 So, if some surface "i" is being hit by radiation from some other surface "j", then the radiation energy incident on surface "i" is "E"e,"ji" "A""i" = "F""ji" "A""j" "J"e,"j" where "F""ji" is the "view factor" or "shape factor", from surface "j" to surface "i".So, the irradiance of surface "i" is the sum of radiation energy from all other surfaces per unit surface of area "A""i"