Windows can adjust the brightness of the display depending on the amount of light that is reaching the system's ambient light sensor. This feature is referred to as 'adaptive brightness' and it can be disabled in Windows.

• For Windows 10
  1. Press and hold the Windows () key, and then press the q key.
  2. In the Search box type, settings.
  3. In the search results list, touch, or click Settings (App).
  4. Touch or click System.
  5. On the Display screen, find the Adjust my screen brightness automatically option.
     Note: Some notebooks do not support this feature, and the option is not available. You can manually adjust the brightness using the slider bar under Change brightness for the built-in display, after making any desired changes, close the System window.
  6. Touch or click the slider to toggle the option on or off.
  7. Touch or click Apply.
  8. Close the System window.

• For Windows 8(8.1)
  If you are using the Windows Modern User Interface (MUI):
  1. Swipe from the right and touch Settings.
  2. Touch Change PC Settings.
  3. On the PC Settings screen, touch PC and Devices.
  4. Touch Power and Sleep.
  5. At the top of the screen, there is a toggle for Brightness. Slide the toggle to the Off position to disable adaptive brightness.
  
  If you are using the traditional Windows desktop:
  1. Touch and hold the start button to bring up the Apps screen and touch the Control Panel app.
  2. Touch Hardware and Sound.
  3. Touch Change Plan Settings.
  4. Touch Change Advanced Power Settings.
  5. In the advanced power options, touch the + next to Display to open the options.
  6. Locate the setting to Enable Adaptive Brightness and set the appropriate options to Off.