When an optical system focuses light through a transparent cover sheet, refractive-index variations create spherical aberration which can be partially compensated by adjusting the cover-sheet thickness. In practice, a wide range of refractive index can be tolerated by a diffraction-limited objective lens given such thickness optimization. For example, a system with a numerical aperture of 0.6 and a nominal cover-sheet thickness of 1.2 mm can tolerate a refractive-index range from 1.45 to 1.60.
© 1986 Optical Society of America
Alan B. Marchant, "Index variations in cover sheets," Appl. Opt. 25, 490-492 (1986)