The dimensionless parameter
where a is the characteristic size ("radius") of the aperture, is the wavelength, and R is the distance from the aperture. The regime is called Fraunhofer diffraction, while produces Fresnel diffraction. Intermediate regimes require more difficult analysis, but can be treated using scalar diffraction theory.
Diffraction, Fraunhofer Diffraction, Fresnel Diffraction, Scalar Diffraction Theory
