NCJ Number
218783
Date Published
January 2001
Length
5 pages
Annotation
This study evaluated the Federal Bureau of Investigation’s (FBI’s) Laboratory glass database to determine whether the distribution of refractive indices (RI) had substantially changed since the 1960s when glass production shifted markedly to the float process.
Abstract
Results indicated that the more recent sheet glass samples contained a greater proportion of glass made by the float process than did the older glass samples, many of which were made using the drawing and polishing processes widely employed prior to the 1960s. Moreover, the results revealed that the older glass samples had a greater number of samples with RIs that were substantially different, particularly when compared to the newer glass samples that tended to have smaller differences in their RI distributions. The findings suggest that when using large databases of samples collected over long periods of time, variations over time may occur, presenting problems for statistical calculations. The authors urge caution when large databases are used to estimate the frequency-of-occurrence values or to perform precise statistical calculations. The findings also support the notion of improved quality control among glass manufacturers. The FBI Laboratory glass database consisted of RI measurements on 2,337 samples of glass. Only those RI measurements made at the sodium D-line wavelength were analyzed for this study. A variety of types of glass samples analyzed between 1964 and 1997 were included. The analysis used a single in-house glass RI standard to assess every glass sample over the 34-year period. Figures, references