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 September 2007
Tech News

Committee on Glass Develops Standard for Choosing Refraction Test

A new ASTM International standard describes a variety of methods that can be used to determine the index of refraction and dispersion of glass. The standard, C 1648, Guide for Choosing a Method for Determining the Index of Refraction and Dispersion of Glass, will be useful to manufacturers of glass and glass products as well as designers of products in which glass is a critical component.

C 1648 was developed by Subcommittee C14.11 on Optical Properties, part of ASTM International Committee C14 on Glass and Glass Products.

According to Herbert L. Hoover, a C14 member now retired from Corning Incorporated, knowledge of refractive index and dispersion is necessary for manufacturing glass for certain applications. “For example, the refractive indices and dispersions of optical glass are the basis for their selection for specific applications, such as telescopes, microscopes, lenses and prisms,” notes Hoover.

Hoover says that the accuracy of values, the ranges of wavelengths covered and speed, cost and convenience all must be considered when selecting a method of measuring refraction.

“C 1648 provides information on methods of measurement that will enable a prospective user to choose appropriately what method should be implemented,” says Hoover.

Descriptions of the following test methods are included in C 1648:

• Becke line (method of central illumination);
• Apparent depth of microscope focus (method of the Duc de Chaulnes);
• Critical angle refractometers (Abbe type and Pulfrich type);
• Metricon system;
• Vee-block refractometers;
• Prism spectrometer; and
• Specular reflectance.
In addition, four other methods are briefly detailed in Annex A1:
• Immersion refractometers;
• Interferometry;
• Ellipsometry; and
• Method of oblique illumination.

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