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

Abstract

Greenhouse owners highly value the hemispherical light transmittance (T_HEM) 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 T_HEM 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 T_HEM 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 T_HEM 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 T_HEM remains unaffected by the sphere rotation center. The system provides a consistent and reliable method for T_HEM measurement and offers essential guidelines for future users to construct a similar setup.