b l o o d s patt e r
Properties of blood

Teacher Background Information
Blood is considered to be a fu
with no fxed shape and is su
pressure. A fuid can be either
liquid is a fuid that has a fxed
fuid that can expand indefni

Viscosity is defned as a fuid’s
more viscous a substance is, th
fow. The SI unit for viscosity is
viscosity is compared to water
one. Blood is thicker than wate
due to the cellular component
viscosity of some common su

id. A fuid is a substance
bject to external
a liquid or a gas. A
volume while a gas is a


resistance to fow. The
e more slowly it will
the Pascal second. Fluid
that has a viscosity of
r and is viscous primarily
(see FSB04). The

bstances, including blood:

Liquid Viscosity (mP·s-1)

Milk (25oC) 3

Blood (37 oC) 3-4

Glycerin (20 oC) �420

Mercury (�5 oC) �.55

Water (20 oC) �.0

Water (�00 oC) 0.28

Surface tension
Surface tension is the force that pulls the surface
molecules towards the interior of a liquid, decreasing
the surface area and causing the liquid to resist
penetration or separation.

Surface tension is the tendency of the surface of a
liquid to contract to the smallest area possible. The
fuid is able to do this as the cohesive forces are
stronger on the surface of liquids as there are no
neighbouring molecules above. As a result there are
stronger attractive forces between molecules and their
nearest neighbours on the surface; the surface tension
force actually exerts an upward force. Surface tension is
like having an elastic flm over the surface.

Figure �: A water strider standing on water.
Citation: Water strider: David Cappaert,

In Figure �, the surface tension of the water allows the
water strider to walk on the water without sinking.
This is because the upward force from surface tension
balances the insect’s weight.

Defnition of surface tension: the surface tension γ is
the magnitude F of the force exerted parallel to the
surface of a liquid divided by the length L of the line
over which the force acts:

γ = F _

Surface tension is measured in force per unit length:
newtons per metre: (N·m-�). The old unit is dynes per cm.

The surface tension of some common liquids:

Liquid Surface tension N·m-�

Benzene (20oC) 0.029

Blood (37 oC) 0.058

Glycerin (20 oC) 0.063

Mercury (20 oC) 0.47

Water (20 oC) 0.073

Water (�00 oC) 0.059


b l o o d s patt e r
Properties of blood

Surface tension is important in bloodstain pattern
analysis as;

• the gravitational force must overcome
the surface tension of blood before
a drop of blood can fall, and

• drops of blood remain intact as they move
through the air due to surface tension.

Figure 2: Complimentary efects of adhesion,
cohesion and surface tension on a single blood
Image courtesy UWA PhD research student Mark Reynolds.

Density is defned as mass per unit volume. The
density of water is �000 kg/m3. The density of blood
is proportional to the total protein concentration or
cellular component of blood and is infuenced only
to a minor extent by other ions, gases etc. that are
dissolved in the plasma.

The density of blood plasma is approximately �025
kg/m3 and the density of blood cells circulating in
the blood is approximately ��25 kg/m3. The average
density of whole blood for a human is about �060

Blood Droplets
The application of a force to a mass of blood causes
the mass to break up into droplets. As a blood droplet
travels through the air it retains a spherical shape due
to surface tension. Smaller drops (�mm diameter and
less) are almost perfect spheres while larger drops
oscillate due a range of other forces acting on the
droplet. [Smaller droplets do oscillate but the time
required to dampen the oscillations is far less than

larger droplets.] Droplets do not “break up” whilst in
motion; another force would need to be applied to
cause the droplets to further divide. The oscillations
generally have no efect on the resulting spatter
pattern except for instances where there are only a
few stains and they are present on surfaces less than
�00cm from the source.

When a droplet of blood strikes a horizontal surface
at 90o it produces a circular stain. If the surface texture
is smooth, such as glass or a polished tile, the surface
tension will hold the droplet in the circular pattern.
Essentially the surface infuences the outfow. Surface
tension ensures that the droplet collapses uniformly
however the smooth surface means that the rim
outfow is uniform.

Figure 3: Several blood droplets that have fallen onto
a rough surface.
Image courtesy DUIT Multimedia: Paul Ricketts.

