Abstract: Based on Fresnel diffraction theory, the diffraction field of a spherical wave through a plane transmission grating is studied in detail. The amplitude distribution of Fresnel diffraction and a calculation formula for Fresnel diffraction angle of a grating are given. The waves in each order of Fresnel diffraction are spherical waves under ideal conditions. The diffraction angles can still be given by the grating equation of Fraunhofer diffraction after Fresnel diffraction angle is redefined.

Key words: plane transmission grating, Fresnel diffraction, spherical wave, grating equation