If you want a realistic result the solution is to move
the light source further away (or use the parallel keyword).
Of course this also affects the shadows so you may need to
compensate by moving the objects closer to the plane.
A different approach might be to set a high value for
diffuse (even > 1.0) so the entire backplane saturates.
However, if you use soft shadows these will still show
some asymmetry.
