>>> If you are using fresnel, is it valid (from a PBR point of view) to even
>>> scale the results using a maximum reflection other than 1?
>>
>> Absolutely - if, for instance, your surface is littered with
>> non-specular material at a microscopic level. Fine rust on iron steel,
>> for instance, or a thin coating of dust.

But are you sure simply scaling down the Fresnel reflection term in 
those cases gives a physically correct result for those materials?

>> In those cases it might be better, however, to use an average of two
>> materials.

Yes that's my concern, that if you are using the reflection scaling to 
"fake" a different effect (roughness, absorbtion or scattering etc) then 
it *might* not be physically correct to just scale it.

In other words, if you created a geometrically microscopic rough surface 
(or added physical dust, sub-surface scattering or whatever) and 
rendered it with max reflection 1, would it be possible to get the same 
result by rendering a perfectly smooth, clean surface with max 
reflection set to some lower value?

> What about plastic? It reflects, but it's not highly reflective. I don't
> think I would go higher than 0.5 maximum reflection on plastic.

Agreed completely, but does using the Fresnel equations to calculate 
reflection (which gives an absolute value) and scaling the result to 
simulate some other effect (surface roughness, sub-surface scattering 
etc) give a physically correct result? Or is it just a "fudge factor" to 
give something that looks correct?

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