Materials certainly reflect colors. I think we can all agree on this. But they are more complex than just color. Surface properties can lead to complex effects outside of simple color reflectance. There are many ways of handling the reflectance \( F \) term in the rendering equation. Actual reflected intensity depends on incoming angle, outgoing angle, and wavelength (other variables are sometimes considered).


Reflectance is how surfaces react to incoming light energy. Depending on the material properties, the light can be absorbed, reflected like a mirror, or reflected in a scattered fashion.

The absorption and reflection of light determines the light color that reaches our eyes. For example, a red surface absorbs all light wavelengths except red. If a light source emits white light, then a red material would have reflectance values that only allow red to be reflected. Multiplying the incoming intensity by the reflectance results in the output intensity:

$$ I = [1, 1, 1]\ R = [0.9, 0, 0]\ \ I_o = I^T * R = [0.9, 0, 0] $$

Phong reflectance

Phong reflectance is a very simplified model based on fake global illumination (ambient reflectance), surface roughness (diffuse reflectance), and surface shininess (specular reflectance). We will use this reflectance model, but there are many others that work.

Ambient reflectance controls the approximation for global illumination effects. Diffuse reflectance controls the intensity reflected by the dull or rough part of the surface. It is most often the color we perceive an object being. Specular reflectance controls the intensity reflected by the smooth or shiny part of a surface. The bright shiny spot on a ball is caused by specular reflectance and is called a specular highlight.

In Phong reflectance, a material has three sets of reflectance coefficients: ambient, diffuse, and specular. These reflectance values control the resulting ambient, diffuse, and specular intensities. A materials total intensity will then be:

$$ I = I\mathrm{a} + I\mathrm{d} + I_\mathrm{s} $$

with terms for ambient, diffuse, and specular.