The image processing technique is simple and, in principle, can handle particles with
various shapes since it is based on direct visualization. Moreover, a wide measurement
area can be covered with appropriate optical arrangement. In the present paper, various
techniques of image processing for sizing and counting particles are reviewed and
recent developments are introduced. Two major subjects are discussed in detail:
identification of particles (i.e., boundary detection and pattern recognition) and
determination of in-focus criteria. Finally, an overall procedure for image processing of
spray particles is suggested.