Quote - Dont mean to be a ball buster but my question was ignored once again... I must have the plague LOL

 

It's compiled into the scene and calculated like anything else. It's just when light rays strike the surface, nothing happens to them. The path is continued from that point exactly as it arrived. It also consumes a bounce of your total maximum (Reality uses 25 by default, or at least it has in all the scene files i've seen). The reason it's done like this is because null is generally not used by itself, it's combined with some other shader somehow to cause only part of the object to become invisible. Things like alpha mapping, sheer materials, volume-only objects, etc. See: http://www.luxrender.net/wiki/LuxRender_Materials_Null