THE MEASUREMENT OF BLOOD DROPLET STAINS

• Jurisdiction: Australia

Introduction

Overview ....................................................................................................... [97.600]

Two-dimensional reconstruction .................................................................... [97.620]

Three-dimensional reconstruction ................................................................. [97.650]

Trajectory modelling

String line modelling ...................................................................................... [97.660]

Mathematical modelling and the tangent method ......................................... [97.670]

Computer assisted modelling........................................................................ [97.680]

Detection, enhancement and collection

Introduction ................................................................................................... [97.800]

Visual detection and enhancement ............................................................... [97.810]

Chemical detection and enhancement ......................................................... [97.830]

Catalytic tests ................................................................................................ [97.840]

Luminescent techniques ............................................................................... [97.850]

Staining techniques ....................................................................................... [97.860]

Confirmatory testing ...................................................................................... [97.870]

Collection ....................................................................................................... [97.900]

Introduction

[97.600] Overview

Blood is spilled at many violent crime scenes. This is invariably accompanied by blood droplets leaving or being forced to leave their source and subsequently landing on and staining an object, the victim, the assailant, the floor, the ceiling or the walls of a structure or building. Horizontal and vertical surface stains are the primary focus of this section.

A droplet of blood moving through space is influenced both by the action of gravity and by air resistance. The velocity vector of that droplet may be broken down into a horizontal component (influenced by air resistance) and a vertical component (influenced both by gravity and by air resistance) at any point of the trajectory. The air drag vector in still air is always in the opposite direction to the direction of travel of the droplet.

The reconstruction of crime scenes requires a forensic practitioner to take measurements of the room and measurements of stain dimensions caused by individual droplets on items and surfaces in the room. To make quantitative analyses of these stains requires a mathematical framework: Fischer (1994); Griffin, Anderson and Gardner (1993); Gardner (1992; 1994); Wolson (1995); Raymond and Ashley (1997); Bevel and Gardner (1997 and 2002); Fischer (1999a); Carter (2001); James et al (2005); Reynolds (2008). This framework must allow the practitioner to describe the position of the droplet in space, and, by extension, to describe the motion of the droplet through space prior to impact on an object or surface and causing the remnant bloodstain. This is accomplished by assigning a system of co-ordinates and applying a variety of simple mathematical concepts. In short, a practitioner draws extensively on Euclidean geometry principles.

It is possible to reconstruct a crime scene by determining the "point" or, more correctly, "region" of origin of a set or sets of droplets which have been generated by an event or events associated with the crime. This means that it is necessary to be able to reconstruct the possible paths of a droplet from the impact site on a wall, floor or ceiling back to its source. To do this, the examiner must consider the shape, size and distribution of the stains, the surface upon which they impacted and be able to apply a number of mathematical concepts to those aspects of the scene. The location of the blood source is generally approximated either through:

• a two-dimensional area of convergence; or

• a three-dimensional region of origin.

[97.620] Two-dimensional reconstruction

Stains on walls, floors or other vertical and horizontal surfaces may be encountered at a scene and provide suitable characteristics for investigation. A set of blood droplets impacting on a non-porous, smooth surface will, by nature of their shape, leave some evidence of the origin and direction of propagation of those droplets. The resulting bloodstains will be round or elliptical and may include a thin spine or exclamation-like point at the conclusion of the stain which shows the forward direction of travel of the droplet at impact: see Figures 3 and 4.

Figure 3. An elliptical bloodstain illustrating the classic exclamation mark shape

Figure 4. Elliptical blood stains on a wall illustrating exclamation mark shapes

Lines drawn back through the long axis of the set of ellipses will intersect at a point or area which reflects the origin of the bloodstains in a two-dimensional plane: see Figure 5; Wonder (2001); MacDonell (2005); James et al (2005).

Figure 5. Two-dimensional origin reconstruction using elliptical shaped blood stains on a wall

For stains on walls and other vertical surfaces it is standard practice to only choose those stains that were created by fast upward moving droplets. The trajectories of these droplets will not have yet exhibited the characteristic parabolic loop or curve (Figure 6) of a body moving through air under the influence of gravity and air resistance. In other words, for these fast, upwards moving droplets, the direction of travel assumes a closer-to-straight-line trajectory. (This, in fact, is never quite the case as has been demonstrated by the work of Carter: see [97.680].) The result of choosing stains on a surface for which the droplet trajectory is significantly curved is that the impact velocity vector is greatly different from the initial velocity vector, leading to ambiguity of the droplet source position.

Figure 6. Illustration of stains made on a vertical wall by droplets propagated from a point in space

[97.650] Three-dimensional reconstruction

This analysis should be able to locate more precisely the spatial region from which the bloodstains originated. Droplet stain shape becomes increasingly elliptical the more acute the angle of impact between the droplet and the deposition surface. Droplets that contact the surface at near right angles to a plane leave a circular stain. There is an empirically determined mathematical relationship between the shape of the bloodstain and the angle at which it impacts on the surface. This allows the practitioner to make calculations as to the height above the convergence area from which a droplet set originated.

The "impact angle" may be defined as the angle at which a blood droplet strikes the target surface (usually a plane); that is, the angle between the flight path of the droplet at the point of contact and the plane. Impact angles are calculated from the measurement of the stain width and length. This analysis relies traditionally on the assumption that the droplets are spherical when they strike the surface, the trigonometric implication being that the stain width is the same as the droplet diameter. Although experimentation has shown this is, in fact, not the case, the trigonometric relationship has been shown to hold true: Raymond (1997). The apparent derivation of the calculation as described by MacDonnell and Bialousz (1971); MacDonell (1982); Eckert and James (1998); Carter (2001); Bevel and Gardner (2002); Chafe (2003); James et al (2005) is shown in schematic Figure 7.

Figure 7. A classical view of a droplet sphere impacting on a plane and the trigonometric relationship of the resultant ellipse with that sphere

It is important to note that while this diagram implies that the diameter of the blood droplet in flight equates to the final length of the resulting stain, it is well known that this is not the case.

In essence, the ratio of width to length approximates sin. Difficulties are experienced in determining the relevant measurable...