Flow imaging (FI) is useful for an approximate particle range from 2 to 100 µm, depending on the instrument and its configuration, covering particle sizes from the accepted subvisible size range to hundreds of micrometers (see Measurement of Subvisible Particulate Matter in Therapeutic Protein Injections 〈1787〉 and Methods for the Determination of Subvisible Particulate Matter 〈1788〉). Specialized instruments or configurations can cover expanded size ranges. FI instruments provides both individual microscopic images of observed particles and reports of particle size distribution (PSD) and particle concentration at selected size thresholds. Values of morphological parameters and sample images corresponding to the stated size bins or thresholds may be added to the report. Particle size can be reported as an equivalent circular diameter [similar to light obscuration (LO)] or based on Feret's diameter, which captures some information about the shape. Algorithms categorizing particles are usually based on optical density and morphological parameters such as circularity, allowing them to differentiate to some degree between elongated fibers, dense symmetrical air bubbles, circular silicone oil droplets, and amorphous protein particles, etc.