1. The amount of a given type of radiation that crosses a specified area within a specified period; such as, the number of photons passing through one square centimeter of a target in one second.
2. The electric flux passing through a surface, divided by the area of the surface.
3. A vector field that represents the differential flux of field lines per unit area.

The electric field intensity multiplied by the permittivity (measure of the ability of a nonconducting material to retain electric energy when placed in an electric field) or the property of a dielectric medium which determines the forces that electric charges placed in the medium exert on each other.

A process by which radiation flux density is tracked within a reactor or other radiation source.

The number of gamma rays passing through a unit area in a unit of time.

1. A radiometric term for the rate at which radiant energy is transferred across a unit area of a surface, commonly measured in watts per square meter.
2. A radiometric term for the rate at which radiant energy in a radiation field is transferred across a unit area of a surface (real or imaginary) in a hemisphere of directions.

In general, irradiance depends on the orientation of a surface. The radiant energy may be confined to a narrow range of frequencies (spectral or monochromatic irradiance which is characterized by a single frequency) or integrated over a broad range of frequencies.