Change your Mac display’s brightness
You can adjust the display’s brightness manually or automatically.
Use the brightness function keys
If the screen seems too light or too dark, you can adjust your display’s brightness.
On your Mac, press the increase brightness key
or the decrease brightness key 
(or use the Control Strip).
Automatically adjust brightness
If your Mac has an ambient light sensor, choose Apple menu
Select “Automatically adjust brightness”.
If you don’t see the “Automatically adjust brightness” tickbox, you can manually adjust the brightness.
Manually adjust brightness
On your Mac, choose Apple menu
Drag the Brightness slider to adjust the brightness of your display.
Depending on the type of display connected to your Mac, you may also see a Contrast slider that you can use to adjust the display’s contrast.
You can’t adjust the brightness of some older Apple displays. In OS X 10.10 and later, the Brightness slider may no longer appear in the Display pane of Displays preferences.
For specific information about the brightness of your display, check the documentation that came with your display.