I think that the lights are further away from the "darkening" side you speak of.
As I see it, I'm standing in the z axis looking at the y axis right? if we look from the left this is what I imagine is happening:
| >0 (camera)
| _[surface]<-- dark side_________
|____________________________________________________________________________________ z axis
I don't know If you get what I mean. If I'm right, that's a normal behavior, but if I'm wrong, then we'll need to wait for anyone with more experience
Could you provide the coords of the lights and that surface-object-thing?