In this study, emission spectra of three different commonly used xenon irradiation sources were analyzed and compared for the first time to ascertain the most suitable setup to simulate natural solar radiation. In order to demonstrate setup differences, absolute photon fluxes of irradiation sources were received by actinometry. Verification was done by measuring quantum yields of the model compounds Penconazole, Terbutryn, and Mecoprop in every setup. Differences regarding kinetic aspects and the formation of transformation products (TPs) was evaluated by analyzing direct phototransformation and additionally photolysis in presence of Nitrate as a photosensitizer in one irradiation setup (optical bench). Results showed that a precise setup characterization is needed to estimate whether irradiation sources are suitable to simulate terrestrial sunlight. This was found to be especially important for weakly sunlight-absorbing substances. In comparison with direct photolysis, indirect photolysis led to an enhancement of degradation rate constants for all substances and in case of Mecoprop to different types of TPs that were formed during irradiation. This study underlined that there are big knowledge gaps regarding irradiation sources setups and conditions. It is therefore absolutely necessary to consider those factors while simulating substance degradation and the TP formation under environmental conditions.