Abstract
Thermal imaging has been a mainstay of military applications and diagnostic engineering. However, there is currently no formalised procedure for the use of thermal imaging capable of standing up to judicial scrutiny. Using a scientifically sound characterisation method, we describe the cooling function of three common shoe types at an ambient room temperature of 22 °C (295 K) based on the digital output of a consumer-grade FLIR i50 thermal imager. Our method allows the reliable estimation of cooling time from pixel intensity values within a time interval of 3 to 25 min after shoes have been removed. We found a significant linear relationship between pixel intensity level and temperature. The calibration method allows the replicable determination of independent thermal cooling profiles for objects without the need for emissivity values associated with non-ideal black-body thermal radiation or system noise functions. The method has potential applications for law enforcement and forensic research, such as cross-validating statements about time spent by a person in a room. The use of thermal images can thus provide forensic scientists, law enforcement officials, and legislative bodies with an efficient and cost-effective tool for obtaining and interpreting time-based evidence.
Highlights
There are many examples of the use of imaging techniques to record reflected visible (400–700 nm) [1] and invisible radiation, including ultraviolet-A (UVA) (320–400 nm) [2] and near-infrared (NIR) (780–1200 nm) [3], for visualising and collecting information useful as evidence in court rooms [3,4]
The recording of various types of infrared radiation (IR), including near, intermediate, and far wavelength, is possible through a wide range of techniques, including photographic recording of reflected NIR and thermal imaging for visualising longer wavelength infrared radiation emitted by objects heated above absolute zero (0 K) [9]
We propose a method for quantitatively estimating time by using as a predictor the temperature of an object as recorded by a portable thermal imaging device in a typical house environment, without knowledge of proprietary information, to provide forensic evidence that can stand up to rigorous cross-examination in a court of law in line with contemporary guidelines for determining the reliability of evidence [5]
Summary
There are many examples of the use of imaging techniques to record reflected visible (400–700 nm) [1] and invisible radiation, including ultraviolet-A (UVA) (320–400 nm) [2] and near-infrared (NIR) (780–1200 nm) [3], for visualising and collecting information useful as evidence in court rooms [3,4]. Uses of reflected infrared photographic recordings include revealing the latent marks of laser-removed tattoos for identification purposes [3,10] and pigment identification in heritage artworks [11], whilst thermal imaging can be used to assess potential damage to DNA samples on the outside of fired cartridge cases [12], observing cavitation formation.
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