The key word is 'ambience'. As far as I can tell, this feature only works for materials with the ambience level activated. To get an object to appear to cast light onto another object, both objects must have materials with some ambience level. The light that is cast comes only from the ambient aspect of a material - not from 'bounced' light from its diffuse channel. Only the ambient aspect of the 'receptor' object's material is affected by 'true ambience'. In the image, sunlight has been disabled and the sun colour set to black. There are no diffuse colours in the materials, and there are no lights in the scene. The light being cast onto the floor comes from the ambient materials of the wall and sphere. The light being cast onto the top of the sphere comes from a brighter yellow object just out of view. The most obvious affect is from lighter colours onto darker colours, but only down to a certain darkness level - very dark or black doesn't appear to receive light at all. The light that appears to be cast from 'light' colours is realistic in its fall off rate. Looking at the image, you'd think that the darker colours were having no affect on the lighter colours at all. This is because their fall off rate seems to be very slight. The wall and ball are darkened slightly by the presence of the dark floor. It's almost as if the fall off rate for light colours onto dark colours is squared with distance (as with real light), while the rate for darker onto lighter seems linear with distance. I find the effect far too limited to be of any real use. In Bryce4, I'd have simulated the 'radiosity' with lights (positive, negative and coloured) and the scene would be more realistic and would have rendered faster. This is another quirky Bryce trick that someone, somewhere will find useful occasionally. The rest of us will fiddle with it to see what it does, baulk at the render overhead, and move onto something more practical. I'm not aware of there being a global illumination feature in Bryce5. The image in the link provided by rbanzai is rendered in Bryce4 I believe. 'blueba' put up a whole series of Bryce4 'trick' renders to show what could be achieved by simulation methods.