Fluence

This redirect is about the concept of fluence in physics. For the car, see Renault Fluence.

In physics, fluence is the flux (either particle or radiative flux) integrated over time. For particles, it is defined as the total number of particles that intersect a unit area in a specific time interval of interest, and has units of m^–2 (number of particles per meter squared). Fluence can also be used to describe the energy delivered per unit area, in which case it has units of 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.

It has two equivalent definitions:

1. Suppose N particles pass through an area A. The particle fluence for the area A is defined as:

$\Phi ={\frac {N}{A}}$.

In the limit of infinitesimal area, this is:

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

2. Imagine an infinitesimal volume dV with particles passing through it. The particle fluence can be defined as

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

where $\sum {\rm {d\ell }}$ is the sum of all the path lengths of the particles that traverse the volume. These definitions are equivalent as can be seen by multiplying the first definition by ${\rm {d}}x/{\rm {d}}x$ where dx is the typical path length of a particle in the volume. The numerator (${\rm {d}}N{\rm {d}}x$) then gives the total path length traced out by the dN particles in the volume (${\rm {d}}N{\rm {d}}x\simeq \sum {\rm {d\ell }}$) while the denominator (${\rm {d}}A{\rm {d}}x$) gives the volume dV.

References

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