If the droplet falls onto a rough surface such as
cardboard, carpet or concrete it will produce an
irregular and distorted stain pattern. The rough
surfaces results in an irregular rim outfow.



b l o o d s patt e r
Properties of blood

Phases of impact
There are 4 distinct phases of impact:

�) Contact /collapse
The droplet contacts the target surface and collapses
from the bottom up. The part of the drop that has not yet
collided with the surface remains as part of the sphere.

Figure 4: Flight of a single blood droplet.

mage used with permission from Tom Bevel & Ross Gardner, June 2006I .

Figure 5: A diagram showing blood being pushed
into a rim on contact with a receiving surface.

As the collapse occurs, the blood that has come in
contact with the surface is forced outward creating a
rim. The rim gets bigger as more of the droplet comes

in contact with the surface and more blood is forced
into the rim.

The angle of impact afects the collapse as it defnes
the nature of the rim and the blood fow into it. For
example: if the droplet impacts at 90o the blood fow
into the rim is equal on all sides. If the impact angle is
more acute, the blood fows into the area of the rim
opposite the direction from which the droplet came.

2) Displacement
In this stage, the blood droplet has collapsed against
the target surface and nearly all of the blood has
moved from the centre of the droplet to the rim. The
actual area of displacement will be the same size as
the eventual stain.

At the edge of the rim will be dimples or short spines.
In this stage the movement of the blood is lateral or to
the sides.

Figure 6: Displacement phase of a blood droplet in a
90o impact.

Image used with permission from Tom Bevel & Ross Gardner,

June 2006.



b l o o d s patt e r
Properties of blood

The surface texture is important. Surface tension is
responsible for keeping the shape of the droplet as
it moves through the air. When the droplet hits the
target surface, the ‘skin’ of the droplet, created by
surface tension shifts its shape. The droplet doesn’t
actually burst.

If the surface is rough, the blood fows irregularly into
the rim so the spines or dimples that form will also
be irregular in shape. This will result in a distorted or
asymmetrical shape.

3) Dispersion
In this phase, most of the blood is forced into the rim.
The spines and dimples continue to rise upward and
in a direction opposite to the original momentum. As
the amount of blood in the rim and spines increases
they become unstable.

Figure 7: Early dispersion phase of a blood droplet
impacting at 90o.

Image used with permission from Tom Bevel & Ross Gardner,
June 2006.

4) Retraction
The last phase results from the efect of surface
tension attempting to pull the droplet back. If the
forces trying to pull the droplet apart are overcome by
surface tension, the resulting stain will be reasonably
circular and symmetrical in shape. If the forces pulling
the droplet apart overcome the surface tension,
the droplet will ‘burst’ and create an irregular stain

An excellent animation showing the impact behaviour
of a blood droplet (November 2006).

The higher the droplet falls from the ‘more’ blood
satellite spatter occurs. Blood spatter is a broad term
essentially meaning blood distributed through the air
in the form of droplets. Satellite spatter, or spatter on
the receiving surface may or may not be formed.

If two similar sized droplets fall from diferent heights
the resulting stains have diferent sizes. E.g. a droplet
falling from �0cm will produce a diferent stain than
a droplet falling from �00cm. The stain diameter
from the �00cm height will be larger than the pattern
from the �0cm height. The reason is that the velocity
of the droplet will be greater the longer the droplet
is airborne [until it reaches terminal velocity.] Above
a fall distance of 2.2m there is little change in the
diameter of the blood spot.

Force, Velocity and Droplet Size
The size and appearance of the bloodstains depends
on the force that was used to create them. When an
object comes into contact with blood, the force of the
object moves the blood. The blood must respond to
this energy transfer in some fashion. The response is
often by the distribution of blood through the air in
the form of droplets.

Velocity is measured in meters per second. At a crime
scene there may be evidence of low, medium or high
velocity blood spatter or a combination of these. For
example, dripping blood (low velocity) has a velocity
of �.5 metres per second. Blood droplets produced
from a bullet shot from a gun will have much greater
energy and will travel faster.



b l o o d s patt e r
Properties of blood

Low velocity blood spatter
A low velocity force is usually the result of blood
dripping from a person who is still, walking or
running. Blood drops may be free falling and only
moving due to the force of gravity. At low velocities
larger bloodstains are produced. Sometimes low
velocity bloodstains are a result of weapon cast-of of
from blood dripping from a victim.

