clay, I reread the first chapter of Cohen , and I think I derived a working definition, perhaps incomplete or partially incorrect: When a light strikes an object the reflected light carries with it some of the color of the object it reflects off of - in addition the light has it's own color. these colors in combination will be imparted to any other object the light strikes. It is not just that - particles in the air (having their own color will add to the blend and the visable light produced from that. they quote Renoir as saying that Shadows are never black(hey I knew that) and imply that radiosity applies to shadows as well. further any mirrored surfaces are included in the definition, so I can see how refraction and relection play into the HDRI and how that relates to radiosity. It is then a catch all phrase, the grand unification theory of the art world ? Clay, now that I (think I) know what to look for I can take your advise, but correcting any errors and ommissions I made would be helpful. - TJ