An inverse method was conducted to obtain the spectral optical properties of four greenhouse covering materials, (Low Density Polyethylene (LDPE), Polyolefin (PO), Polyvinylchloride (PVC) and Fused Silica Glass). Diffuse reflectance and transmittance of the covering materials were measured using spectrophotometric method; the complex index of refraction in the range between 0.22 and 25 mu m was deduced by inverse calculation using Radiative Element Method by Ray Emission Model (REM2). At longwave radiation, the optical constants of opaque glass material were found by utilizing Kramers-Kronig method resulting good correlation with results obtained by other investigations. A rigorous model for radiative heat transfer analysis to an agricultural greenhouse was developed. The greenhouse covering material was analyzed as a non-gray one-dimension plane-parallel medium subjected to solar and thermal irradiation using REM2. Specular reflectance and diffuse incident irradiation were treated at the boundary surfaces and absorption and emission were taken into account. Thermal performance was evaluated for the above mentioned covering materials. (c) 2013 Elsevier Ltd. All rights reserved.