Transcript File

Blood Spatter
Analysis
What impact patterns can tell us
about a crime.
Characterization of Stains

The first step when dealing with blood in
any aspect of forensics/criminology is
to characterize the stain.

What 3 questions must you ask?
3 Major Questions



Is it blood?
From what species did the blood come
from?
If the blood is of human origin, how
closely can it be associated to a
particular individual?
If it is human blood…

The pattern should be investigated
 Location of the pattern
 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.
PASSIVE
TRANSFER
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 4mm in size and greater.
Gravitational pull up to 5 feet/sec.
MEDIUM VELOCITY
Preponderant stain size 1 to 4mm in size.
Force of 5 to 25 feet/sec.
HIGH VELOCITY
Preponderant stain size 1mm or greater.
Force of 100 feet/sec. or greater.

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The smaller the size of blood spatters, the
greater the energy required to produce
them.
Low, medium, and high velocity impact
spatter may be identified by their respective
sizes but exceptions must also be
considered.
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

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 = 5.9cm
WIDTH = 2.6cm
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 2D
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 (2D)
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

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): 90cm
AOI (calculated from AOI formula): 30°
SOLUTION:
PO = TAN (30°) x 90cm
PO = .577 x 90cm
= 52cm
In practice (at a
crime scene),
strings, tapes
and protractors
are commonly
used.
Point of Origin
(3D --- 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.