Before revision: Mathematically, we may represent the object plane field as: i.e., as a sum over weighted impulse functions, although this is also really just stating the sifting property of 2D delta functions (discussed further below).
After revision: Mathematically, we may represent the object plane field as: i.e., as a sum over weighted impulse functions, although this is also really just stating the shifting property of 2D delta functions (discussed further below).