Feret diameter

In general, it can be defined as the distance between the two parallel planes restricting the object perpendicular to that direction.

This measure is used in the analysis of particle sizes, for example in microscopy, where it is applied to projections of a three-dimensional (3D) object on a 2D plane.

In such cases, the Feret diameter is defined as the distance between two parallel tangential lines rather than planes.

[1][2] From Cauchy's theorem it follows that for a 2D convex body, the Feret diameter averaged over all directions (〈F〉) is equal to the ratio of the object perimeter (P) and pi, i.e.,〈F〉= P/π.

[5] It is also used in biology as a method to analyze the size of cells in tissue sections.

Illustration of horizontal and vertical Feret diameters of a particle, Fh and Fv, respectively.
The diameter of an object measured with a caliper is sometimes called the caliper diameter; it is the same as Feret diameter.
Feret diameter applied to a projection of a 3D object.