AppleScript : Dim brightness below factory setting
As was noted in the comments, brightness cannot go below a setting of 0. If you use the brightness keys on your mac, you'll note that if you keep going, you hit screen off, which entirely turns off the screen backlight. From experience, I would assume this is zero. Below zero would mean your screen is actively attracting light from it's surroundings. You can adjust some settings by 1/4th notches by holding the option key and pressing any of the adjustment buttons, but that's as close as you'll get. What shady is doing is merely darkening all of the colors on your screen, which does decrease brightness because of how well made lcds work, but I'm not sure if that's exactly what you're after, as that can definitely affect how colors look otherwise. EDIT: If 1/4 brightness increments are not enough, shady and those small increments should stack in effect, but some older macs can't dim their brightness in 1/4th increments after you get down to one notch.