Revised ASTM Standard Test Method for Measuring Roll Wave Optical Distortions to Minimize Inadvertent Errors
November 18, 2011

By Sahely Mukerji, smukerji@glass.com

The ASTM Committee C14.11 recently revised the C1651 - Standard Test Method for Measurement of Roll Wave Optical Distortion in Heat-Treated Flat Glass to C1651-11 to make it clearer and minimize inadvertent measurement errors, says Joel Feingold president of Strainoptics Inc. in North Wales, Pa.

The test method is a procedure for determining the peak-to-valley depth and the wavelength of roll wave in flat glass and then calculating the optical distortion resulting from that roll wave. Donivan Shetterly, development engineer IV for Reliability Physics at First Solar Inc. in Toledo, Ohio, notes that shortly after the standards development in 2008, it was recognized that there were errors in the mathematical nomenclature in the evaluation equations. "The general mathematical formulation was correct, as were the instructions on how to practice the method, so it was only necessary to correct the subscripts in the equations for evaluating this characteristic of glass. That was accomplished and the new standard is both correct in general, as well as in detail," he says.

Shetterly and Feingold were the co-authors of the original standard.

Feingold says that other than changes to measurement values in tables, the updated standard includes the following changes:

  • The non-roll wave distortions present that may be detected by the gauge should be ignored. This will help users obtain a more accurate average wavelength, because it will eliminate the non-roll wave distortions from the calculation.
  • A flat bottom gauge should be used when average wavelength cannot be calculated due
    to irregular waves that may or may not be roll wave. Using the 3-point gauge in these instances may result in an average wavelength value that is not representative of the actual roll wave distortion.
  • The glass to be measured should be placed on a flat supporting surface with any edge/end kink facing upward. The direction of the edge/end kink may be determined by using visual or optical inspection techniques (such as the reflection of a Zebra board) and/or production documentation. The supporting surface should have dimensions equal to or exceeding the dimensions the specimen to be tested. This will eliminate any mechanically induced load on the glass caused by it being supported by a downward-facing curl at the edges (edge kink) that could change the surface distortion characteristic. Sometimes the direction of the roll wave or edge curl is not known, e.g., end user or testing lab, so the reader is advised that it can be determined by using the inspection methods indicated.
  • To determine the direction of the roll waves using visual or optical inspection (such as the reflection of a Zebra board) and/or production documentation, place a measuring tape on the glass surface perpendicular to the roll waves. The measuring tape shall extend from leading or trailing edge and extend the entire length of the substrate where the roll wave peaks and valleys will be determined.
  • While the above specifies only a single traverse of the glass, several traverses will better represent the distortion over the face of the glass. It is common practice, for instance, to make three to five traverses across the glass in order to better represent the distortion of the entire glass surface. This will help to minimize measurement errors introduced by inadvertent pressure on the gauge as it is being moved across the glass and/or local
    deformations that do not correspond to roll wave.
  • When using the 3-point gauge, the first peak or valley (as well as the last) should not be considered when there is no previous or next peak or valley able to be measured. Also, the results of measurements using the 3-point gauge are only valid if all three contact points are evenly supported on the glass, i.e., not off the edge.

"For now, inexpensive roll wave gauges used in conjunction with this standard test method can help assure fabricators, architects, specifying engineers and end-users that the tempered glass used in their products or delivered to their jobsite meets their design requirements," Feingold says.

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