Thermal emissivity of coated glazing-simulation versus measurements

A large variety of coated glazing products are available on the market today. These are used in energy efficient low emissivity (low-e) or solar control windows. Not only the solar optical properties, but also the thermal emissivity of these coated glazing materials are of importance for the performance of such energy efficient windows. The thermal emissivity is calculated from the IR reflectance. A problem is that for accurate determination of the emissivity according to international standards, the reflectance needs to be known between 2000 and 200 cm−1, and many FTIR spectrophotometers cannot measure below 400 cm−1. In this paper some different strategies for the extrapolation to 200 cm−1 are discussed. A sensitivity analysis for different types of materials is presented for a few different extrapolation algorithms. The simplest extrapolation procedure assumes a constant reflectance value throughout the extrapolation interval. This appears to work well for surfaces with high reflectance values. A procedure based on a linear relation between the values at a starting wavelength and at the end point of the extrapolation interval or one using a simple second-degree polynomial function can be used when coatings on glass having medium or low reflectance values are evaluated. A guide on how to extrapolate the spectra, according to the different strategies, is included in the Appendix.