Fluence

In physics, fluence or integrated flux 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 Laser Medicine or Laser Dentistry, the fluence usually refers to the "Power Density" or "Energy Density" of a laser at the emitter tip. 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 d"a" is impinged upon by d"N" particles of a certain type. Then, the fluence is

::$Phi\; =\; frac$ m d} N} m d} a}.

2. Using the same image of a small sphere as above,

::$Phi\; =\; frac\{sum\; \{\; m\; d\; ell\}\; \}$ m d} V},

where $\{\; m\; d\}\; V$ is the infinitesimal volume and $sum\; \{\; m\; d\; ell\}$ is the sum of all the path lengths of the particles that transverse the volume.

References

* ICRU 33: Radiation Quantities and Units, April, 1980. (www.icru.org)
* http://www.dtra.mil/rd/programs/rtfc/defin.cfm