A DirectionalLight node defines an oriented light with an origin at
infinity. It has the same attributes as a Light node, with the
addition of a directional vector to specify the direction in which the
light shines. A directional light has parallel light rays that travel
in one direction along the specified vector. Directional light contributes
to diffuse and specular reflections, which in turn depend on the
orientation of an object's surface but not its position. A directional
light does not contribute to ambient reflections.
