Irradiance is the power of radiation per unit area. The closer you are to a source of radiation, the greater the irradiance. As you move away from the source the irradiance will decrease, in other words there is less radiation per unit area. If you continue to move away from a radiation source then the irradiance will reduce as the square of the distance from the source. This means that if you double the distance from the source of radiation then the irradiance will be four times less.