OS X El Capitan: ambient light sensor

Many Mac computers and displays have a built-in ambient light sensor, which makes automatic brightness adjustments on the display based on current lighting conditions. To find out if your Mac or display has an ambient light sensor, see the manual or documentation that came with it.

If your Mac has an ambient light sensor, you can use it to adjust the screen’s brightness by selecting the “Automatically adjust brightness” checkbox in the Displays pane of System Preferences.