Characterizing High Numerical Aperture Microscope Objective Lenses

Abstract

Testing and characterization of high quality lenses have been perfected into fine art with the advent of lasers, phase-shifting interferometers, CCD cameras and computers. A bewildering array of techniques is described in Malacara’s classical reference book on the subject [1]. Several of these techniques, in particular the Twyman–Green interferometer and the star test, are applicable to testing of microscope objective lens.

Characterizing high-numerical aperture (NA) objective lenses presents unique challenges. Many of the standard approaches, including Twyman–Green interferometry, are in fact comparative techniques. They require a reference object – an objective or a concave reflective surface – of the same or larger numerical aperture and of perfect (comparatively speaking) optical quality. This is problematic. Even if two lenses of the same type are available a problem still remains of appropriating the measured aberrations to the individual lenses.