Diffraction criterion for a slit under spherical illumination

A quantitative criterion to determine the type of diffraction produce by a slit, which is an aperture that cannot be circumscribed, is defined. The slit is illuminated with a spherical wave that eliminates the alignment problems produced by filamentary light sources. The determination of the first minimum of intensity in the diffraction pattern was used to define the criterion that differentiates between Fraunhofer and Fresnel diffraction. The criterion is defined in terms of a parameter γ, which is connected to the number of Fresnel zones. Simulations are used to define the criterion and experimental results are presented to corroborate it.