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 \( \bar{n} \) and extinction coefficient \( \bar{\sigma } \)) of PE-LD sheets during exposure to an arid climate. Values of \( \bar{n} \) and \( \bar{\sigma } \) 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 \( \bar{n} \) and \( \bar{\sigma } \) were investigated. Exposing films to the arid climate for 1 year increased the values of \( \bar{n} \) from 1.58 to 2.86 and \( \bar{\sigma } \) 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 \( \bar{n} \) and \( \bar{\sigma } \) for PE-LD covers were developed.