in the stream yesterday (#96), Casey underlined "illumination is constant along a line", which is correct of a 1d world (like a wave tube) , but for 2d and 3d worlds, illumination is decreasing because of conservation of energy,see here http://en.wikipedia.org/wiki/Inverse-square_law. you can say that for a perfect ray it doesn't matter, but it does matter for a finite sized ray, and also for understanding.
Casey answered in stream about a 2 mirror system, and he said that the diminishing effect of the higher order reflection is because the mirrrors aren't perfect. in the real world, a mirror returns about 97% i think of the light, so it really does has an effect, but the effect of the propagation of light is much more noticeable, diminishing light in a non linear way. i won't go into details because of the format of the posts, but the second reflection is about 25% of the original, the third 11%, and so on.
Thanks for reading.
edit: i watched till the end now, and i saw that what he called brightness is not intensity. he went into it at the end, but he talked about something like brightness=intensity/area of something,sorry about it.