Abstract
Transmittance of a material depends on the type of radiation impinging on the material (direct or diffuse), the angle of incidence of the sun’s rays (in direct radiation conditions) and the structure and characteristics of the material itself. The aim of this study was to evaluate the performance of nine agricultural screens of different densities, colours, thread diameters and porosities. A simple metal frame was used to quantify global, diffuse and photosynthetically active radiation (PAR) transmission to determine their transmittance values (as a function of the incidence angle of solar rays in direct radiation conditions) for global radiation and PAR, as well as the diffuse radiation transmitted characteristics of the screens (ratio of diffuse radiation to global radiation transmitted by screens). Non-coloured (translucent) screens contributed to a higher and more efficient proportion of diffuse radiation (Di/Gi ≈ 90%). The green screen behaved similarly to the non-coloured 20×10 type as far as global radiation was concerned, although the PAR transmittance values were lower (up to 12.4% at 15º). Variation in transmission according to diameter and density of the threads was greatest in the black screens. The non-coloured-thread screens may be the best option for maximising the transmission of diffuse radiation and the 6×6 green screen for vegetables cultivation during the summer in inland areas with latitudes close to 36ºN.
Highlights
During recent years growing in screenhouses vegetable crops has taken on considerable socio-economic importance especially for small-growers in semiarid and arid countries
Transmittance of a material depends on the type of radiation impinging on the material, the angle of incidence of the sun’s rays and the structure and characteristics of the material itself
A simple metal frame was used to quantify global, diffuse and photosynthetically active radiation (PAR) transmission to determine their transmittance values for global radiation and PAR, as well as the diffuse radiation transmitted characteristics of the screens
Summary
During recent years growing in screenhouses vegetable crops has taken on considerable socio-economic importance especially for small-growers in semiarid and arid countries. The green screen behaved to the non-coloured 20 × 10 type as far as global radiation was concerned, the PAR transmittance values were lower (up to 12.4% at 15°).
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