contrast Enhancer) and set to Off