This PART of IEC 62788 provides a method for measurement of the optical transmittance of encapsulation materials used in photovoltaic (PV) modules. The standardized measurements in this procedure quantify the expected transmittance of the encapsulation to the PV cell. Subsequent calculation of solar-weighted transmittance allows for comparison between different materials. The results for unweathered material may be used in an encapsulation manufacturer’s datasheets, in manufacturer’s material or process development, in manufacturing quality control (material acceptance), or applied in the analysis of module performance.

This measurement method can also be used to monitor the performance of encapsulation materials after weathering, to help assess their durability. The standardized measurements are intended to examine an interior region within a PV module, e.g., without the effects of oxygen diffusion at the periphery of the cells. Subsequent calculation of yellowness index allows for quantification of durability and consideration of appearance. The change in transmittance, yellowness index, and ultraviolet (UV) cut-off wavelength may be used by encapsulation or module manufacturers to compare the durability of different materials.