Smallest Focal Spot

Abstract
According to diffraction theory the size of a focused spot is determined by wavelength, numerical aperture, and aperture shape. But even when these parameters are fixed it is possible to achieve super-resolution and reduce the spot size even further. It is shown, however, that the three-dimensional spot size cannot be arbitrarily reduced. The minimum focal spot able to probe high-resolution volumetric information in an optical system is about half of the diffraction-limited spot size.

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