I might add there are several terms and keywords, besides ambient, in POV,
parameters that can not be translated directly into the physics of light. They
are there basically as adjustments and control knobs not any different than
adjusting the brightness or contrast on your TV. Specular, diffuse, metallic,
noise -- while these have optical counterparts they are terms and concepts
developed by computer graphics, even in scanline, where the goal is just to get
as quickly as possible, by whatever means, a result that looks right. Perlin did
not come up with his famous noise factor because of physics but because by
chance it just happened to be a good way of visualizing certain things. It
wasn't like he
said I want to find an equation for how clouds look, but more like "hey, this
formula makes pictures that look just like a cloud." POV is a raytracer but it
is also a computer graphics tool and they do not use completely the same
techniques.

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