Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!aries!mcdonald
From: mcdonald@aries.scs.uiuc.edu (Doug McDonald)
Newsgroups: comp.graphics
Subject: Re: Lambert's Law & the Moon
Message-ID: <1990Nov19.140818.5095@ux1.cso.uiuc.edu>
Date: 19 Nov 90 14:08:18 GMT
References: <27331@cs.yale.edu> <10506@ubc-cs.UUCP>
Sender: news@ux1.cso.uiuc.edu (News)
Organization: School of Chemical Sciences, Univ. of Illinois at Urbana-Champaign
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In article <10506@ubc-cs.UUCP> fournier@cs.ubc.ca (Alain Fournier) writes:
>As this is one of my favourite trick question about reflection, I'll bite.
>Consider the moon when full (that is the sun -the light source- is in the same
>direction as the observer -the eye. It is pretty obvious that it appears
>essentially as a disk, that is the reflected light is about of same
>luminance all over the visible part of the moon (I ignore the effect
>of the surface details such as the maria). That shows that it is neither
>a specular reflector (the centre would be a lot brighter than the periphery,
>nor a diffuse reflector (the centre would be somewhat brighter than the 
>periphery (try this on your favourite renderer, a perfectly diffuse white
>sphere illuminated from the direction of the eye). So what's going on.
>Well, as most of the computer graphics types (us) ignore, the world is not all
>between totally diffuse and totally specular, there are surfaces outside
>of this. In the case of the moon, it so happens that a Phong model
>(using the expression loosely) with an exponent of 0.5  for the cosine
>of the angle normal/light gives the right appearance of a disk at full moon.
>I can find exact references back in my office, if anybody is interested.
>Credit where credit is due: Bob Woodham, of UBC, first pointed that out to me,
>and has worked on the subject of models for the surface reflectance of the
>moon (and other objects in the solar system).


Remember that the moon has hills and valleys. The hills near the terminator
are what one sees, and the actual surface is seen at an angle much less than
90 degrees. The actual surface itself is sort of Lambertian. At least 
when I actually examined some small (~5 cm) size pieces of the Moon in a lab
they looked rather Lambertian. On the spot reports agree. 

The word "fractal" comes to mind.

Doug McDonald