Dripping blood often falls at a 900 angle and forms
a round bloodstain that is often 4mm in diameter or
larger: up to approximately �0mm. If droplets are,
however, falling from a moving object or person
(walking or running) they fall to the ground at an
angle (see angle of impact) and the direction of the
movement can be established.

Identifying Blood Trail Motion

Droplets dripping from a moving object or person do not
drop straight down. As they are in motion themselves, they
fall to the ground at an angle.

Blood-trail motion is defned by considering the
directionality of the individual droplets present in the blood
trail pattern.

Figure 8: A blood-trail pattern.
Image used with permission from Tom Bevel & Ross Gardner, June 2006.

Figure 9: Passive bloodstains falling onto a smooth
surface at approximately 90°

Image courtesy UWA PhD research student Mark Reynolds.

Medium velocity blood spatter
A medium velocity force moves blood between
fve and 50 metres per second and the resulting
bloodstains at 90o are between one and three
millimetres in size. The size of the bloodstain depends
on the angle of impact with the receiving surface. An
oblique stain can be greater than �0mm but would be
long and thin. Medium velocity blood spatter might
result from blunt force trauma, for example, beating
with fsts, baseball bats, whips, bricks or hammers.
Medium velocity blood spatter can also occur when a
body collides with rounded or edged surfaces.

Figure 10: Spatter deposited on a wall as a result of a
‘blunt force’ beating.
Image courtesy UWA PhD research student Mark Reynolds.



b l o o d s patt e r
Properties of blood

High velocity blood spatter
A high velocity force moves blood greater than 50
metres per second and the bloodstains are usually
smaller than �mm and appear as fne spray or misting.
High velocity blood spatter can be caused by high-
speed machinery such as chain saws and wood

Figure 11: Spatter deposited on a wall as a result of a
Image courtesy Stuart James, February 2007.

Crime scene investigators can determine the direction
that a blood droplet was travelling in as droplets
impact surfaces in a consistent manner. The droplet
will keep moving along the same path that it was
travelling before hitting the surface. When it impacts
a surface, the blood in the droplet moves outwards
during the collapse phase creating either an elliptical
or circular stain.

The long axis of the stain (major axis) provides an
indication of the direction the droplet was travelling in
prior to contact with the receiving surface and hence
the direction that it came from. The droplet always
travels in the long axis, but it is sometimes difcult to
tell the actual direction as shown in Figure �2.

Figure 12: Spines, scallops and satellite spatter help
to identify the path of the blood droplet.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

A crime scene investigator will look at other features
of the bloodstain to determine which direction the
blood droplet was travelling in. Bloodstains also
usually have features such as satellite stains, scallops
or spines. The stain will have a higher number of these
features on one side. This is due to the way the droplet
collapses on impact. As discussed previously, blood
fows into the area of the rim opposite the direction
from which the droplet came. In many instances the
dimples on the rim break slightly from the droplet
structure creating spines, scallops or if it breaks
entirely away, satellite stains.

Figure 13: Scallops, spines and satellite stains are
always in the direction of travel.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

The pointed end of the bloodstain always points in
the direction of travel.



b l o o d s patt e r
Properties of blood

Angle of Impact
There is a relationship between the length and
width of a bloodstain and the angle at which the
droplet impacts on a surface. It is therefore possible
to calculate the angle of impact on a fat surface by
measuring the length and width of a stain.

The angle of impact is the acute angle that is
formed between the direction of the blood drop
and the surface it strikes. This is an important
measure because it is used to determine the area of
convergence and the area of origin.

Figure 14: The angle of impact of a blood droplet on
a receiving surface.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

When a droplet of blood impacts a surface at 90o,
the bloodstain will be circular. The more the angle of
impact decreases, the more the stain is an ellipse. The
angle of impact can be measured by the degree to
which the shape of the drop changes from a circle to
an ellipse.

An excellent animation showing the angle of impact
(November 2006).

When measuring the length and width of a stain, no
part of the spines, tails or satellite spatter are included
in the measure. Round the stain to an elliptical shape
when making measurements.

