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Define Diffuses: Creating Properties for Rough Finish That Diffuses Light

Question

I would like to model a plastic object with a rough finish that diffuses light. How can I create the properties for this in TracePro?

Synopsis

Creating properties for a rough finish that diffuses light

Solution

The plastic material with the rough diffusing surface can be modeled in TracePro, but as two separate properties (select Define/EditPropertyData).

MATERIAL PROPERTY

A TracePro Material Property can be created, and the index of refraction of the material is entered here, as well as an absorption coefficient if it is known (this would be for absorption losses just from propagating through a given thickness of a clear polished sample of the material, unrelated to the diffusing surface finish. Entering zero for the absorption coefficient is probably a reasonable estimate for a transparent material.

The Material Property is applied to the Object (Define/Apply Properties).

SURFACE PROPERTY

A TracePro Surface Property can specify 5 possible outcomes for light incident on a surface - Specular Transmission, Specular Reflection, Scattered Transmission (BTDF). Scattered Reflection (BRDF), and Absorption.

For your property, I expect that Specular Transmission, Specular Reflection, and Absorption would all be 0, leaving only Scattered Transmission (BTDF). Scattered Reflection (BRDF). BTDF and BRDF are controlled by 3 coefficients - A, B, and g. For an initial estimate, I would suggest entering BTDF(g) = BRDF(g) = 0, which defines a Lambertian distribution.

The challenge is to determine the proper ratio of Transmittance vs Reflectance. You can enter a value for BRDF(A) and then use the Solve For Feature to solve for BTDF, and the editor will display the resulting Integrated BTDF and Integrated BRDF values. You want these to sum to 1, and to have a ratio that matches the behavior of your surface. If the ratio of T vs R is dependent on the Incidence Angle, you can use the Add button in the property to add Incidence ANgles to the table, and then define the BTDF and BRDF separately for each incidence angle.