Interstellar radiation field model featuring a stellar (I), warm dust/SHG (II), cold dust (III), and cosmic background component after Zucconi et al. (2001). The total field is shown in blue with the different components as magenta lines. A warm dust component was added to fit the data by Black (1994; plus signs). The green (orange) curve shows our low(high)-mass ISRF model, respectively (notations of the component factors: I-II-III). The red curve shows the ISRF determined for L1689B in Ophiuchus containing B stars, but not near the core (Steinacker et al. 2016).