Figure 15: The measurement of the length and width
of stains.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

Calculating the angle of impact
The angle of impact formula relies on the relationships
that exist between the angles of a right triangle
and the length of its sides. These are trigonometric
functions called sine, cosine and tangent.

Imagine a right triangle formed between the droplet
and the target surface as the droplet strikes. A blood
droplet in fight is the same shape as a sphere.
Therefore, the width of the stain is equal to the length.
By measuring the length and width of the stain,
the droplet’s impact angle, i can be calculated. NB:
convention is to refer to the impact angle as the alpha

Figure 16: The relationship of the droplet to an
imagined right angle.
Image used with permission from Tom Bevel & Ross Gardner, June 2006



b l o o d s patt e r
Properties of blood

The diagram below (Figure �7) represents a stain that
has impacted on a surface.

Figure 17: The width and length of a bloodstain can
be used to calculate the angle of impact.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

As a result, we have two known quantities from the
crime scene, the width and length of a bloodstain,
which can be applied to the following formula:

The sine of the angle of impact = width divided by
the length.

Sine i = Width (ab) / Length (bc)

The result of the division is a ratio.

Look for the ratio on a trigonometric function table
– the closest angle will be identifed, OR

The “inverse sine” or the arc sine function on a
scientifc calculator (ASN) converts the ratio to an

Inverse Sine (ASIN) i = Angle of Impact

The steps are:

• Accurately measure the width and length
of a given bloodstain. This should be
measured to the nearest millimetre.

• Divide the width of the stain by the length of the
stain in order to obtain the width to length ratio.

• Calculate the inverse sine of this ratio.

• This value is the angle of impact.

An example

Width = 3mm
Length = 5mm
Sine i = width / length

Sine i = 3mm / 5mm = 0.6

Angle = 37o

Using the calculator

Inverse Sine i (0.6) = 36.8

The angle of impact is between 36-37o

It is important to note that this method gives an
estimate of the impact angle rather than a precise
result. The accepted variance is between 5-7o.
Computer ftting of theoretical ellipses has refned
the measurement process to sub-degree levels of


Area of Convergence
Consider a simplifed crime scene where there are
two elliptical bloodstains on a foor, forty centimetres
apart. Lines are drawn from the centre of the long axis
of each bloodstain and extended until the two lines
from the separate stains meet. The point where the
lines meet is called the Area of Convergence. (NB _
this is also called the POINT of convergence – for our
purposes the term will be the AREA of convergence
as accuracy is not sufcient to determine the actual

In addition to two stains having a coincidental intersecting
point, it is also possible to have several patterns overlap.
If this condition is not considered it might well result in a
mistaken point of convergence.

Figure 18: The area of convergence.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

This area of convergence is possibly the source of
both bloodstains, but the path crossover may also be
completely coincidental if the two stains were created
by unrelated events.

In the fgure below there are 3 stains with diferent
angles of impact. When lines are drawn from the
stains, (the centre of the long axis of the stain) the
lines converge at an area (of convergence).


b l o o d s patt e r
Properties of blood

Figure 19: Measuring the distance from the
loodstain to the area of convergence.

efore drawing lines it is important to determine the
irectionality of the bloodstain. The lines must be
rawn away from the direction of travel towards the

lways work via the centre of the long axis and extend



the line from the back of the bloodstain.

Figure 20: Establishing the direction of travel.

Area of Origin
At a crime scene with several bloodstains, crime
scene investigators attempt to determine the origin
of the blood. In essence the investigator is trying to
determine from which location in a 3-dimensional
space the blood originated, from 2- dimensional
measurements. Figure 2� below attempts to show the
point in space where the paths converge.



b l o o d s patt e r

The base of each stain’s present position, the point in two-
dimensional space where the paths converge (c), and their
point of origin (o), defne another right angle.

Figure 21: A representation of the area of origin
established from 2-dimensional calculations.
Image used with permission from Tom Bevel & Ross Gardner, June 2006

Calculation Methods
The angle of impact and length of the convergence
line can be graphed for each stain and the area of
origin (of the blood) established OR it can be done
mathematically through the relationships that exist in
a right triangle OR it can be done using a protractor
and string.

