Obliquity effects in interference microscopes

Abstract

A theoretical analysis is made of the influence of oblique rays on the fringes produced with interference microscopes. The convergency of the light, which is determined by the numerical aperture of the objective, results in the fringes being displaced from the position for ideally collimated light at normal incidence. The analysis accounts fairly well for some recent measurements of step heights with micro-scopes of different numerical apertures by Tolman and Wood. The correction due to obliquity is of 10% and over a limited range of step heights, a simple correction rule of a2/4/unit is satisfactory to about 2%, where [alpha] = sin-1(n.a). The more rigorous formula shows a nonlinear relation between obliquity-correction and step-height.