Hi wARmAcH1n3,
The Source Length of a Spot Light only adjusts the focal length of the light beam. The reason it extends behind the light as well is because it is basically a child of the Point Light.
I would recommend using a Point Light to create a fluorescent light. You can change the Source Radius and Length to mimic the light and use a casing to direct the light just like a real fluorescent light by making the inside a reflective material.
Thanks, TJ
