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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 \(m^{2}\)

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.

\(I=\frac{k}{d^{2}}\)

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:

\(I_{1}d_{1}\,^{2}=I_{2}d_{2}\,^{2}\)