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Rayleigh distance in optics is the axial distance from a radiating aperture to a point at which the path difference between the axial ray and an edge ray is λ / 4. A good approximation of the Rayleigh Distance is , in which Z is the Rayleigh distance, D is the aperture of radiation, λ the wavelength. Actually, Rayleigh distance is also a distance beyond which the distribution of the diffracted light energy no longer changes according to the distance Z from the aperture. It is the reduced Fraunhofer diffraction limitation.
In electronics, a distance equal to four Rayleigh distances is used to denote the distance at which the beam radiated by a reflector antenna is fully formed.