* What is a light ramp?
A light ramp controls the impact of light on the model. 
In this case, it controls the transition from indirect to direct light, 
or shadow to fully lit.

* What are the specifications for a light ramp?
A light ramp should be a power-of-two sized texture with
black on the left and white on the right and any kind of gradation
between them. 

You can also choose to avoid going to fully dark if you want your model
to always be partially lit.

Also, when imported into Unity, the texture wrapping mode must be set to
clamp. You can change this by clicking on the texture file in Unity and going
into the inspector.