Eric Hegstrom | Grading Visibility of Distortion and Anisotropy in Heat-Treated Coated Glass

Session: Tempering Event: GPD Finland 2025 Grading Visibility of Distortion and Anisotropy in Heat-Treated Coated Glass This presentation builds on our “Controlling Glass Quality During Heat Treatment” talk from GPD 2023 and includes an overview of the science and technology behind objective and subjective measurement and grading systems, particularly as they apply to coated glass. Today, nearly all high-end architectural glass products feature at least one, often multiple, coated surfaces. A well-defined and repeatable grading system is required to be able to specify an acceptable level of optical quality and objectionability in coated glass after heat treatment. Although there has been extensive research and discussion over the past 20 years, most work has focused on uncoated glass and does not account for the specific characteristics of coatings. Currently, the industry lacks widely accepted standards for objectively grading the optical quality of coated glass products after heat treatment. As a result, architects and end-customers are not able to specify a meaningful objective quality threshold. This presentation will explore how incorporating the visual impact of modern coatings offers significant opportunities for improvement. Here we’ll examine measurement and analysis techniques that quantitatively characterize coatings and model how they impact the visibility of distortion and anisotropy. We’ll delve into the physics and optics behind optical distortion from physical deformation, as well as optical retardation —often referred to as anisotropy, iridescence, or quench marks— resulting from stress imbalances imparted by heat treatment. We’ll demonstrate a subjective analysis method that incorporates coating characteristics (transmission and reflection) to objectively grade the visibility of these effects. Additionally, we’ll review psychophysical testing, including human studies on perception and objectionability, showing how subjective analysis supports the objective calculations. The presentation will conclude with practical guidance on implementing these grading methods in real-world facilities, including essential equipment and procedures for efficient quality control. Glass Performance Days - Forum dedicated to the development of the global glass industry through education. https://gpd.fi/