@Dimas@social.edu.nl Short answer: No. The product of multiple spectra can simply be a more complicated function than one on its own. Either you accept strong approximation errors or you use so many coefficients that you are not gaining anything by moving away from Monte Carlo integration. That is already true for products of reflectance spectra. And emission spectra need more coefficients to begin with, see Fig. 9 in my SIGGRAPH 2019 paper: https://momentsingraphics.de/Siggraph2019.html