When any kind of radiation (radioactive particles from a source, or electromagnetic waves) is , 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:
$I_{1}d_{1}\,^{2}=I_{2}d_{2}\,^{2}$