Beam stop and vignetting effects in particle size measurements by laser diffraction.

In the measurement of size distribution by forward light scattering, flare in the collection lens can cause unacceptable noise at low particle concentration. To avoid this problem use of a beam stop is proposed. A study is made of the effect of beam stops on the measured energy distribution from a homogeneous particle field. In addition it is shown how this same approach can be applied to correct for vignetting, an effect which occurs when the particle field is too far from the collection lens.