Standard Active Last Updated: Apr 09, 2021
ASTM C1901-21e2

Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass

Significance and Use

5.1 Stress may be applied intentionally through a heat treatment or tempering process to increase mechanical strength and improve safety characteristics of glass sheets. The process itself makes it practically impossible to achieve a homogenous residual stress profile over a full glass panel. These variations are due to variations in type of glass (clear, tinted, coated, etc.), the fabrication, sheet geometry, heating, quenching, and cooling. Even though the level of inhomogeneity may not interfere with the global mechanical property of the glass sample, it can produce optical patterns called anisotropy (often commonly referred to as leopard spots). Today to evaluate this stress homogeneity people may use the subjective, non-standardized method of viewing through a polarized filter or employing a polariscope. The present test method provides guidelines for measuring a physical parameter, the optical retardation, directly linked to the local residual stress, at many locations on each heat-treated glass sheet.

5.2 Through this test method one can obtain in a non-destructive manner, on-line to the tempering furnace equipment, a map of the retardation value of all glasses. That information can then be used:

5.2.1 By the tempering operator to adjust the settings of the heat treatment process to optimize/tune both the levels optical retardations and its homogeneity on heat treated glass sheets.

5.2.2 To provide a standardized way to measure optical retardation values for each glass panel that can be archived and communicated when desired.

5.2.3 By customers and other stakeholders to develop/write specifications for the optical retardation values (not the visibility of the pattern) that are independently verifiable.

5.3 This test method can also be used off-line to evaluate the optical retardation level and homogeneity of any heat-treated glass, for quality assurance or other purposes.


1.1 This test method addresses the measurement of optical anisotropy in architectural glass.

1.2 This test method is a test method for measuring optical retardation. It is not an architectural glazing specification.

1.3 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.

1.4 This test method applies to monolithic heat-treated (heat-strengthened and fully tempered) clear, tinted and coated glass.

1.5 This test method does not apply to:

1.5.1 Glass that diffuse light (that is, patterned glass, sand blasted glass, acid etched, etc.), or

1.5.2 Glass that is not optically transparent (that is, mirrors, enameled or fritted glass).

1.6 The optical measurement is integrated through the glass thickness, and therefore cannot be used to assess the level of tempering. It does not give information on the surface stress or center tension.

1.7 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

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Book of Standards Volume: 15.02
Developed by Subcommittee: C14.08
Pages: 10
DOI: 10.1520/C1901-21E02
ICS Code: 17.180.99; 81.040.20