The angular distribution of scattered light in the main lobe of the Fraunhofer diffraction pattern of a particle changes rapidly with its size, but is largely independent of its refractive index. Even when the particle is so small (diameter less than a few wavelengths) that its forward lobe must be calculated from the Mie theory, the relative angular distribution within the central part is found to be given approximately by the Fraunhofer formula with obliquity factor, even for particles smaller than one wavelength, except near a minimum in the particle extinction efficiency factor calculated from the Mie theory. A measurement of the ratio between the scattered intensities at a pair of convenient angles within the lobe can thus give a useful estimate of the size of spherical and nonspherical particles without knowing their refractive index. The sizing error caused by the narrowing of the lobe near an extinction minimum is acceptable for many practical purposes, and can in any case be detected by comparing the size estimates derived from measurements at two different wavelengths. Families of curves for sizing by this simple procedure have been prepared. Sideways scattering instruments have been preferred hitherto because of their easier construction. A realization of their possibilities for sizing unknown particles should, however, stimulate the development of suitable forward scattering instruments.
J. Raymond Hodkinson, "Particle Sizing by Means of the Forward Scattering Lobe," Appl. Opt. 5, 839-844 (1966)