I'm using an external monitor, so most of the time I've got the MacBook Air's internal screen dimmed to 0. But it doesn't switch off completely at 0 brightness as I can still see some graphical elements. They're hardly discernible but still there.
I was wondering if this is normal.
Unfortunately I couldn't find any other way to disable the built-in screen.
Best Answer
This is normal for LCDs that have the backlight off (which is what brightness at zero does), but which are still turned on. As far as I know, it's a side-effect of how LCDs work. The LCD screen itself turns the pixels on or off, but a CCFL or LED backlight behind the screen shines through to illuminate them. So while the backlight is off, the pixels are still active, just very hard to distinguish them.
I find on my old 2006 MBP, the Apple light from the lid leaks enough light that I can read a bit of text in the centre of my screen, even at zero brightness.