Whichever method is used for the calculation, the
initial steps for all methods are the same:

• Identify stains that have a common
area of convergence.

• Draw lines through the central long axis of
the stain away from the direction of travel.

• Identify the area of convergence.

• Measure the distance (cm) from the back edge
of the stain to the area of convergence.

• Calculate the angle of impact of each of
the stains (measure width and length of
stains in mm and apply the formula).

• Use a minimum of 3 stains.

Once the angle of impact has been calculated and the
distance from each stain to the area of convergence
has been measured, either of 3 methods can be used.

Properties of blood

Defning the Area of Origin by Graphing
A graph is prepared that has the following features.

�. The X-axis represents the target plane and
graphs the distance from the back-edge
of the stain to the area of convergence.

2. The Z-axis represents the height above
the target plane – in this example
the target plane is the foor.

3. The scales of both axes, X and Z
are scaled the same (cm).

Do the following.

4. Mark on the X-axis of the graph the
convergence distance (cm) for each stain.

5. Using a protractor, draw a line from the
mark on the X-axis, at the calculated
angle of impact, to the Z-axis.

6. Repeat this procedure for each stain.

7. The area at which the lines from the X-axis
converge on the Z-axis establishes the probable
height of the area of origin. See below.

Figure 22: A graph showing the method for
estimating the area of convergence.


Defning the Area of Origin with
the Tangent Function.
The same steps as above are followed to determine

• Area of convergence (AOC)

• Distance from the stain to the AOC

• Angle of impact of the selected
stains – a minimum of 3 stains.

The following formula is used to determine the point
of origin.

TANi = H/D

i = angle of impact

D = distance from stain to area of convergence

H = unknown distance above target surface

Line bc = H – height above the target: unknown.

Line ca = D – distance to the area of convergence:

i = angle of impact: known.

TAN i = H/D
To solve for unknown H

H = TAN i * D



For example:

Distance to the AOC = D = 30cm

Angle of impact = i = 35o

H = TAN i * D
H = 0.7002 * 30cm

= 2�cm


b l o o d s patt e r
Properties of blood

Defning the Area of Origin by Triangulation.
The same steps as above are followed to determine

• Area of convergence (AOC)

• Distance from the stain to the AOC

• Angle of impact of the selected
stains – a minimum of 3 stains.


• Ring stand

• Protractor

• String

• Masking tape

• �m rule

• pencil


• Place the ring stand on the area of convergence

• Write the calculated angle of
impact next to each stain.

• Using string, masking tape and a protractor,
raise the string to the calculated angle
and attach it to the ring stand.

• Do the same for a minimum of 3 stains.

• The place on the ring stand where the string
from each stain meets is the ‘area of origin’.

• Measure the height of the area of origin.

NB: the value for the TAN of angle 35 can be found
from the Table of Trigonometric Function or by using a
scientifc c alculator.




b l o o d s patt e r
Properties of blood

The methods described above have limitations but are
able to give an investigator a good approximation of
the origin. This helps to identify the general location
where an application of force to a source of blood

Crime scene investigators now tend to use computer
software applications to analyse blood stain patterns
including area of origin calculations however many
investigators prefer to use traditional methods. Their
choice of method depends on a range of factors.

The information from area of origin calculations can
be used to verify or refute various claims made about
a crime scene. For example, if all of the blood spatter
evidence points to a certain height that equates to
a area low to the ground this would not back up
a suspect’s claim that it was self-defence from a
standing position.

The scenario in this program of work requires students
to analyse stains and calculate the area of origin of
bloodstains on one target surface which is a wall.
Students will then review 3 statements: suspect,
victim and witness and determine which statement
verifes the forensic evidence.

Bloodstains in other parts of the room (foor, walls,
stove and ceiling) are not measured in the activity
but the characteristics of the spatter can be used to
further support or refute a statement.


Bevel, T. & Gardner, R.M. �997 Bloodstain pattern analysis. CRC Press Ltd, LLC.

James S.H., Kish, P.E. & Sutton, T.P. 2005 Principles of bloodstain pattern analysis : theory and practice. Boca Raton,
CRC Press LLC.

Thanks to Mark Reynolds, UWA PhD student for verifc ation of information and supplying a number of images.


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