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:

<math>\Phi = \frac{{\rm d} N}{{\rm d} a}</math>.

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

<math>\Phi = \frac{\sum {\rm d \ell} }{{\rm d} V}</math>,

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

ICRU 33: Radiation Quantities and Units, April, 1980. (www.icru.org) Template:WikiDoc Sources