There is often great difficulty in measuring mean droplet size and drop size distribution in sprays because of sampling problems. This is particularly true if the spray is produced in the practical environment, for example in a high pressure combustion chamber, and the spray density is high. In recent years there has been a greater need to evaluate the characteristics of atomisers under such conditions and hence greater interest shown in optical methods which present the most convenient means for examining spray characteristics. It is not intended in this brief note to discuss in detail the principles involved with diffractive scattering methods of measuring mean droplet size, - this has been done very effectively elsewhere - but merely to outline some of the practicalities of producing a reliable instrument based on one method which has considerable promise. One of the early papers describing and evaluating this method was published by Dobbins, Crocco and Glassman in 1963, but the theoretical background on which the method is based was available much earlier … [cont.].