Simplistic Physics of Light Acting on Plastic Surfaces

To understand the physics that the I-DEAS display model is attempting to simulate, we must follow a photon of light as it interacts with a "plastic" surface. When that photon strikes the surface of a "plastic" part, it may act in one of three ways: (1) it may act in a diffuse manner, (2) it may act in a specular manner or (3) it may be absorbed into the part with the energy of the photon changing into another form, namely heat. The following illustrations, which are intended to represent material at the atomic level, should help illustrate what "diffuse" and "specular" mean.

Diffuse Behavior

Diffuse behavior results in our perception of the color of a part. The photon is effectively interacting with the molecules of the surface. Because the photon is "interacting" with the molecules of the surface, the direction in which the photon "leaves" the part is not dependent upon any angle-of-incidence-equals-angle-of-reflection physics. Such behavior is happening at the molecular level and is not predictable and must be evaluated statistically rather than physically. Diffuse behavior results in our perception of the color of a part. The photon is effectively interacting with the molecules of the surface. Because the photon is "interacting" with the molecules of the surface, the direction in which the photon "leaves" the part is not dependent upon any angle-of-incidence-equals-angle-of-reflection physics. Such behavior is happening at the molecular level and is not predictable and must be evaluated statistically rather than physically.

Green photons are absorbed by red parts (below), whereas red photons are "rejected" by red parts (above). Also, in the "real world" nothing is purely red, so even some red photons are absorbed by red parts and some green and blue photons are "rejected" by red parts. Also, dark red parts are dark red because they don't "reject" all red photons.

Specular Behavior

Specular behavior is generally what communicates a portion of the information about how "shiny" a part is. In this process, the photon merely "bounces off" the surface of the part much like a billiard ball bounces off the side of the billiard table. Predicting what direction the photon goes is applying the physical concept of "angle of incidence equals angle of reflection". In this model of photon behavior there is no presumption of color-of-photon or color-of-surface. The photon bounces off regardless of such issues.

The Microscopic Surface

The specular behavior of angle-of-incidence-equals-angle-of-reflection should be considered with respect to the microscopic level at which the photon exists. Even though a surface may seem "smooth", at the microscopic level there is always some level of "roughness".

This means that although the photon follows the angle-of-incidence-equals-angle-of-reflection physical law, the angle in question is determined at a microscopic level and is therefore variously predictable based on the microscopic "smoothness" of the surface. Consider the concept of "microscopic smoothness" as the level of "roughness" which is too small to be seen with the naked eye. In other words, if it is visually perceptible, it is not "microscopic smoothness", it is "texture".