The luminous flux at a surface divided by the area of the surface. (See also "illuminance" and "luminous exitance".)
(2) (radiant): The radiant flux at a surface divided by the area of the surface. (See also "irradiance" and "radiant exitance".)
(3) flux density (of particle radiation): The time rate of flow of radiant-energy particles emitted from or incident on a surface, divided by the area of that surface.
NOTE 1 The equation "flux density = N/A∙t" applies, where N, A, and t represent the quantities number of particles, area, and time.
NOTE 2 The unit symbol (e.g., cm‑2·s‑1) does not identify particle type. The particle name may be placed before the term, e.g., "neutron flux density", or in the spelled-out unit name, e.g., "neutrons per square centimeter second".
NOTE 3 Flux density is maximized when the surface is perpendicular to the direction of the emitted or incident particle flow.
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