Multidimensional model to correct PV device performance measurements taken under diffuse irradiation to reference conditions

In the domain of solar irradiance measurements, a wide range of irradiation conditions exist in indoor and outdoor environments. While the primary calibration of reference solar irradiance detectors is usually performed using only direct irradiance, secondary calibrations are conducted with a diffuse incident irradiance component. If the optical detector properties vary between the reference and the device under test, spectral and angular dependencies of incident irradiance and the detectors' responsivities may result in a mismatch when referring to defined reference conditions. This becomes particularly significant when photovoltaic (PV) devices are measured under global natural sunlight. In the field of PV metrology, diffuse solar irradiance is commonly considered to be isotropic, which is generally not the case. In this work, we present a novel multidimensional model that introduces spectral-angular effects to this field, with a focus on quantifying their impact on high-accuracy device calibration. Our enhanced approach allows anisotropic solar spectral radiance to be considered for any device orientation, including spectrally resolved ground reflections. To account for these effects, we consider both the spectral radiance of the source and the angular dependent spectral responsivities of the detectors. Angular mismatches of more than 1% were found for the examples investigated. This allowed detailed studies to be carried out on the impact of the detectors' optical losses and on the resulting mismatch of spectral and angular properties. Our model allows the spectral and angular effects to be determined and corrected, resulting in the measurement uncertainties for high-accuracy outdoor measurements decreasing significantly. It can be also used in other areas of photometric and radiometric applications, where a wide range of irradiance characteristics have an effect on measurements. As a consequence of this study, we propose assigning a directional property to the reference condition defined in international standards (i.e., direct beam only).