ThanksI used white as background with a strength of 2. There are 3 windows. The two you can see, and one on the right side of the camera. And basically thats it. The lamps on the blue wall are IES Lamps. But here's the trick I found on blenderartists: dl.dropboxusercontent.com/u/58… use this shader for all the walls and the floor. This will light the scene from everywhere, thus preventing noise. But after the first bounce the material changes from AO back to diffuse.