New method helps to reconstruct events at crime scenes

Impact and spreading of blood droplets

12 November 2015

It may be possible to reconstruct events at a crime scene by analysing bloodstain patterns. A new method makes it possible to determine the height of the source of blood droplets more accurately.

This allows one, for example, to determine whether a victim was in a seated or standing position – information that can be crucial in distinguishing between self-defence or an attack. The method is the most important result of doctoral research conducted by physicist Nick Laan, who investigated the impact and spreading of blood droplets. Laan will defend his doctorate on Wednesday, 25 November at the University of Amsterdam (UvA).

Bloodstain Pattern Analysis is a forensic methodology used by investigators to determine, among other things, where the blood shedding event happened and how. Forensic researchers, for instance, do such an analysis to determine where the victim was when a specific bloodstain pattern was created. For this they use a so-called straight line approximation in which, however, they ignore the effects of gravity and air resistance. As a result, they overestimate the height of the source. This height overestimation depends on the distance between the origin and the bloodstain pattern. Nevertheless, from a distance of one metre from the wall, it is not possible to determine the exact position of the victim and perpetrator. 

Hammer in a pool of blood

Laan investigated whether it is possible to determine – from a dried bloodstain – the impact velocity of a blood droplet and, subsequently, calculate the curved flight trajectory from where it originated. He created actual bloodstain impact patterns at the Netherlands Forensic Institute (NFI) by using a hammer on a spring setup, which was placed at a distance of 50 cm, 100 cm and 150 cm from the wall. ‘By also taking into account the effect of gravity, I was able to show that the accuracy in the reconstructed height of the origin of the bloodstains was greater when compared to results obtained from the straight-line approximation’, says Laan. ‘I then took it one step further by investigating if downward directed stains could be taken into account in the analysis. With the straight-line approximation, downward directed bloodstains are discarded because they greatly overestimate the height due to the parabolic nature of the actual trajectory. By taking gravity into account, we can now also use these kinds of stains to determine the origin’s location.’

blood drop's trajectory

Practical application

It is expected that Laan’s findings will be of great importance for forensic research at a crime scene. ‘We are currently validating the method before going ahead and using it at a crime scene. We’re hoping that once this happens, the method will be applied by forensic researchers across the world’, says Laan. ‘By using 3D visualisation, we can improve the implementation of our method. We’re confident that this method can improve the reconstruction of events at a crime scene.’     

Details

Nick Laan, Impact of Blood Droplets. Supervisor: Prof. D. Bonn. Co-supervisor: Dr K.G. de Bruin.

Time and place

The PhD conferral ceremony will take place on Wednesday, 25 November at 10:00. Location: Agnietenkapel, Oudezijds Voorburgwal 231, Amsterdam.

Published by  UvA Persvoorlichting