Blood Spatter Analysis What impact patterns can tell

Blood Spatter Analysis What impact patterns can tell us about a crime.

Characterization of Stains n The first step when dealing with blood in any aspect of forensics/criminology is to characterize the stain. n What 3 questions must you ask?

3 Major Questions Is it blood? n From what species did the blood come from? n If the blood is of human origin, how closely can it be associated to a particular individual? n

If it is human blood… n The pattern should be investigated n Location of the pattern n The way the blood has been spattered o o It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion of the stain. The shape of the stain tends to change depending upon the angle of impact which caused the stain.

§ Experiments with blood have shown that a drop of blood tends to form into a sphere in flight rather than the artistic teardrop shape. § The formation of the sphere is a result of surface tension that binds the molecules together. § This elastic like property of the surface of the liquid makes it tend to contract.

n. PASSIVE n. TRANSFER n. PROJECTED

DEFINITION: drops created or formed by the force of gravity acting alone.

EXAMPLES: Drops Drip Patterns Pools Clots

PICTURES:

DEFINITION: created when a wet, bloody surface comes in contact with a secondary surface.

EXAMPLES: Contact bleeding Swipe or Smear Wipe Smudge

PICTURES: A recognizable image of all or a portion of the original surface may be observed in the pattern.

DEFINITION: created when an exposed blood source is subjected to an action or force, greater than the force of gravity.

EXAMPLES: Arterial Spurt / Gush Cast-Off Impact Spatter

PICTURES:

The spherical shape of blood in flight is important for the calculation of the angle of impact (AOI) of blood spatter when it hits a surface.

When a droplet of blood strikes a surface perpendicular (90 degrees) the resulting bloodstain will be circular. That being the length and width of the stain will be equal. 90○ ANGLE

Blood that strikes a surface at an angle less than 90 degrees will be elongated or have a tear drop shape. 70○ ANGLE

45○ ANGLE 10○ ANGLE 30○ ANGLE 5○ ANGLE

Low Velocity Medium Velocity High Velocity

LOW VELOCITY Relatively large stains 4 mm in size and greater. Gravitational pull up to 5 feet/sec.

MEDIUM VELOCITY Preponderant stain size 1 to 4 mm in size. Force of 5 to 25 feet/sec.

HIGH VELOCITY Preponderant stain size 1 mm or greater. Force of 100 feet/sec. or greater.

The smaller the size of blood spatters, the greater the energy required to produce them. n Low, medium, and high velocity impact spatter may be identified by their respective sizes but exceptions must also be considered. n

Influences of Surface Texture The blood drop at the right struck a plastic wall tile falling again from 42 inches. On the left, the drop fell from the same distance on to a piece of heavy, unusual textured wallpaper.

Determining Direction

ANGLE of IMPACT is the acute angle formed between the direction of the blood drop and the plane of the surface it strikes.

Angle of Impact

Angle of Impact n The greater the difference between the width and length, the sharper the angle of impact.

By utilizing trigonometric functions, it’s possible to determine the impact angle for any given blood droplet.

By accurately measuring the length and width of a bloodstain, the impact angle can be calculated using the SIN formula below: AOI = -1 SIN (W / L)

Finding AOI LENGTH = 5. 9 cm WIDTH = 2. 6 cm

SOLUTION: AOI = SIN-1 W / L AOI = -1 SIN 2. 6/5. 9 AOI = SIN-1 (. 44) AOI = 26. 2°

DEFINITION: The common point, on a 2 dimensional surface, over which the directionality of several bloodstains can be retraced.

Once the directionality of a group of stains (one or two stains is not sufficient) has been determined, it's possible to determine a 2 D point (or area) for the group of stains.

By drawing a line through the long axis of a group of bloodstains, the point of convergence can be determined. LONG AXIS / LENGTH

Point of Convergence (2 D)

DEFINITION: lies at a point in space above the point of convergence. Measurement of the impact angle allows for translation of the 2 -D image (convergence) into a 3 -D one (origin).

Determining Origin n Origin can be determined by using the angles to find the area of convergence. This can be done with string or a computer model.

TO DETERMINE WHERE THAT POINT IS LOCATED: 1) First measure the distance from each blood stain along its central axis to the POC (distance = y) 2) Then take the TAN of the degrees AOI.

3) Third, multiply the TAN of the AOI by the distance. 4) Measure that distance from the floor up the perpendicular axis and you will arrive at the Point of Origin (PO) FORMULA: PO = TAN (AOI) x y

Finding PO GIVEN: DISTANCE FROM BLOODSTAIN (to POC): 90 cm AOI (calculated from AOI formula): 30° SOLUTION: PO = TAN (30°) x 90 cm PO =. 577 x 90 cm = 52 cm

In practice (at a crime scene), strings, tapes and protractors are commonly used.

Point of Origin (3 D --- use Z axis)

Computer programs can also be used.

Assignment: Investigate how different heights, direction of travel and angle affect the shape of a blood drop’s impact on a hard surface.