The Shifting Screen
Modern laptops running Ubuntu often come equipped with an Ambient Light Sensor (ALS), located near the webcam. Ubuntu utilizes this hardware to implement an “Automatic Brightness” feature. When you are in a dark room, the operating system dims the screen to reduce eye strain and save battery life. Conversely, if you move into direct sunlight, it automatically blasts the brightness to maximum. While this sounds ideal in theory, the sensors are often overly sensitive. A shadow falling across the laptop or a slight change in overhead lighting can cause the screen to constantly flicker and shift brightness levels, which is highly distracting when trying to read text or edit photos.
Taking Manual Control
To lock your screen at a consistent illumination level and stop Ubuntu from automatically changing the display brightness based on the ambient light sensor, you must disable the automatic power management feature.
Click the system menu cluster in the top-right corner of your screen (where the battery, network, and volume icons reside). In the dropdown menu, click the gear icon to open the main Settings application. In the left-hand navigation sidebar, scroll down and select the Power tab. On the right-hand side of the screen, look for the section labelled “Power Saving Options.” You will see a toggle switch explicitly named Automatic Screen Brightness (or simply “Automatic Brightness” on older GNOME versions). Click this toggle switch so it turns from active (usually orange or green) to grey (Off). From now on, Ubuntu will completely ignore the ambient light sensor. Your screen brightness will only change when you manually adjust the slider or press the brightness keys on your keyboard.