Methods and instruments for the measurement of numerical aperture for microscope objective lens: A mini review

Numerical aperture (NA) of objective lens is an important parameter for the design of microscope systems and evaluation of imaging characteristics. The present mini review presents and summarizes the methods and instruments used in the NA measurement of objective lens. Five different categories of methods are introduced, which are original versions of apertometer measurement for angular aperture, method based on the working of Abbe apertometer and its modified versions, geometry-based methods, focal-plane (FP)-imaging-based methods, and back-FP-imaging-based methods, respectively. The methodology, devices, applied scenarios, and characteristics of methods (instruments) are summarized. Finally, some issues and potential areas of application are indicated as well.