In
physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
,
fluence is the
fluxIn the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
(either particle or
radiative fluxRadiative flux, or radiative flux density, is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m2. It is used in astronomy to determine the magnitude and spectral class of a star...
) 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/m
^{2}. It is considered one of the fundamental units in
dosimetryRadiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation...
.
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:

 .
In the limit of infinitesimal area, this is:

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

 ,
where
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
where d
x is the typical path length of a particle in the volume. The numerator (
) then gives the total path length traced out by the d
N particles in the volume (
) while the denominator (
) gives the volume d
V.