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In physics, fluence 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.

It has two equivalent 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:

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

2) Using the same image of a small sphere as above:

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

where dV is the infintesimal volume and \sum {\rm d \ell} is the sum of all the path lengths of the particles that transverse the volume.


ICRU 33: Radiation Quantities and Units, April, 1980. (

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluence". A list of authors is available in Wikipedia.
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