Blood presentation

Determination of the Impact angle anddirectionality of a bloodstain

A diagram of a collapsing droplet impacting at an acute angle. Thisdemonstrates how the long axis of the ellipse develops and is oriented along thepath of travel. Unless impacting at 90°, the axis associated with the direction oftravel is always longer as a result of the skimming action of the droplet on thetarget. (Bevel and Gardner, 2002)

The presence of spines, scallops, spatter, or tails help the analystidentify the path the droplet was traveling at impact. These structures are foundopposite (or at least are concentrated opposite) the side of the stain thatimpacted first. (Bevel and Gardner, 2002)

A 90° impact. Small scallops appear around the entire periphery of the stain.

A 70° impact. Note the greater concentration of large scallops on one side of the stain. They are opposite the droplet’s origin and point in the general direction the droplet was travelling. Directionality in circular stains isnot as clear as in elliptical stains because the spines are not concentrated along a single vector.

(Bevel and Gardner, 2002)

A 50° impact. In this instance the scallops have spawned significant tadpole tails. Once again the tails align with the long axis of the stain and pointin the direction the droplet was travelling. Although directionality is not as specific as when we find a single tail, the analyst can clearly define the droplet’sdirectionality.

A 20° impact. This stain has a tail and detached satellite. In combination with the long axis of the elliptically shaped stain, it provides a very specific indication of the droplet’s path of travel.

(Bevel and Gardner, 2002)

15º Impact on Office Paper40º Impact on Office Paper

Results from previous work

Sin θ = opp (A) (or B to A) so Sin θ = width hyp (C) (or B to C) length

By accurately measuring the length and width of a bloodstain, the impact anglecan be calculated using the SIN formula below

e.g. length 3.0 cmwidth 1.5 cm

Sin θ = opp (A) (or B to A) so Sin θ = width hyp (C) (or B to C) length

For the exemplar figures above:

Sin θ = 1.5cm/3.0cm so Sin θ = 0.5 using the inverse of SIN we get 30º

References

Jackson R W and Jackson J (2004). Forensic Science. Pierson.

Bevel T and Gardner R M (2002). Bloodstain Pattern Analysis (2nd Edition). CRC Press.

Slemko J (2005). Bloodstain pattern Analysis Tutorial. http://www.bloodspatter.com/BPATutorial.htm page last accessed 7th February 2009