Abstract: Different from Fraunhofer diffraction, Fresnel diffraction has been considered to have no the property of similarity. However, our study shown that Fresnel diffraction patterns also have similarity property under certain conditions. In this paper the general requirement for this property is given, and the relation between the scale of the Fresnel diffraction pattern and the parameters of the diffraction system is quantitatively derived. As an inferences, some special cases such as the circular aperture diffraction and straight-edge diffraction are discussed and a series of important conclusions are obtained.

Key words: Fresnel diffraction, similarity, distance transform, scale transform, Fresnel number