Fluence

Not to be confused with Renault Fluence.

In physics, fluence is the radiative flux integrated over time. It is defined as the number of particles that intersect a unit area. Its units are m^–2 (number of particles per meter squared). In particular, it is used to describe the strength of a radiation field, in which case the unit used is J/m². It is considered one of the fundamental units in dosimetry.

In light based medicine and dentistry, fluence, which may be more properly referred to as radiant exposure, is a measurement of energy over area. The area is usually the spot size of the light device. The higher the fluence, the more "cutting power" a laser has.

It has two 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 infinitesimal volume and $\sum {\rm {d\ell }}$ is the sum of all the path lengths of the particles that traverse the volume.

References

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