The trick is not to lower the brightness but to invert it. Under Mac OS, press Cmd-Opt-Ctrl-8 or get one of the many apps that do virtually the same thing but add more options. Basically, it just inverts the gamma curve and what was white becomes black etc. Under Linux... I have no idea.
Check out this post on lifehacker. You can see what the screen looks like and you can maybe replicate it on Linux. Someone in comments has a suggestion for Linux too:
You can get the same thing in Linux installing Compiz/Beryl and it's "Negative" plugin.
Once active just press Super+m (Super is the "Windows" key) to "negate" the screen.
