This research aims to investigate the potential of utilizing particle-based luminescent coatings for light management in greenhouses. Light management includes solar spectral conversion, transmittance, and scattering. Phosphor materials perform energy conversion from high-energy radiation to low-energy radiation through photoluminescent mechanisms. These phosphor materials are applied in the form of a sprayable coating intended to be employed on greenhouse roofing. Through energy conversion, a luminescent phosphor coating can achieve spectral conversion, adding light for crop growth, and will at the same time affect both light transmittance and scattering in greenhouse roofing. The application of phosphors as luminescent coatings on greenhouse roofing is a young research topic with very few published results; therefore, it presents several research challenges. The first is the lack of practical work on luminescent greenhouse coatings, demonstrating what type of spectral conversion is beneficial to plants and even the general feasibility of phosphors for greenhouse applications. The second challenge is to define clear and standardized measurable quantities that determine the success of a greenhouse coating, for example, the coating’s UV-to-PAR conversion efficiency. The third challenge is to design and build a measuring system that can characterize these quantities, especially the hemispherical light transmittance of small-scale luminescent coating 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.

Solar spectral conversion by a low-cost luminescent coating for greenhouse applications increases crop yield and can contribute to addressing the food crisis. A luminescent coating based on cheap SiO2 particles doped with Eu2+ and Al3+ demonstrated extra photosynthetic active radiation (PAR) in this work. To optimize the efficiency of this phosphor for greenhouse applications, three phosphor series with varying Al/Eu content in SiO2 were synthesized via a sol–gel approach and characterized by luminescence decay time, absorption, luminescent excitation, emission, and quantum yield measurements. With increasing the Eu%, at a fixed Al%, the decay time and quantum yield decreased while the emission shifted to the red. The effect can be explained by a more and more efficient resonance energy transfer to lower energy Eu2+ ions and quenching sites. While increasing the Al% at a fixed Eu%, the decay time and quantum yield increased, and the red-shift was reduced. Both effects can be explained by an enhanced Eu2+ solubility (reduced Eu clustering) through the Al3+ co-doping, causing the average Eu2+-Eu2+ distance to be longer and the onset of concentration quenching to shift to a higher Eu%. Specifically, we found that for 1 mol% Eu2+, a minimum of 4 mol% Al3+ was required to avoid concentration quenching. Two indicators were developed to quantify the UV to PAR converting efficiency and to quantify the PAR transmission enhancement. Both indicators were determined in a real coating sample based on the optimized phosphor. The result showed an additional PAR was provided by our luminescent coating. A general discussion about all factors that can bring the conversion efficiency of a phosphor coating closer to the theoretical maximum will be presented.