fluences

Fluence is a quantity used in physics to describe how much something passes through a surface area. The exact definition depends on context. In radiometry and radiography, energy fluence (also called radiant fluence) is the total energy of electromagnetic radiation crossing or deposited in a surface, per unit area, with units of joules per square meter (J/m^2). In particle dosimetry and materials science, fluence more commonly refers to particle fluence: the number of particles crossing a surface per unit area, with units of particles per square meter (m^-2).

When the amount is integrated over time, fluence is related to rate quantities such as irradiance (for photons) or particle flux density (for particles). The time-derivative of energy fluence yields an irradiance (W/m^2); the time-derivative of particle fluence yields a particle flux (particles/(m^2 s)).

Fluences are used to quantify exposure and dose in medical, industrial, and research settings. In radiography and therapy, energy or particle fluence is used to estimate energy deposition and dose in tissue or materials, accounting for interaction cross sections and stopping powers. In ion implantation and surface processing, fluence represents the total number of ions arriving at a surface, typically expressed as ions per square centimeter or square meter. In practice, the term fluences may refer to multiple measurements or categories, such as energy fluences and particle fluences.

Because fluence can refer to different physical quantities depending on context, practitioners specify the type (energy fluence vs. particle fluence) and the units to avoid ambiguity.