This part of IEC 60904 applies to the following photovoltaic devices for terrestrial applications: solar cells with or without a protective cover; sub-assemblies of solar cells; modules; and systems. NOTE The term test specimen is used to denote any of these devices. The principles contained in this standard cover testing in both natural and simulated sunlight. Photovoltaic conversion is spectrally selective due to the nature of the semiconductor materials used in PV solar cells and modules. To compare the relative performance of different PV devices and materials a reference standard solar spectral distribution is necessary. This standard includes such a reference solar spectral irradiance distribution. This standard also describes basic measurement principles for determining the electrical output of PV devices. The principles given in this standard are designed to relate the performance rating of PV devices to a common reference terrestrial solar spectral irradiance distribution. The reference terrestrial solar spectral irradiance distribution is given in this standard in order to classify solar simulators according to the spectral performance requirements contained in IEC 60904-9.