When any kind of radiation (radioactive particles from a source, or electromagnetic waves) is incident on a surface, its irradiance, \(I\), is defined as the power per unit area. This relationship is represented by the following equation:

\( I = \frac{P}{A}\) where:

  • \(I\) is irradiance \(Wm^{-2}\)
  • \(P\) is power \(W\)
  • \(A\) is area \(m^{2}\)

Irradiance and distance

As the distance from a point source of radiation increases, the irradiance decreases. The relationship between irradiance, \(I\), and distance, \(d\), can be shown to follow an inverse square law.


The product of irradiance and the square of the distance from the source is a constant, \(k\).

As this product is constant, it follows that for two points at distances \(d_{1}\) and \(d_{2}\) from a point source of radiation:


Remember that a point source emits radiation in all directions.