. Introduction
In solar cell design and ratings, the conversion efficiency under standard testing conditions (STC) has always been the main figure of merit. However, realistic outdoor operating conditions may deviate to a substantial degree from standard testing conditions. These deviations concern the irradiance level (i.e., light intensity), spectral composition, device temperature and other effects. These deviations can have a pronounced impact on the power generation of solar cells, especially tandem solar cells which are well known to be more sensitive to spectral variations due to current mismatch losses (Faine et al., 1991 and Philipps et al., 2010).