The Directional light functions are an infinitely distance light source and creates an effect much like the sun illuminating on the surface of the earth. Rotation is the only transform value that will affect this light type since it originates from an infinite location (figure 1).


Directional Light

Figure 1: The Directional light type and it's assocated parameters


Directional Light Parameters

Emission - By default a Texture Emission node is connected as the emission type. A Black Body can be used as well. Please see their respective topics for parameter details.

Light Transform - The only parameter in this roll out that affects the light source is Rotation since the light source is infinite.

Light Sample Spread Angle - Larger value will create a larger light source, as a result, the shadows will become softer. 

Use in Post Volume - Enables or disables the light in post volume rendering.

Object Layer - Contains the standard object parameters. See the Object Layer Node section for more details.