Material technology of diffuse greenhouse covering materials - influence on light transmission, light scattering and light spectrum

During the last years it has been shown that diffuse light is advantageous for many greenhouse crops. Modern greenhouse covering materials are able to transform direct sunlight into diffuse light. After entering the greenhouse sunlight is scattered and penetrates into the crop, where it is absorbed and used for photosynthesis. Experimental research with different crops (e.g., tomato, cucumber, roses) over the last years showed an improved production under diffuse light conditions, expressed in growth rate or yield of up to 10% in The Netherlands. In the last five years different glass producers have put a lot of effort in devel-opment of diffusing greenhouse covering materials. Different structures have been developed and applied on glass for greenhouse application. Glass surface treatments result in regular or irregular patterns on glass such as matt/matt, prismatic, satin, texture and v-grooves. Next to that micro structures are combined with nano-structures, such as anti-reflection treatments (AR), in order to improve light transmission into the greenhouse. It can be shown that material development was able to improve hemispherical light transmission of diffuse glasses by at least 5% in 2013 compared to before 2007, glasses with larger hemispherical transmission and highly diffusing properties are available. Earlier an evaluation method for diffuse materials had been proposed. That method is mainly based on a measurement of the so-called hemispherical light trans-mission and haze. New possibilities in material technology and increasing knowledge of the relation of diffuse light and crop reactions caused a need to extend evaluation methods. Measuring spatial distribution of transmitted light is suggested next to measuring angular and spectral light transmission. Different new diffuse materials of different producers are measured and evaluated. The results show that diffuse glasses differ in haze and hemispherical light transmission. As well, the choice of basic glass material as the type of anti-reflection treatment are main causes of light spectrum changes. Diffuse structures alone do not cause differences in spectral light transmission. Different structures and AR treat-ments result in different angular light transmission, which explains differences in hemispherical light transmission. Measurements of spatial light transmission show that many diffusing glasses only cause a narrow light scattering pattern so far. Full Text (PDF format, 1057673 bytes