If particles (disperse phase) are embedded in a surrounding medium (continuous phase) – e.g. as grains, drops or bubbles – their so-called equivalent diameter can be determined by measuring them so that they can be assigned to specific classes according to their size. The particle size distribution is obtained by determining the proportions with which each particle class participates in the disperse phase.
Different types of quantities can be used: If particles are counted, the quantity type is the number. In the case of weighing, however, it is the mass or in case of homogeneous density it is the volume. Other quantity types are derived from length specifications, projection surfaces and surfaces. The particle size is usually given in micrometer (micron).