Blood Spatter Analysis
Bloodstain pattern analysis (BPA) can be defined as the analysis and interpretation of the
dispersion, shape characteristics, volume, pattern, number, and relationship of bloodstains
at a crime scene to reconstruct a process of events (Houck and Siegel 252). Not just
anyone can be involved in bloodstain pattern analysis, in fact, there are several
requirements required in order for an individual to be considered certified.
Blood stains can be grouped into three main classes: passive, transfer, and projected or
impact stains. Passive bloodstains include clots, drops, flows, and pooling. Transfer
bloodstains include wipes, swipes, pattern transfers, and general contact bloodstains.
Projected or impact bloodstains include spatters, splashes, cast-off stains, and arterial
spurts or gushes. Thus, since our focus is blood splatter analysis, our investigations will
focus primarily on projected or impact bloodstains.
Spatter is a term in bloodstain pattern analysis that describes a stain that results from
blood hitting a target. Two types of spatter are recognized. A forward spatter results
when blood droplets are projected away from the item creating the impact, such as a
hammer. A back spatter is caused by droplets being projected toward the item. In
general, back spatter will be lighter and the stains smaller than forward spatter.
Blood, like other liquids, is governed by the laws of physics as it falls. It will accelerate
to the earth as other objects as a rate of 9.8 m\s\s and the force of air resistance will slow
its fall. If the droplets fall at a 90 degree angle to a relatively non-porous surface the
resulting pattern will be an almost perfect circle. (e.g. Try dropping red paint from an
eyedropper directly above a white piece of paper).
Blood droplet with an angle of impact at 90 Degrees
Width of droplet = 2.5 cm
Length of droplet= 2.5 cm = 1
What angle has the sine of 1? 90 degrees
As the angle of impact decreases a distortion of the droplet results. The droplet travels as
a projectile and the bottom of the droplet will hit the floor first while top will continue
along its path and result in a spatter elongation and sometimes a tail. The tail of the
droplet corresponds to the direction the blood travels.
Sin x = width of stain\ length of stain
Example
Width of droplet = 2.5 cm
Length of droplet = 4.5 cm
2.5 cm \ 4.5 cm = .55
arcsin of .55 = 33 degrees
Why do we study blood spatter analysis? Often physical evidence is more reliable than
eyewitness accounts. The use of mathematics and physics are the best and most unbiased
witnesses to the sequence and origin of blood spatter patterns at a scene. The use of these
tools may help reconstruct a physical conflict and thereby support of impeach a
suspect\witness statement in a pending case. It may even point to valuable information
about an injury someone sustained at the scene.
Blood Spatter Evidence Activity
Case History—Stephen Scher
A man banged on the door of a cabin in the woods outside Montrose, Pennsylvania. His
friend, Marty Dillon, ha just shot himself while chasing after a porcupine. The two had
been skeet shooting at Scher’s cabin, enjoying a friendly sporting weekend, when Dillon
spotted a porcupine and took off out of sight. Dillon’s friend, named Stephen Scher,
heard a single shot and waited to hear his friend’s voice. After a few moments, he chased
after Dillon and found him lying on the ground near a tree stump, bleeding from a wound
on his chest. Scher administered CPR after locating his dying friend, but he was unable
to save Dillon who later died form his injuries. Police found that Dillon’s untied boot
had been the cause of his shotgun wound. They determined he had tripped while running
with his loaded gun and shot himself. The grief-stricken Scher aroused no suspicion, so
the shooting was ruled an accident.
Shortly thereafter, Scher moved from the area, divorced his wife, and married
Dillon’s widow. This was too suspicious to be ignored.
The crime scene was reconstructed to show that Scher’s boots bore the unmistakable
spray of high-velocity impact blood spatter, which is evidence that Scher was standing
within an arm’s length of Dillon when he was shot. This pattern of blood stains cannot
be created while administering CPR, as Scher claimed. This also clearly refutes his claim
that he did not witness the incident. In addition, the tree stump near the body bore the
same type of blood spatter, in pattern that indicated Dillon was seated on the stump and
not running when he was shot. Finally, Dillon’s ears were free of the high-velocity blood
spatter that covered his face, but blood was on his hearing protectors found nearby. This
is a clear indication that he was wearing his hearing protectors when he was shot and they
were removed before investigators arrived. This and other evidence resulted in Scher’s
conviction for the murder of his long-time friend, Marty Dillon.
Blood Pattern Analysis
Supplies
Simulated blood
Disposable plastic pipettes
Protractor
Measuring tape
Smooth cardboard
Blotter paper
Role of white paper, 36” wide, cut into 6-foot lengths
Digital or film camera
Procedure
Part 1 Cast-Off Spatter
1. In a designated area, place a 6-foot by 3-foot piece of paper on the floor.
2. Stand on the side of the paper, and, using a disposable pipette filled with
simulated blood, walk along swinging your arum by your side in a natural motion.
3. “Blood” should stream from the pipette as you walk, landing on the paper. Do
not flick or fling the “blood,” as this will not produce usable results!! This is
meant to simulate the “cast-off” spatter created when on walks carrying a bloody
item.
4. Record the approximate height of the pipette from the floor (this will vary with
the heath and arm length of the individual).
5. Allow your paper to dry.
6. Photograph the individual spots in detail. Be sure to include scale and an arros
indicating the drop’s orientation to the direction of travel (i.e. which way was the
person walking).
Part II Impact Angle
1. Prop a smooth piece of cardboard (or other rigid surface) on a stack of books until
the angle formed with the floor is 20 degrees.
2. Dispense a single drop of simulated blood from the height of 24 inches. Avoid air
bubbles in your dropper as they will deform the drop before it lands. If you need
to try again, make sure your next drop is at least 3 inches away from the first!
3. Carefully remove the paper (do not disturb the shape of your drop!) to your bench
and photograph it with scale.
4. Repeat procedure at 48 inches.
5. Change incident angle to 65 degrees and repeat above at 24 and 48 inches.
6. Change incident angle to 90 degrees and repeat above at 24 and 48 inches.
7. Repeat above procedure on blotter paper. This illustrates the effect that substrates
of various textures have on the shape of a blood drop.
Part III Calculations
Stain shape vs. Impact Angle. Elongated stains have a distorted or disrupted edge that
easily describes the direction of travel of blood drop. The location or origin of bloodshed
may be established by determining the directionality of the stain and the angle that blood
impacted with the landing surface. The angle of impact is readily determined by a stain’s
length to width ratio and by applying the formula:
Sin A = Width of bloodstain \ Length of bloodstain
Where A = the angle of impact
Example The width of a stain is 11 mm and the length is 22 mm.
Then, Sin A = 11mm \ 22mm = .50
A scientific calculator having the trigonometric function will calculate that a sine of 0.50
is equal to a 30 degree angle.
Note: There is a 5-degree error factor with this formula. This means that your
calculations are good to plus or minus 5 degrees of the actual value of the angle of
impact.
Stain Shape vs. Impact Angle
Measure the stain length and width in millimeters of the nine bloodstains shown on page
77. Use the previously described formula to calculate the angle of impact for each
bloodstain. Record your findings in the following table.
Stain
Number
1
2
3
4
5
6
7
8
9
Width
Length
Sine
Estimated
Impact Angle
Part IV Examining Your Work From Part II
1. Develop a numbering system to catalog your “blood spatter evidence” and assign
a unique identifier to each “blood drop” and\or each photograph. Refer to your
evidence in your report using its unique identifier.
2. Using the formula described in the previous section, calculate the angle of impact
of the blood spatter patterns you created in part II.
3. Do you calculations match the angles you used in your experiment?
4. Show your work in the space below.