This will stop the light from a light source.
light_source{<0,20,30> color White}
sphere{<0,20,30>,1+40*Value pigment{Red} hollow no_image}
Change Value to any thing ya want.
It will light only up only things in the sphere so if you want to see
the objects of a scene before the light wave hit use another light.
Like:
light_source{<0,30,0> color White shadowless}
