The axial response of confocal microscopes with high numerical aperture objective lenses

We consider the axial resolution of confocal optical systems employing high numerical aperture objective lenses for the cases when the specimen refractive index is both higher and lower than that of the immersion medium. In the latter case a spherical aberration-like phase shift is introduced when the objective lens numerical aperture is greater than the refractive index of the specimen which leads, inter alia, to an asymmetrical axial response with higher side lobe levels. Theoretical and experimental results are presented.