-> ambient light sensor not working at all
-> light sensor only works for keyboard illumination but not for display brightness
-> light sensor only works for display brightness but not for keyboard illumination
-> keyboard illumination works the wrong way round (ie. gets dimmed when room darkens)
-> after migrating an existing user account from another Mac
-> may occur on freshly installed MacBook Air with no account migration
-> after wake from sleep
-> create a new user account named "test" with password "test"
-> make sure this user is allowed to administer the computer
-> log out and then log in as user "test"
-> open the Terminal and type this one line command:
sudo cp ~/Library/Preferences/com.apple.BezelServices.plist /Users/YOUR_USER_NAME/Library/Preferences/
-> replace "YOUR_USER_NAME" with the short account name of the problem account
-> enter the admin password ("test") which will not be shown
-> log out and log in again using your own account
-> you may now delete the user account "test" if you want to
The ambient light sensor will now work properly on both the display and keyboard.
How to test if your ambient light sensor is operating properly:
-> make sure you are in a well lit (full daylight) room
-> make sure you enable "Automatically adjust brightness" in the Displays system preferences
-> make sure you enable "Illuminate keyboard in low light conditions" in the Keyboard system preferences
-> set display brightness to max using the F2 key
-> set keyboard brightness to max using the F6 key
-> hold your hand over the tiny holes to the left of the iSight camera
-> the screen will now slowly dim and the keyboard will now slowly light up
When putting the MacBook Air to sleep in a well lit room, and then waking in a dark room, the keyboard illumination will work the wrong way round. This should be solved by Apple in a future software update.
You can work around this by always waking and sleeping your MacBook Air in a well lit room.