ASTM E1967-11a

This technique modifies the sample, in that the glass fragment must be crushed, if it is too large, and immersed in oil for the analysis. Some sample handling, however, would enable the analyst to recover the sample in the crushed form, if necessary.

This test method is useful for accurate measurement of η_D^t from a wide variety of glass samples, where most glasses of interest have η_D^t in the range between 1.48 – 1.55 in η_D^t units.

The objective nature of the match point determination allows for a better standardization between laboratories, and therefore, allows for the interchange of databases between laboratories.

It should be recognized that surface fragments, especially from float glass samples, can result in η_D^t,s measurably higher than fragments from the bulk of the same source (5).

The precision and bias of this test method should be established in each laboratory that employs it. Confidence intervals or a similar statistical quality statement should be quoted along with any reported η_D^t value. For instance, a laboratory may report that the error for the measurement, using a reference optical glass is 0.00003 units.

It should be recognized that this technique measures the refractive index of the glass at the match point temperature, which will be higher than ambient temperature, and thus, may give different η_D^t values from those obtained by other methods, which measure the refractive index at room temperature.

1.1 This test method covers a procedure for measuring the refractive index (η_λ^t) of glass samples, irregularly shaped and as small as 300 μg, for the comparison of fragments of a known source to recovered fragments from a questioned source.

1.2 This test method does not include the measurement of optical dispersion or the measurement of refractive index (η_λ^t) at any other wavelength other than the Sodium D line (η_D^t). This method employs a narrow band pass filter at 589 nm, but other filters could be employed using the described method and allowing the η_λ^t to be determined at other wavelengths, therefore, also allowing for the dispersion value to be calculated.

1.3 Alternative methods for the determination of η_λ^t are listed in Refs (1-5).

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

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