Fluorescence imaging of petroleum accelerants by time-resolved spectroscopy with a pulsed Nd-YAG laser

Abstract

In this paper, fluorescence of petroleum accelerants such as kerosene, motor gasoline and diesel fuel were measured by the nanosecond time-resolved spectroscopy with a pulsed Nd-YAG laser. The excitation wavelengths are 266 nm (fourth harmonic generation) and 355 nm (third harmonic generation). Fluorescence was detected by a cooled CCD camera with an image intensifier. From these measurements, fluorescence lifetime was determined. We have also shown that the principal component of fluorescence spectra of kerosene is dimethylnaphthalene when excited with 266 nm laser light. For gasoline and diesel fuel, there exist other hydrocarbons which emit fluorescence. There is a significant difference among fluorescence spectra of kerosene, gasoline and diesel fuel. These results can be used for the identification of petroleum accelerants. We have also measured fluorescence spectra of various background materials such as newspaper, cardboard, carpet, cotton glove, wood board and various kinds of paper. We have shown that there exists a significant difference between petroleum accelerants and these background materials in fluorescence spectra and their lifetimes. By using such a difference, petroleum accelerants can be detected effectively even when they are on the fluorescent background materials which sometimes disturb the detection by emitting strong fluorescence. We have shown that petroleum accelerants on various background materials can be visualized by the time-resolved fluorescence imaging. Taking actual fire cases into account, the influence of heat to kerosene was studied. By heating, the fluorescence intensity of kerosene reduced for 266-nm excitation, but it increased for 355-nm excitation. For spectral profile, no change was observed for 266-nm excitation but there is a difference between non-heated kerosene and heated kerosene for 355-nm excitation. The lifetime seems to increase slightly by heating for 266-nm excitation but it is almost constant for 355-nm excitation. Imaging heated kerosene on burned background materials was examined and it is confirmed that heated kerosene can be detected on burned materials by the time-resolved method with an appropriate band-pass filter.