In this work, silicone on glass (SOG) Fresnel lenses with different designs and sizes have been investigated. Indoor measurements were performed as a good way to characterize an optical component under monochromatic light. Outdoor measurements were used to demonstrate the effects of the varying solar spectrum and the spectral responses of the solar cells on the optimum cell-lens distance and optical efficiency. Therefore, three solar cell structures (lattice matched triple-junction solar cell, middle and top component cell) were investigated. Indoor and outdoor measurements compare well when looking at the same range of wavelength. However, the experiments presented in this paper show that outdoor measurement are essential to determine the optical efficiency for a CPV module in operation.