Oh, OK. I got it. Sort of like a 'test tube' or a test cube maybe. You get to control everything as you said. I'm actually trying to study how light DOES come into a "room" with controlled openings. The light comes into this room from UNDERNEATH the walls and is filtered through the water. Check out the shadows on the walls and ceiling from the surface of the water. Light also enters through the slits along the top of the walls. There is just a bit of sky and cloud visible. This setup only seems to work with a volumetric atmosphere. There is a streak on the water surface also reflected in the ceiling that I dont understand. any clues about it? Thanks.