Re: The reflector problem

[T3D John Bercovitz <bercov@xxxxxxxxxxx>]

Joel wrote:

> Now, a question: John, can you describe "a Lambertian surface"
> in layman's terms, (or are you simply trying to limit the
> discussion on this topic ;)  ???

A "Lambertian surface" obeys Lambert's Law.  (Surprise! Surprise!)  
Lambert's Law says that the flux from a unit surface goes as the 
cosine of the angle between where you are observing from and a 
normal to that surface.  I should just copy this from a book 
before I tie my tongue in knots.  Well anyway, imagine a circle 
tangent to a line.  Draw a line from the point of contact to any 
point on the circle.  The length of that line is proportional to 
the amount of light coming from a unit area of that surface.  'Zat 
make better sense than talk of cosines?  So now what if we have an 
infinite surface or at least one which is larger than what our 
light meter can see?  Well, as you tilt the surface relative to 
the line from the light meter to the surface, the amount of 
surface the light meter sees goes up but the amount of light from 
a _unit_ surface goes down.  These two effects exactly cancel out 
and so no matter what direction you look at a surface from, the 
flux per unit solid angle is the same as long as it fills the 
field of view of the light meter.  

Of course this doesn't work for specular surfaces, only totally 
diffuse surfaces.  Most surfaces are somewhere in between.  My 
garage door, with its weathered white paint, is as nearly 
perfectly Lambertian as I can measure.  So it does happen.  My 
brick wall is not Lambertian.

John B

Hey!  Now we're talking viewer construction!  Good.

And thanks for puttin my picher up, I'll havta take a look.


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End of TECH3D Digest 5
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