Measurement of Glass Refractive Index Using a Hot Stage and Its Application to Identification of Windshield Glass

Abstract

Glass fragments often give useful information as trace evidence to investigation of crimes such as hit-and-run, murder, burglary and so on. Trace elemental analysis is considered adequate for discrimination of glass evidence but it is destructive and time-consuming. In contrast, refractive index (R.I.) measurement is nondestructive and timesaving. Measuring R.I.s previous to chemical examination as a screening test is regarded as beneficial to rapid and precise discrimination for selecting suspicious samples from many pieces of glass. In the present paper, R.I. measurement using hotstage and silicone oil is described. The procedure is examined in respects of the following : using an optical filter to obtain monochromatic light, simple treatment to purify silicone oil, easy judgment of match temperatures, calibration with optical standard glass available in Japan, and handling of samples. The proposed method is applied to windshield glass samples of Japanese cars. Intra- and inter-sample variations are examined and 370 out of 406 pairs are discriminated each other by comparing their R.I.s It is proved that accuracy of this method is satisfactory for the discrimination of glass fragments. This technique is practical for discrimination of small glass fragments and to samples of any size as a screening test prior to trace elemental analysis.