calculating penumbra frustum from polygon light source

hi, I'm having trouble working out how to construct the outer-most planes of a frustum (penumbra) formed by two arbitrary polygons in 3-space, if you consider the first polygon (A) to be a light source and the second polygon (B) to be an occluder then I am looking to find the volume formed from the penumbra of the occluder as a set of planes defining a frustum. The only way I can see to do this is by constructing each 3d line formed from the two polygons i.e:

for all points in A

for all points in B

line(a, b)

this would give me a set of 3d lines but how do I find the outer-most convex set that defines the penumbra frustum? or is there another way.