ASTM C1901 Test for Measuring Optical Retardation in Flat Architectural Glass

ASTM C1901 is used to measure optical anisotropy in flat architectural glass. Optical anisotropy is measured by measuring optical retardation. This test is used for quality control of the end use transparent glass products. The values stated in SI units are regarded as standard.

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    ASTM C1901 is used to measure optical anisotropy in architectural glass used as a building material. Anisotropy is the non-uniform spatial distribution of properties, which results in different values being obtained when specimens are probed from several directions within the same material.

    During heat treatment, stresses are applied to the glass to improve its mechanical performance. This leaves residual stress. The residual stresses are not spread evenly across the glass due to which properties are not spread evenly. For instance, one part of the glass may be stronger than the other part. This is called anisotropy or leopard spots. 

    To evaluate anisotropy, ASTM C1901 measures optical retardation at many locations on heat-treated glass sheets. To measure optical retardation, light is passed through a glass. Optical retardation measures the relative change in the speed of light across the glass surface. Optical retardation is linked to residual stresses. Different residual stresses on the glass lead to different optical retardation values. This test provides a map of optical retardation across the glass, and therefore of residual stresses and anisotropy. 

    Data obtained from this test can then be used:  

    1. For quality control.
    2. To adjust the heat treatment process to get the desired level of optical retardations and homogeneity on glass sheets.
    3. To provide a standardized way to measure optical retardation values. 
    4. By customers and other stakeholders to write specifications for the optical retardation values that are independently verifiable.

    ASTM C1901 test method does not apply to glass that is nontransparent such as a mirror and glass that diffuses light such as a sandblasted glass.

    Test Procedure:

    ASTM C1901 test helps to analyze the Residual Stresses resulting from heat treatment that make the material optically anisotropic. These residual stresses are not spread evenly across the surface. 

    Polarized light is introduced into the heat-treated glass. The speed of the light changes due to residual stresses. Since residual stresses are not spread evenly across the glass, the change in speed of light is different at different parts. The differences in the speed of light rays vibrating along the maximum and minimum principal stress introduce relative retardation between these rays. This relative retardation is proportional to the measured stresses, and it is measured using compensators.

    Specimen Size: 

    For ASTM C1901, the test specimen should be a flat piece of architectural glass. 


    Optical Retardation calculation for ASTM C1901:

    Retardation, caused by the difference of the speed of light between the two directions is a function of time difference i.e. Δt = (t1-t2).

    R = cx Δt


    R = Optical retardation

    c = speed of light in vacuum

    t= time difference in the axial and transverse direction


    ASTM C1901 is used to measure optical anisotropy in flat architectural glass. Optical anisotropy is measured by measuring optical retardation.


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