A fiber optic spatial filter assembly for laser diffraction particle sizing apparatus utilizing a laser to generate a monochromatic light beam which is coupled to an optical fiber (36) operating substantially in a monomode and creating a beam of light (56) having a high degree of spatial coherence which is then passed through collimating lenses (64, 66) to interrogate and impinge upon the particles of matter (86) through which the laser diffracted light passes. The light scattered by the particles is focused onto a Fourier plane (96) and thereafter impinges upon a photooptical detector array (98), positioned coincident with the Fourier plane, for measuring the light intensities of the scattered light by scattering angle, thus enabling the computation of particle size.