Introduction

Several authors in the 1970s suggested ways of inferring the size distributions of fine particles from the angular distribution of the intensity of forward-scattered coherent light (Cornillault, 1972; Weiss & Frock, 1976; Swithenbank et al., 1977). This work led to a first generation of commercial laser diffraction sizers from CELAS (France), Leeds and Northrup (USA), and Malvern Instruments (UK). Subsequently several more manufacturers have produced instruments – Horiba, Coulter, and Fritsch – while the original makers have improved their products. The original machines used the Fraunhofer diffraction approximation which de Boer et al. (1987) point out is only applicable to particles that are large relative to the wavelength of light. McCave et al. (1986) reported rather poor performance of one instrument at the lower end of the size range with samples containing a significant clay fraction. Nowadays instruments are mostly (not all) using a combination of Fraunhofer and the full Mie scattering theory, which deals properly with fine particles. However, many instruments still have detectors with relatively few elements (e.g., 18 in one machine) whose results after processing may be presented as multielement histograms (e.g., up to 56 bars) conferring a somewhat spurious air of precision to multimodal size distributions. However, the method is fast, reproducible, nondestructive for weak particles (e.g., floes), and is constantly being improved.

One potential advantage of the method that has not yet been much exploited is that suspended particles can be sized in situ in the air, sea, or rivers.