Based on diffraction integral formula and selfreproducing principle,diffraction integral equation for transverse modes in a circular symmetric resonator with an aperture diaphragm was disintegrated to finitesum matrix equation,and field distributions of transverse modes in the resonator were calculated by solving matrix for eigenvalues and eigenvectors.

According to the basic relation E =φ/A, a direct optical method for aperture area measurement was provided, based on known constant irradiance over the aperture.