What is blood spatter? And how do drops of blood play a role in crime scene investigation?
Actually, it is called BloodStain Pattern Interpretation.It involves reconstructing the events that must have happened to produce the bleeding. It is not something that most law enforcement officials can do–it usually requires a specialist.
The first step when dealing with blood in any aspectof forensics/criminology is to know the characterization of bloodstains.To determine this, 3 questions must be answered.
Is it blood?
From what species did the blood come from?
If the blood is of human origin, how closely canit be associated to a particular individual?
To answer these questions, a variety of test can be used. While I could go into each test, it would take attention fromthe subject of this page–blood spatter interpretation. If you are interested in forensics, please refer to a book called: Criminalistics, Richard Saferstein.
Once it has been determined that the blood is human and whatnot, it’s pattern can be investigated. However, most law enforcement or police officers do not do this. But they should not discount the fact that the location, way it has been spattered, and the way it looks can be extremely useful in determining its origin. Since the procedure is so complex, a specialist is usually called in.
The specialist will try to determine what the position and shape indicate. He/she take measurements to determine the trajectory as well as execute carefully controlled experiments. These experiments will use surface materials like those found at the scene.
Herbert Leon MacDonell is the leading authority on blood stain interpretation. It was his observations that lead the way. In his published study, he gives the following tips to investigators:
It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion ofthe stain. In other words, the shape of the stain tends to change depending upon the angle of impact which caused the stain. For example, the more the angle decreases, the more the stain is less circular and more long.
Surface texture is one of the key components in determining spatter type. By this, MacDonell means that the harder the surface is, the less spatter will result. It is therefore extremely important to duplicate the surface in a controlled test.
When a droplet of blood hits a surface which ishard as well as smooth, the blood usually breaks apart upon impact. This in turn causes smaller droplets. The smaller droplets will continue to move in the same direction as the original droplet.
Now let’s take a look at some photos of these tips in action.
1. Determination of Angle of blood
The drop on the right fell 42 inches at an angle of 60-degrees on tohard smooth cardboard, while the drop on the left fell the same 42 inches and on to the same surface, but at a 10-degree angle.
Example based on: Flight Characteristics and Stain Patterns of Human Blood, Herbert L. MacDonell (Washington DC: US Government Printing office,1971), pp. 44,49.
2. Influences of Surface texture
The blood drop at the right struck a plastic wall tile falling againfrom 42 inches. On the left, the drop fell from the same distance on to a piece of heavy, unusual textured wallpaper.
Example based on: Flight Characteristics and Stain Patterns of Human Blood, Herbert L. MacDonell (Washington DC: US Government Printing office,1971), pp. 36,37
3. Impact on Hard Surface
This drop of blood was tested traveling 4 feet per second, on to ahard cardboard surface. It was traveling from right to left at an angleof 56-degrees. Although not clear in the example, MacDonell’s test showed the smaller drops aimed toward the large one.
Example based on: Flight Characteristics and Stain Patterns of HumanBlood, Herbert L. MacDonell (Washington DC: US Government Printing office,1971), pg. 39
To learn more about the scientific application of bloodstain pattern interpretation, read:
Flight Characteristics and Stain Patterns of Human Blood by Herbert L. MacDonell (Washington D.C.:US Government Printing Office,1971)
To read about cases Herbert L. MacDonell has solve dusing his techniques, you should read:
The Evidence Never Lies
The Casebook of a Modern Sherlock Holmes by Alfred Allan Lewis,Holt, Rinehart and Winston, 1984
* This is a GREAT book
To learn more about Forensic Science, please read:
Criminalistics An Introduction to Forensic Scienceby Richard Saferstein, Prentice Hall, 1990