This part of ISO 16232 defines methods for determining the size and number of contaminant particles, which have been extracted from components and deposited on the surface of a membrane filter, as determined by using either a light optical microscope (LM) or a scanning electron microscope (SEM). The result of this measurement is the particle size distribution on the membrane filter. As the function of parts and components can be impaired by the presence of a single or a few critical particles, a complete analysis of the total membrane filter surface is essential. These analyses can be performed either manually or automatically using Image Analysis (IA) techniques if the appropriate equipment is available. NOTE 1 Manual full-surface counting is a difficult and tiring task associated with errors. For this reason, an automatic counting system is recommended if the membrane filter is prepared in a suitable way as described herein. NOTE 2 The results of counting and sizing depend on many parameters, such as type and model of microscope, magnification, illumination, and other settings used.