I had the same kind of problem last week with volumeric lights. It was in my tower image where you have a parallel tube light pointing upwards. Then I put a normal spotlight pointing to the doorway at the base of the tower. What happened was that the volumeric light at the top took on the color of the spotlight at the bottom. I vaguely remember something about objects and textures, when being higher with the same texture it affects the lower texture. Not a very good description but does anyone have an idea on this being the problem, ie. similar objects taking on the qualities of the lower object? Catlin