Quote from ChickenMobile on August 27, 2011, 2:04 am

try an env_projectedtexture. This is what Valve used for sharp shadows and lighting. The only catch is that only one projected texture can be active at one time, so you need to put it in the most dramatic lighting position in the current room.

If I remember correctly: inside Valve's example SDK map coop_doors they have a projected texture inside the observation room which you can just copy.