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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.

[edit] 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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