Directional light adds the angular diameter to the common 3Delight attributes.
Angular Diameter
A directional light emulates a point light source that is placed at infinity. It is handy to be able to specify objects that are extremely far but are not well simulated by a single point (e.g. the Sun). It would be impractical to specify the footprint of a far way object using absolute values of distance and size so we use instead the measure of angular diameter.
The attributes is specified in degrees.