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• SAA
• SALS
• SAXS
• SC
• scattering
• SD
• SdFFF
• secondary organic aerosol
• SEM
• SERS
• SI
• single-scattering albedo
• small
• SML
• SMPS
• snake photon
• SNR
• SOS
• spatial filtering
• speed of light
• SPF
• SPR
• ST
• STEM
• SVM

Spatial filtering - a form of Fourier-transform filtering. Spatial filtering is performed by inserting an aparture in the Fourier plane (focal plane) of a lens where the directional distribution of light intensity in the object space of that lens is converted into a two-dimensional irradiance distribution. This aperture limits the angular extent of the scattered light admitted by an optical system. It is a typical way of "cleaning-up" a collimated laser beam from light scattered by imperfections of the laser cavity mirrors and windows. Indeed, the collimated light is focused to a very small spot in the Fourier plane of the lens while the scattered light generates a broad distribution of irradiance in that plane. In this latter application, the Fourier lens is followed by another lens, with its front focal plane coplanar with the Fourier lens back focal plane. This additional lens restores the collimation of the laser beam.