Note 1: A far-field[?] pattern exists at distances that are large compared with s 2/λ, where s is a characteristic dimension of the source and λ is the wavelength. For example, if the source is a uniformly illuminated circle, then s is the radius of the circle.
Note 2: The far-field diffraction pattern of a source may be observed at infinity or (except for scale) in the focal plane of a well-corrected lens. The far-field pattern of a diffracting screen illuminated by a point source[?] may be observed in the image plane of the source. Synonym Fraunhofer diffraction pattern. Contrast with near-field diffraction pattern.
Source: from Federal Standard 1037C
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