How to obtain only induced fields with MWS?

lets say a plane wave is incident on a rectangular screen. And there is a small hole in the screen. How can I obtain only the induced fields? is there a simple way?

You mean fields before the rectangular screen with hole?
Maybe I'm wrong but what to subtract the complex values of whole field - plane wave field?
I saw on some presentation picures (from MWS) with sphere illuminated with plane wave - showing total field and field excited by illuminated sphere only..

This exactly the thing I want to calculate

This exactly the thing I want to calculate