Change display brightness in Windows with Boot Camp on Mac
You can change the brightness of your built-in display and your Apple Cinema Display.
You can adjust your display’s brightness in Windows or with the brightness keys on your Apple keyboard. You may need to hold down the Fn key as you press the brightness keys. For information about configuring the Fn key, see Use Windows keys on a Mac keyboard.
Change your Apple Cinema Display settings
In Windows on your Mac, click
in the right side of the taskbar, click the Boot Camp icon 
, then choose Boot Camp Control Panel.If a User Account Control dialog appears, click Yes.
Click Cinema Display.
You can choose what happens when you press the display’s power button and the brightness buttons.
Change your display’s brightness in Windows 10
In Windows on your Mac, click the Start button, choose Settings, then click System.
In the list on the left, click Display.
Set brightness options as desired.