how to visualize a field of view?

I'm trying to have an overview map of my level with a visualization of the field of view of my npc. I'm thinking of something similar to "Commandos".

Any ideas on how to go about this? I tried to this using spotlights attached to the heads and aimed on the floor. However I don't want the lights to show up in my main game window, just on the radar/map and couldn't figure out how to do this.

Maybe there is another way to do this?

 

I'm looking for the same thing.
Just inspire here. http://www.emanueleferonato.com/2007/09/26/create-a-survival-horror-game-in-flash-tutorial-part-1/ its flash, but could be helpful.