It is the power of electromagnetic radiation of any kind that affects a surface unit. It is measured in watts or watts per square meter. It is used to know how much solar energy reaches Earth.
It is the amount of radiation received on a surface for a certain time. For the international system the W 8901;m2 (watt per square meter) unit is used.
IRRADIANCE Irradiation . Luminosity. Amount of luminous energy emitted by a body.