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 with Boot Camp.
Change your Apple Cinema Display settings
In Windows, 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 7
In Windows, click the Boot Camp icon
in the system tray, then choose Boot Camp Control Panel.Click Brightness, then use the slider.
If your Mac has both a built-in display and an external display, you may see brightness controls in both the Boot Camp Control Panel and the Power Options Control Panel. In that case, the brightness control in the Boot Camp Control Panel is for the external display, and the brightness control in the Power Options Control Panel is for the built-in display.
Change your display’s brightness in Windows 8
In Windows, position the pointer in the upper-right or lower-right corner of the screen.
When the Charms bar appears, click Settings.
Click Brightness, then use the slider.
Change your display’s brightness in Windows 10
In Windows, click the Start button, choose Settings, then click System.
In the list on the left, click Display.
Set brightness options as desired.