Hi, the question sounds strange: what is it flux density? The letter "B" is commonly used to designate magnetic induction. Magnetic induction flux is a surface integral of BdS over the choosen surface ; both "B" and "dS" are vectors, dS - differential vector perpendicular to the surface). One can introduce magnetic induction value into the radiative transfer equation in place of the intensity. Intensity depends on the square of the amplitude of the vector B or E (which one you prefer) of the electromagnetic wave.

Thanks Dr. Patyk. Your response is very insightful indeed!