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Diffuse roughness is how you would simulate a powdery surface like dried clay or dust. 
When roughness is 0.0,  PxrSurface uses the Lambertian model to calculate the diffuse response. 
When roughness is > 0.0, the Oren Nayar model is used instead.

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The diffuse exponent controls the diffuse falloff.  It is a power exponent.  A higher number gives us a sharper falloff. 
For example, on the moon's surface, you can set a high exponent to produce a sharper falloff.

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If on, illuminate both sides of the surface for this diffuse lobe, that is, this will illuminate the surface whose normal is pointing away from the camera (2-dimensional objects) as well.
This is so the backface will be shaded instead of black. This is off by default.

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