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three-dimensional diffraction pattern

(Science: optics) The diffraction pattern (of a point source) that appears in the three-dimensional space in and near the focal plane. For an aberration-free, diffraction- limited system, the slice of the diffraction pattern in the focal plane is the airy disk and its surrounding diffraction rings. Above and below focus, the pattern changes periodically along the axis of the light beam so that bright and dark airy-disk-like patterns appear alternately. The axial period of repeat is spaced twice as far apart as the radial period of repeat in the Airy disk and its diffraction rings