LOCAL WAVE-FRONTS AT DIFFRACTIVE ELEMENTS

Equations for calculating the principal curvatures and principal direc tions of local wave fronts in an optical system that consists of diffr active curved or plane surfaces between isotropic and homogeneous medi a are presented. The equations for refractive and reflecting surfaces are obtained as special cases of the equations for diffractive surface s. This method is a kind of generalized ray tracing and permits an eas y determination of the position of the local focus or the position and direction of the local focal lines in the case of local astigmatism; it must be emphasized that elements and systems with spherical aberrat ion or coma also have local astigmatism.