Diffuse is the percentage of light from light_sources reflected 
"diffusely" from the surface, ambient is the same thing, except for the 
ambient "light source" defined by ambient_light in global_settings. The 
default ambient light is white, so you can use "ambient 1 diffuse 0" to 
specify all points on your are illuminated with a white ambient light 
(that is, a light with no position or direction, that fills space)
Or you could use "ambient 0 diffuse 1" to specify that the only 
illumination comes from light_sources. In that case, completely shadowed 
areas will be pure black.

Basically, ambient specifies a "minimum illumination", and diffuse 
specifies the effect of light_sources. I generally try to make sure 
ambient+diffuse+reflection <= 1 to get more realistic 
textures...otherwise, your surface is reflecting more light than it 
absorbs. Also, giving everything an ambient of 0 and using radiosity 
gives very realistic shading...

And though these are percentages (with 1 being 100%), you can go outside 
the [0, 1] range...while this is usually not realistic, it is useful in 
radiosity, where you can make an object glow by giving it a very high 
ambient value.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

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