This part of ISO 13322 is applicable to the analysis of images for the purpose of determining particle size distributions. The particles are appropriately dispersed and fixed on an optical or electron microscope sample stage such as glass slides, stubs, filters, etc. Image analysis can recover particle images directly from microscopes or from photomicrographs. Even though automation of the analysis is possible, this technique is basically limited to narrow size distributions of less than an order of magnitude. A standard deviation of 1,6 of a log-normal distribution corresponds to a distribution of less than 10:1 in size. Such a narrow distribution requires that over 6 000 particles be measured in order to obtain a repeatable volume-mean diameter. If reliable values are required for percentiles, e.g. D90 or other percentiles, at least 61 000 particles must be measured. This is described in Annex A.