Thermal irradiation is the rate at which radiation is incident upon a surface per unit area. It is measured in watts per square meter. Irradiation can either be reflected, absorbed, or transmitted. The components of irradiation can then be characterized by the equation
where,  represents the absorptivity,  reflectivity and  transmissivity. These components are a function of the wavelength of the electromagnetic wave as well as the material properties of the medium.