Disable Adaptive Brightness ( I tested this on Windows 8.1, but should work on both 8 and 10 )
- Open the Windows Control Panel ( Do Windows key + C on your keyboard to open the Windows Charms, click on Settings and select Control Panel ).
- From the Control Panel, navigate to Hardware and Sounds > Power Options.
- On your selected Power Plan, click on Change Plan Settings.
- Next, find and go to Change advanced power settings.
- On Power Options, look for Display and click on the plus sign to expand the available options.
- Expand Enable adaptive brightness and turn off for either On battery or plugged in, or turn off for both.
- Click on Apply to save changes.
Note: After doing the steps above, you may need to restart your PC for changes to take effect.