This example demonstrates the use of Source Length on reflective surfaces. The purpose of Source Length is to give lights an elongated or tube-like reflective shape, as if the light were coming form a fluorescent light fixture. It should be noted that light emission is still calculated as if it is coming from a single point. The effect exists only within reflections.
The Source Length runs down the X-Axis of the light, and can be rotated by rotating the light. Also, while the light is selected, Source Length can be visualized by a yellow line stretching out from the light itself. When rotating the light the Source Length will continue along the Local Space X-Axis, but when checking the World Space, the light may not follow a direct axis.