Particle size usually refers to a certain length dimension through the interior of a particle, without implying any regular geometric shape.

[Reference] Particle sizes can be divided into two major categories.

One category is measured and defined directly according to the geometric properties of the particle, such as particle sizes determined by microscopy.

The other category is measured and defined indirectly according to a certain physical property of the particle, such as particle sizes determined by the sedimentation method or the photoelectric method; these are in fact a kind of equivalent diameter.

[Reference] In air cleaning technology, particle size is commonly measured with a light-scattering particle counter; the particle size then refers to the composite effect obtained by making an equivalent comparison between the scattered light intensity of the measured particle and that of a standard particle, and it is a kind of equivalent optical diameter.