Fluence

Overview
In physics, fluence is defined as the number of particles that intersect a unit area. Its units are m-2. In particular, it is used to describe the strength of a radiation field. It is considered one of the fundamental units in dosimetry.

It has two equivalent definitions:

1) Imagine that an infinitesimal sphere of cross sectional area da is impinged upon by dN particles of a certain type. Then, the fluence is:

$$\Phi = \frac{{\rm d} N}{{\rm d} a}$$.

2) Using the same image of a small sphere as above:

$$\Phi = \frac{\sum {\rm d \ell} }{{\rm d} V}$$,

where $${\rm d} V $$ is the infintesimal volume and $$\sum {\rm d \ell}$$ is the sum of all the path lengths of the particles that transverse the volume.