# Fluence

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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<sup>&ndash;2</sup> (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<sup>2</sup>. 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:

:[itex]Phi = frac[/itex].

In the limit of infinitesimal area, this is:

:[itex]Phi = frac[/itex].

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

:[itex]Phi = frac[/itex],

where [itex]sum [/itex] 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 [itex] x/ x[/itex] where dx is the typical path length of a particle in the volume. The numerator ([itex]N x[/itex]) then gives the total path...

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