Modeling a cut-off spherical wave in an imaging system

I need to model a "cut-off" spherical EM wave to carry on some energy calculations for an imaging system. Referring to the figure below, let’s assume that we have a dipole source emitting a spherical wave. This wave goes through some optical system that converts it into another spherical wave focused on a pixel detector. The pixel detector has a limited angular field of view (this is what optical system engineers call "acceptance angle"). I would like to derive from first principles the energy transfer equation between the source and acceptance angle limited detector. I am also wondering what is the standard way to represent a cut-ff spherical wave in optical physics. Maybe a sum of plane waves?