Aberration-free width of angular spectrum of a field probing a layered object in confocal microscopy

The effect of the numerical aperture of a light beam probing an object on the value measured by a confocal interference microscope when determining the thickness of a layered object for the case of a significant effect on the signal of the spherical aberration arising in the probe beam at its propagation through an object has been theoretically and experimentally investigated. Taking into account the influence of spherical aberration on the signal the dependence of the measured value on the numerical aperture has been obtained. The criterion of the “aberration-free” propagation of the beam probing an object has been established and the condition under which the effect of spherical aberration on the signal of a confocal microscope can be neglected has been determined. An approximate analytical expression which allows to determine the “aberration-free” width of angular spectrum of the probing light beam taking into account the empirical correction factor has been obtained. The limits of applicability of the analytical dependence of the measured value on the numerical aperture, which was obtained earlier for the case of negligible influence of spherical aberration on the signal of a confocal microscope, have been determined.