Laser Particle Heating Process in a Stand-off Photo-thermal Explosive Detection System

TL;DR

This paper develops an analytical model for laser-induced heating and cooling of particles in a stand-off explosive detection system, aiding in understanding and optimizing the technique.

Contribution

It provides the first theoretical analysis of laser particle heating and cooling in a resonant infra-red photothermal imaging system for explosive detection.

Findings

Derived differential equations for particle and substrate temperatures.

Compared analytical results with experimental data.

Identified negligible effects of radiative and convection cooling.

Abstract

Recent publications have described a method for stand-off optical detection of explosives using resonant infra-red photothermal imaging. This technique uses tuned lasers to selectively heat small particles of explosive lying on a substrate surface. The presence of these heated particles is then detected using thermal infra-red imagery. Although the method has been experimentally demonstrated, no adequate theoretical analysis of the laser heating and subsequent particle cooling has been developed. This paper provides the analytical description of these processes that is necessary to understand and optimize the operational parameters of the explosive detection system. The differential equations for particle and substrate temperatures are derived and solved in the Laplace transform domain. The results are used to describe unexplained cooling phenomena measured during the experiments. A limiting particle temperature is derived as a function of experimental parameters. The effects of radiative and natural convection cooling of the particle and of non-uniform particle temperature are examined and found to be negligible. Calculations using the analytical model are compared with experimental measurements. An analysis of thermal contact heating of the substrate is included in the appendix.