The official L4D blog just posted part 2 of their analysis of the Left 4 Dead art style. This one deals with ‘Stylized Darkness’, and discusses the methods used to light the maps to give the atmosphere they were after. Here’s a sample:

“Flashlights, like car headlights, were a great way to light the game without contradicting the game’s fiction. Even better, we found that by making the flashlight weapon-mounted, the light is just slightly off-center at all times. This created interesting shadows and helped make zombie attacks much more dramatic. From a gameplay standpoint, gun-mounted light sources also had the interesting side effect of disappearing when you reloaded or shoved zombies back.”

Part 1 of the article can be found here.