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Technical Note: Intra-pane refractive index variability and adequate sample sizes in forensic glass analysis.
Emily Haase1, Emily Henry1, Cedric Neumann2
1Federal Bureau of Investigation, 2501 Investigation Parkway, Quantico VA 22135, USA.
Forensic Science International
|April 12, 2023
Summary
Determining the number of glass fragments for forensic analysis requires understanding refractive index (RI) variance. This study found lower RI variability in modern tempered glass, suggesting fewer fragments are needed for reliable forensic comparisons.
Area of Science:
- Forensic Science
- Materials Science
- Statistical Analysis
Background:
- Accurate forensic glass analysis relies on determining sufficient sample sizes for refractive index (RI) measurements.
- Previous research on tempered glass homogeneity is over 20 years old and may not reflect modern manufacturing processes.
- Understanding RI variability is crucial for establishing reliable comparison conclusions in glass fragment analysis.
Purpose of the Study:
- To determine the adequate sample size of tempered glass fragments needed for reliable refractive index (RI) characterization.
- To assess the variability of RI in modern tempered automotive glass.
- To evaluate the impact of sample size on error rates in simulated forensic comparisons.
Main Methods:
- Collected RI measurements from 218 tempered vehicle windows, analyzing 30 edge RI measurements from ten fragments per window.
- Employed a linear mixed effects model to analyze within-source and within-fragment RI variances.
- Conducted simulation studies using varying numbers of RI measurements (3, 9, and 21) to compare known and recovered glass sources.
Main Results:
- Within-source and within-fragment RI variances were found to be similar and lower than previously reported (approx. 4.3e-5 and 4.7e-5).
- Using nine RI measurements (three fragments) for the recovered source resulted in a lower false exclusion rate (1.97%) compared to three RI measurements (6.73%).
- False inclusion rates remained relatively stable across different sample sizes (approx. 4.05%).
Conclusions:
- Modern tempered automotive glass exhibits lower RI variability than previously documented.
- Fewer glass fragments may be sufficient for reliable forensic RI characterization, with nine measurements showing improved accuracy over three.
- The findings provide updated guidance for forensic practitioners on sample size selection in glass fragment analysis.
