ASTM International Publishes Standard Test Method for Anisotropy Measurement

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 based in Collingswood, Ontario, chaired the task group working on these efforts. The industry now has a standard method to measure anisotropy in architectural glass, ASTM C1901-21, Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass.

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

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 growth 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.

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