This note describes a convenient working technique for measuring the minimum size focused spot formed by a lens system. The technique is routinely capable of measurement precision of better than ⅛λ. The theory of knife-edge scanning is presented, and the relationship of the measured spot profile to the actual spot profile is shown to be relatively insensitive to the acceptance solid angle of the photodetector. Finally, some typical results of high resolution microscope objectives are presented.
A. H. Firester, M. E. Heller, and P. Sheng, "Knife-edge scanning measurements of subwavelength focused light beams," Appl. Opt. 16, 1971-1974 (1977)