The optical efficiencies discussed above are determined for the single lenses. In CPV, these lenses are positioned above particular solar cells. Therefore, an effective optical efficiency is calculated for the combination of the lens and solar cell [1]. In this study, this effective optical efficiency is calculated as the ratio of current densities generated in the four subcells without a lens to the current densities with a lens. The current densities (JSC) in the subcells are calculated from the EQE of the 4J solar cells and the AM1.5d ASTM G173-03 spectral irradiance. The EQE is shown in Fig. 2 exemplarily for cell A measured at 25 °C cell temperature.