The spectrophotometer input optics should be designed for diffuse illumination and / or diffuse collection of the transmitted irradiance through the roughened PMMA sub-strate, both with and without the sunscreen layer spread on its surface. An integrating sphere is recommended and, when used, smaller fractional port areas compared with total sphere wall area will lead to greater accuracy. In any case, the spatial response should be close to a cosine response (cosine error smaller than ± 5% for incident angles