# 1.8: Normal Flux Density F

The rate of passage of energy per unit area normal to the direction of energy flow is the normal flux density, expressed in \(\text{W m}^{-2}\).

If a point source of radiation is radiating isotropically, the radiant flux being \(\Phi\), the normal flux density at a distance \(r\) will be \(\Phi\) divided by the area of a sphere of radius \(r\). That is

\[F= \Phi / (4 \pi r^2) \label{1.8.1}\]

If the source of radiation is not isotropic (or even if it is) we can express the normal flux density in some direction at distance \(r\) in terms of the intensity in that direction:

\[F = I/r^2 \label{1.8.2}\]

That is, the normal flux density from a point source falls off inversely with the square of the distance.