Here’s the deal for me. A few of my Macs, more than a few of those I manage in the school where I work, have an external display attached. Apple’s built-in brightness controls work fine for Apple displays, but for connected displays, not so much.
Just Dim The Screen
External displays connected to a Mac usually have their own screen brightness and dim controls and where they are varies from display to display. Some controls are onscreen, others not, and a few are even difficult to find.
Brightness Slider is a free app that lets you adjust screen brightness from the Mac’s Menubar, including attached external displays.
The way Brightness Slider works is just plain remarkable and combines two methods. For Mac displays, Brightness Slider uses controls to adjust the backlight intensity of the screen.
The app also creates a unique semi-transparent black veil that covers everything else, which gives you more control over the dimming process with more intermediate steps; especially evident in a room with little light.
If Brightness Slider cannot manage the screen’s built-in backlight control, it reverts to the semi-transparent black veil which also works (perfect for external displays).
Screen brightness levels can be adjusted right from the Mac’s Menubar, and dimming controls are more granular than those provided in Apple’s own OS X controls. Custom hotkeys can be assigned to lower and raise brightness, too.
Any Mac user with multiple displays knows the problem of keeping brightness at the right levels, especially on the external display. Brightness Slider fixes that and does it for free.