Nearly three years ago, members of the architectural glass and glazing industry, architects, engineers, machinery and equipment suppliers and other international stakeholders gathered together to address anisotropy measurement and develop an ASTM International standard test method. Louis Moreau, head of technology and innovation with AGNORA, was at the helm as chair of the task group working on these efforts. The industry now has a standard method to measure anisotropy in architectural glass, ASTM C1901-21.

The new standard’s purpose is to provide a repeatable test method to measure the optical anisotropy of architectural glass. The optical retardation values may be used to calculate/predict the amount of visible pattern, commonly known as anisotropy or iridescence, present in heat-treated glass.

Line scanners, such as this one at AGNORA, can be used to measure anisotropy in glass.

“Having a worldwide standard allows fabricators to provide clients with an understanding of how much roller wave they will actually see in their heat-treated glass. There’s never been a way to provide a measurement, it was really, ‘what you get is what you see,’ and that was the end of the conversation,” says Moreau. “With this standard, a fabricator can supply an architect, glazier or owner with real results, and they can accept the glass based on real, measured properties.”

According to Moreau, glass, in its annealed state, is isotropic, which means that values are the same no matter what direction they are measured. In this case, the velocity of light transmission remains constant in any direction. As soon as stress is introduced, the transmission values begin to differ, depending on the measure’s direction. The heat-treated glass becomes anisotropic.

“The study of anisotropy is not new and has always been a point of contention between glass fabricators and purchasing stakeholders. Some fabricators are better at managing stress-induced optical distortion than others. Even so, variables such as time of year, humidity, glass thickness, tempering oven performance and even operators, among other variables, can change the heat-treated characteristics of glass day-to-day,” says Moreau, adding that the invention, adoption and furthering ubiquity of glass scanners capable of mapping retardation values has opened the door for proper measurement of heat-treated glass optical properties and paved the way for ASTM C1901-21.