Evaluating optical distortion in high performance glazing

Triple-pane insulated glass units (IGUs) with two low emissivity (low-E) coatings have become popular in cold climates due to their increased thermal performance. While multiple low-E coatings reduce heat transfer through the glazed area, they also increase the IGU’s reflectance, which in combination with thermal loading poses the risk of unacceptable optical distortions.

We demonstrate a novel method for predicting the appearance of distorted reflections in triple-pane IGUs based on climatic conditions and propose criteria for determining acceptability of distorted images. Our method establishes the cavity temperatures using 2D heat transfer modelling and then determines the deflected shape of each glazing layer using finite element analysis. We use ray tracing to visualize the distorted appearance of context objects reflected in a curtainwall arrayed with that deformed geometry. Finally, we compare the distorted image to an ideal, undistorted reflection using feature-based similarity (FSIM) and other image quality assessments. We correlate this quantitative approach with subjective 8095 assessments of façade appearance by 40 building professionals to identify a threshold for visually acceptable glazing deformation.