Methods of Light Transport
© 7 Mar 2013 Luther Tychonievich
Licensed under Creative Commons: CC BY-NC-ND 3.0
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Diffusion, specularity, transparency, fluorescence, phosphorescence, absorption, translucence, and sub-surface scattering.


I spent many years as a teaching assistant in computer graphics and kept up with advances in that field with some regularity. One of the things I discovered in that process is that most people aren’t even aware of how light moves and why things look the way they do. I hope today to review a few of the basic elements of appearance.

When a photon hits an object, there are several things that might happen.

The simplest is that the photon might be absorbed and turned into heat. If all photons are absorbed, an object is black. If only some wavelengths are absorbed other colors result.

Photons can also bounce off of a surface like an elastic ball off of a hard floor. This is called specular reflectance and creates both the shine on shiny things and the reflection on mirrored things. If the surface of the object is not completely smooth then reflections will be jittered a little creating blurry reflections or smooth (rather than crisp) shine spots.

Most materials can be though of a large numbers of specular granules jammed close together. Light encountering such a composite material will effectively bounce off in a random direction, creating diffuse reflectance Diffuse reflectance is also sometimes called Lambertian because of Johann Heinrich Lambert’s early (1760) but incorrect model of how diffuse reflection worked. The much better Oren-Nayar model from 1994 has, as far as I know, not become an adjective. . The non-shiny parts of lighting are mostly diffuse reflectance. Same-wavelength florescence and short-distance subsurface scattering (see below) are visually much like diffuse reflectance.

Some wavelengths of light can travel through some materials just as they can through air. However, the waves propogate more slowly as they work their way around larger molecules, which causes the wave to change direction as it enters and exits the denser material. Together, this creates refraction in transparent materials.

Photons can be absorbed by electrons but then re-emitted, sometimes in different numbers (and hence different colors) than originally absorbed. If the light is immediately re-emitted it is called florescence, while delayed re-emission is called phosphorescence. The most common kind of florescence absorbs high-energy photons (ultra-violet light) and re-emits twice as many lower-energy photons (visible light); this is what makes teeth glow under a black light and is used by most printer paper to make it look whiter.

Some materials are mostly transparent with scattered absorbative or specular particles inside them. Partial absorption creates transluscent materials while partial reflection creates subsurface scattering (SSS). In SSS, a photon enters an object, travels for some distance, then bounces, travels some more distance, bounces again, and so on until it re-emerges somewhere or is absorbed. SSS is quite common, dominating the appearance of most biological matter (skin, blood, muscle, leaves, etc) as well as milk, marble, plastic, clouds, and many other materials. SSS causes materials to appear smooth, since crisp boundaries in lighting and shadows do not create crisp boundaries in the resulting appearance. It also often feels warm and human because flesh exhibits more SSS than do most structural materials.

A simple experiment to observe sub-surface scattering is to hold a flashlight against your hand and observe the reddish light coming from the skin around your hand (and, if the flashlight is strong enough, through the back of it too). Another dramatic example is to shine a laser pointer at snow, since snow is has only SSS, no diffuse reflectance at all; the tiny red dot will be replaced with a broad red blur as the photons are scattered through the snow.

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