where formula_16 is the maximum intensity of the pattern at the Airy disc center, formula_17 is the Bessel function of the first kind of order one, formula_18 is the wavenumber, formula_12 is the radius of the aperture, and formula_20 is the angle of observation, i.e. the angle between the axis of the circular aperture and the line between aperture center and observation point. formula_21, where "q" is the radial distance from the observation point to the optical axis and "R" is its distance to the aperture. Note that the Airy disk as given by the above expression is only valid for large "R", where Fraunhofer diffraction applies; calculation of the shadow in the near-field must rather be handled using Fresnel diffraction.