Estimating irradiance on a square planar surface

[vst98]

Hi, I have a simple problem.

A Lambertian emitter has the shape of a square planar surface, with area A_e=4mm²,
total power P=1mW, and is located in yz plane.
Another square planar surface, with area A_d=6mm², is located in the
xy plane, at a distance d=10cm to the A_e, and is tilted 20 Degrees toward the A_e
(see attached picture).

How do I go about to compute or estimate irradiance at A_d ?

 

[vst98]

Is there really is no one who can answer or give advice on this problem ?
What if instead of A_d I would have a Lambertian point source with the same power,
would this make it easier to give an estimate of irradiance on A_e surface ?