Covered in Section 12.2, Irradiation is the radiation incident (acted upon) on a surface. It is a radiation heat flux represented by

See ME3304 S24 Lecture Note (7) Radiation and 14A - 2nd Class for more info.
There are three main components to irradiation.
For most surfaces you can use Equation 12.2:
For an opaque medium there is no transmission so Equation 12.2 becomes 12.3:
See Section 12.2 and ME3304 S24 Lecture Note (7) Radiation for more infomation.