The Diffuse group applies to the currently selected surface in the surface combo box below the preview.
Color sets the base diffuse color of the surface.
Specifies which illumination model will be used. The drop-down list provides for two choices, including Internal, which is the standard Lambertian model, and the Oren Nayar model, which provides for a roughness variable, allowing the user to go from simple surfaces (0 Roughness, identical to Lambertian), to complex rough surfaces (100% + Roughness, which provides for a surface, such as linen or dirt).
Dictates how rough the Oren Nayar illumination model will be. Zero is no roughness while 100% or greater is very rough. Experimentation is required to fully understand this parameter. This option is only available if Oren Nayar is selected in the Algorithm drop-down list.
Scales the Diffuse Color to produce attenuation in the illumination of the surface (this simply means it makes the color darker when the value is close to zero and brighter when the value is closer to 100% or above). If you are utilizing the Oren Nayar illumination algorithm you may want to bump up the illumination about 10% to 20% because it renders darker usually. Appropriate values are between 0% and 200%.
Provides contrast for the result of the diffuse sample color. Zero causes no effect, 0.0% - 100% causes standard contrast, above 100% causes a wrap contrast (the values that go above 100% wrap around to 0% again, used to create the Candy materials), negative values provide an inverse contrast that simulates materials that seem luminescent, such as silver. Appropriate values are 500% to 500%.