The presented article examines aspects of a PV module testing using natural sunlight in outdoor conditions. The article discusses the physical sense of indexes: atmosphere purity, diffused component content, beam clear sky index. Procedures for their determination are given in relation to both instantaneous and daily values. Their close connection with the values of solar irradiance spectral distribution such as Average Photon Energy and Useful Fraction is demonstrated, as well as their usefulness in module testing in outdoor conditions. Their influence on the conversion of modules made from various absorbers and various technologies is demonstrated