Check out the app called Shades:
Shades is a FREE utility for controlling the brightness of your screen. It runs in the background providing always-available fine-grained control over the brightness of your display via a slick Mac-like interface
Though all modern Apple laptops have brightness controls for dimming the screen in dark conditions, there are several situations where the built in controls are unsatisfactory. The granularity of control is rather coarse, and the minimum brightness setting still quite bright when working in pitch darkness.
Perhaps you need to use your machine in a darkroom, or like to work late at night without disturbing your partner who sleeps beside you?
Perhaps you need to dim the screen on a desktop machine, and the display you are using offers no such facility?
If this is the case for you, then try Shades.
Note: Shades may interfere with colour calibration software, and should not be used if colour accuracy is critical.
My iMac has a "fault" in that the lower I set the brightness, the noisier the screen buzzes, due to an issue with the capacitor that does the work. But using this app I can get the brightness down way further than using hardware controls on the backlight, and the buzzing is not introduced either. Double result!
It's also useful to use this tool for altering the brightness on external displays which do not respond to the brightness key controls.