What are the Solid Model versions of the Transmissive materials and when should they be used?
Transmissive materials use BRDF & BTDF files to characterize how light reflects and transmits when it is incident upon the material. When a ray strikes a transmissive surface in Photopia, the appropriate reaction is applied to the ray using the BRDF & BTDF data. The important point to note is that the measured BRDF & BTDF data takes into account the effects of the full material thickness. So when a ray in Photopia strikes an infinitely thin polygon with a transmissive material assigned to it, the full reaction of the physical material (with a thickness) is accounted for at this single ray/polygon interface. When a CAD model is constructed as a solid, every part has a thickness. Thus, if a prismatic lens or white diffuser material is drawn as a solid, it will be constructed with the thickness of the physical part. When this model is exported to Photopia via a STL file it imports as a mesh of polygons that cover the surface of the original solid model. If a transmissive material is assig
