The Radiative Effectiveness of Plastic Films for Greenhouses

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Abstract
The spectral transmission properties of glass, polyester film, polyvinyl-fluoride film, polystyrene film and polyethylene film are presented, based on laboratory determined spectrograms and field measurements of atmospheric infrared radiation. Glass, polyvinyl-fluoride film and polystyrene film transmit about 0.88 of incident visible light; polyester and polyethylene transmit 0.81 and 0.73, respectively. For ultraviolet radiation, transmission values differ markedly between films. The amount of protection provided by various plant covering materials against nocturnal net radiation (Rn) loss from plants has been determined and is expressed as a protection index (PI) in per cent by: PI = 102 · [1 − (Rn(with cover)/Rn(no cover))] where PI ranges from zero (no protection) to 100 per cent (complete protection). Based on field measurements, PI values have been determined as follows: 1) window glass–93 per cent; 2) polyvinyl-fluoride film-90 per cent; 3) polyester film-87 per cent; 4) polystyrene film-63 per cent; and 5) polyethylene film-26 per cent.

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