Distribution of Refractive Index Values in Sheet Glasses
Koons, Robert D., Buscaglia, JoAnn, Forensic Science Communications
After determining that a fragment of glass is indistinguishable from a window at a crime scene, the glass examiner is then faced with the task of assessing the significance of this result and providing that significance measure to the interested parties. There are two approaches that can be used for this. Some glass examiners will determine that two samples are indistinguishable based on refractive index (RI) and then use a database of RI values to obtain the frequency of occurrence of glass having the particular RI in question. Other glass examiners may use the likelihood ratio approach in which they use databases of RI values to determine the relative probabilities of competing hypotheses as to the origin of a questioned fragment of glass. In either approach, it is essential that the glass examiner have access to an accurate database containing RI values determined from appropriate, representative samples.
It has been noted that the distribution of RI values for sheet glass, beginning roughly in the 1960s, became narrower than that observed previously (Almirall 1996; Buscaglia 1994; Curran et al. 1997; Curran et al. 2000; Koons et al. 1991; Stoecklein 1996). Most of these authors used this observation to support their contention that more discriminating methods of comparison, such as elemental composition, should be used in glass examination. They based their statements on comparisons of recently acquired databases with older databases, usually those compiled by the FBI Laboratory in Washington, DC (Miller 1982), and the Forensic Science Service in the United Kingdom (Lambert and Evett 1984). It cannot be determined from these reports whether the purported changes in RI distributions reflect changes in glass manufacture or are an artifact of the limited source distribution in small geographical areas of individual studies.
Buscaglia and Kubic (1991) took a different approach, breaking down FBI glass case data into five-year intervals, and found that there is evidence of a narrowing of RI distributions about a central value. Because the sources and glass types in their sample population did not change drastically during the five-year periods, the observed narrowing of RI distributions was explained as a change in glass manufacturing rather than an artifact of changing sample selection. Unfortunately, their study was never widely disseminated.
Prior to the early 1960s, drawing and polishing processes were used to manufacture most sheet glass. Since the 1960s, the flat glass industry has converted almost entirely to the float process. As a result, recent glass collections contain a higher percentage of float glass than older collections. Curran and colleagues (2000) stated that an observed narrowing of the range of RI in modern sheet glass results from improved quality control in the manufacture of float glass. This is brought about by computerized delivery of raw materials and a standardization of methods and formulations among manufacturers throughout the world (Curran et al. 2000). Although these statements are possible explanations for observed changes in RI distributions, the authors provide no supporting data. Further, any narrowing of refractive indices of glass from a single manufacturing facility is offset somewhat in the overall distribution by a globalization of the glass products market.
The FBI Laboratory has maintained a database of refractive index values on flat glass control samples received as evidence in casework since the 1960s. In casework, examiners in laboratories throughout the United States use this database to determine, at least in approximate terms, the frequency of occurrence of glass having a given RI range. To assist in determining whether this frequency value can be applied to current glass evidence, an evaluation of the FBI Laboratory data has been made to assess whether the distribution of refractive indices has, indeed, changed appreciably since float glass products began to dominate evidentiary samples. …