Surface and Material Properties - guidelines for when to use one or both

Synopsis

Solution

"The answer to whether or not you need both a Material and a Surface Property is ""sometimes"". There are some effects that occur at an interface that is best modeled as a Surface Property:

• surface roughness or texture (results in reflective or transmissive scatter)
• coatings (thin films, black paint, anodizing, plating, mirror finish, etc) that alter the relative percentage of transmission vs reflection vs absorption

There are other effects that occur either within the volume of the body or at the interface due to the change in index of refraction, that is best modeled with a Material Property:

• refraction
• fresnel reflection (like the 4% reflection that comes from a window)
• bulk absorption (absorption that is dependent on the thickness of the material, like a red filter glass)

An example of a case where both a Surface and a Material Property are needed is an AR Coating on a glass lens. The coating reduces the reflection that would have occurred without the coating (Fresnel reflection based on index), so a Surface Property is needed. But the Material Property is also needed to define the refraction.

An example of a case where the Surface Property is sufficient is Black Paint. Since no light is transmitted, there is no refraction or Bulk Absorption that can occur, and the Fresnel reflection is superceded by the black paint coating.

An example of a case where the Material Property is sufficient is a polished piece of glass. The meaningful effects are refraction and Fresnel reflection, and possibly a small amount of bulk absorption. There is no coating or surface texture that would warrant a Surface Property."