Unreal Engine 5 Documentation has been moved to the Epic Developer Community

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.