NCJ Number
218653
Date Published
January 2005
Length
10 pages
Annotation
This report presents guidelines for measuring the refractive index (the ratio of the speed of light in one media compared to another for a particular transparent medium) of forensic glass samples.
Abstract
Refractive index is the most commonly measured property in the forensic analysis of glass. It is a function of the glass's composition and thermal history. One of the guidelines presented addresses sample handling regardless of the technique used to measure the refractive index of a glass sample. Seven general considerations in analyzing glass are outlined. One consideration notes that glass samples exhibit a range of refractive index values, depending on the sample's chemical composition and its thermal history. Thermal history is typically assessed by measuring either refractive index or density. A second consideration notes that variations in either refractive index or density at fixed values of the other provide some degree of additional distinction between samples. A third consideration notes that because there is a high correlation between density and refractive index, a limited amount of additional significance is provided by an association determined by a combination of analytical techniques. Other considerations are as follows: Samples distinguished by refractive index are from different sources; surface fragments may have a different refractive index than the bulk glass; and replicate measurements must be taken in assessing the extent of refractive index variation within specimens. Guidelines are presented for analyzing glass by means of Emmons double variation, automated glass refractive index measurement, immersion methods (dispersion staining and Becke line), and laboratory annealing. For each of these techniques, guidelines are suggested for the materials used in the technique, the procedure, advantages, and limitations. 8 references and a 13-item bibliography