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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 radient 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
-
.
2. Using the same image of a small sphere as above,
-
,
where dV is the infinitesimal volume and 
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)
- http://www.dtra.mil/rd/programs/rtfc/defin.cfm
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