How to define a Macro accessing rad intensity
 

Hi! I wish to define a reaction rate that involves the radiation intensity (W/m2). How can I define the macro accessing to it using UDF? Like C_T(c,t)is the macro accessing to solver's variable, temperature. Should I define a UDS for the radiation intensity (or incident radiation), but how?

This problem has been troubling me for a long time. Can anyone help me with this?