By Catten Ely
Interpreting blood spatter (not splatter) is an art unto itself. The laws of physics concerning liquids apply; therefore a basic understanding of physics is useful. Math is also handy.
Here’s how it works: The surface texture blood falls on influences the amount of spatter. Smooth surfaces reveal fewer spatters because there is less surface area to deflect the liquid. Fabric absorbs blood, hard plastic does not, and many outdoor surfaces are water-resistant, leaving no blood evidence.
A drop’s velocity, direction, and distance from the point of origin can be determined by its size and shape.
As a general rule, the higher the energy of the impact, the smaller the drops are. Free-falling blood has a low velocity, leaving large drops. Impact spatters, found in cases of beating or stabbing, are medium-velocity. Additional blows typically knock off blood collecting on clothing, skin, or hair, causing low- and medium-velocity spatters to mark the attacker with backspatter. High-velocity impact patterns appear as a mistlike spray of droplets with a few larger ones mixed in. These are the result of gunshots or explosions.
Impact angle is the angle formed between a blood drop’s path and the surface into which it landed. Divide the width of the bloodstain by its length to get the sine of the drop’s impact angle.
Round drops with evenly spaced scalloping at the edges indicate that the drop landed on a surface at 90 degrees. As the angle of impact moves away from the 90-degree point, a drop elongates and one side will show larger scallops or tapering tails. The longer scallops or tails point in the direction the drop was going when it hit the surface.
Once the impact angle has been determined, the point of origin above or away from a flat surface can be found using a spatter program. The investigator enters the impact angles and distance from the floor or wall of several spatters. Accuracy increases with more data, but it is not necessary to input every drop found. The program will display a three-dimensional representation of the common point of origin, showing the investigator where the victim was at the time the attack took place.
Another method of determining the source of spatter is using strings to plot each trajectory from its point of impact. This method is too complicated to describe here but is very impressive in the courtroom.
Pretty cool, huh?
Note: Blood spatter is part of the forensic science discipline of bloodstain pattern analysis/interpretation. Other areas include transfer patterns and any other blood stains that occur at a crime scene.
By Catten Ely