fluence

Fluence is a measure used in radiometry, photonics, and particle physics to describe the amount of something passing through or striking a surface per unit area. In radiative contexts, energy fluence, often denoted F or Φ, is the total radiant energy incident on or transmitted through a surface per unit area. Its SI unit is joules per square meter (J/m^2). Fluence is related to irradiance (power per unit area) I by F = ∫ I(t) dt over the exposure period.

Particle fluence refers to the number of particles crossing a unit area, with units such as particles per square meter (m^-2) or per square centimeter (cm^-2). It is the time integral of the particle flux, N = ∫ Φp(t) dt, where Φp is the particle flux in particles per square meter per second. Fluence is thus a total quantity, while flux is a rate.

Fluence is distinct from fluence rate or flux, which represent rates rather than totals. The concept applies across disciplines, including laser-material interactions, where the energy fluence determines thresholds for ablation or melting, and in radiation protection and medical physics, where fluence relates to dose through energy deposition in matter.

Units and practical use vary: energy fluence uses J/m^2, while particle fluence uses particles per m^2 (or cm^2). Fluence can be measured directly with calibrated sensors or inferred from source characteristics and exposure time. The term derives from Latin fluere, to flow, reflecting its interpretation as a total crossing quantity accumulated over time.