Detection of latent bloodstains at fire scenes using reflected infrared photography

Abstract

Bloodstain evidence is an element of crime scene investigation often found at scenes involving violence. Setting fire to the scene is a method sometimes used by offenders of crime in an attempt to conceal evidence. Fire often produces thick soot as a by-product of the combustion and has the potential to cover bloodstain patterns rendering them latent. There is limited published material offering a method of detecting bloodstains hidden beneath dense soot deposits caused by fire. This project employed a modified digital single-lens reflex (SLR) camera to investigate the application of reflected infrared photography to detect latent bloodstain evidence beneath varying deposited overlaying soot densities. The potential of this technique was examined by photographing blood samples beneath soot from a scaled fire simulation. A qualitative evaluation was completed by comparing images taken of a series of samples using both reflected infrared and standard visible light photography and corroborated with quantitative image analysis to support the findings. Results indicate that infrared photography can reveal latent bloodstains beneath a dense layer of soot in excess of ρ2.3 (550 nm) density with substantial clarity. The success of this technique is dependent on specific optical and specimen parameters. These parameters include i) the reflective properties of the background surface, ii) the spectral absorption properties of blood and iii) the ability of infrared wavelengths to transmit through the soot layer. Reflected infrared photography may provide crime scene examiners with a specialised field recording method that is easily executed and non-destructive to assist in visualising and locating latent bloodstain patterns beneath dense layers of soot.