Irradiance

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.

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}

curriculum-key-fact
Remember that a point source emits radiation in all directions.