Degradation Characteristics of the Optical Constants of PE-LD Film-Covered Greenhouses in an Arid Climate.

This study is to describe the degradation behavior of the optical constants (refractive index ¯ n and extinction coefficient ¯σ ) of PE-LD sheets during exposure to an arid climate. Values of ¯ n and ¯σ are used to determine the solar radiative properties of any PE-LD cover at any time without the need of measurements. Different greenhouse models were covered with PE-LD films and exposed for 1 year to an arid climate, and values of ¯ n and ¯σ were investigated. Exposing films to the arid climate for 1 year increased the values of ¯ n from 1.58 to 2.86 and ¯σ from 3.86 to 8.1 mm−1, resulting in a decrease in transmittance from 0.42 to 0.12. Two correlations for predicting the degradation rate of ¯ n and ¯σ for PE-LD covers were developed.