It has long been recognized that one of the inherent problems in spray application, particularly with airplanes, is that of droplet size control. It is not generally known which droplet sizes or ranges of droplet sizes are the most effective biologically for the various types of treatments. It is known, however, that excessively large droplets result in a serious waste of material, and that very small droplets present a serious drift problem. For these reasons, spray droplet size and the range of sizes in a given spray have become important criterions in evaluating a spray distribution pattern.