Design of tabletop hemispherical light transmittance characterization system for small scale samples

Greenhouse owners highly value the hemispherical light transmittance (THEM) of roofing materials because sunlight rarely projects at a perpendicular angle, especially in high-latitude regions. With growing interest in research of advanced multi-functional greenhouse roofing, a compact and efficient THEM characterization system for lab-scale samples is needed to promote research in the horticulture field. In this study, we developed a tabletop system capable of characterizing THEM of lab-scale samples with a size one-third of that required by the current characterization system. Key designing parameters, such as the beam cross-section area, port area, and port edge thickness were systematically varied to evaluate their impact on THEM characterization. The results indicated that the total port area should be limited to under 1% of the sphere surface area with minimized edge thickness since reflection from the edge area can not be corrected by a double-beam measurement. Furthermore, the collimated beam cross-section area should exceed the port area by a factor of 1.5 to ensure that THEM remains unaffected by the sphere rotation center. The system provides a consistent and reliable method for THEM measurement and offers essential guidelines for future users to construct a similar setup.