of the rays from a directional light source have the same direction, and
no point of origin.
It is as if the light source was infinitely far away from the surface
that it is illuminating.
Sunlight is an example of an infinate light source.
Involved in Diffuse and Specular components of illumination.
We consider the direction of the light source when computing
both the diffuse and specular components of illumination.
With a directional light source this direction is a constant.