Bloodstain analysis requires that a relatively large number of blood droplets be analyzed in order to determine a point of convergence (POC), the location where the blood came from. When these measurements have been made, it is then possible to determine the two-dimensional POC by drawing a line through the long axis of each blood droplet, and extending the line until all the lines converge. The point where all of the lines cross is the POC.
The next part of the puzzle is to determine the angle of impact (AOI) of the droplets with the surface. This requires careful and accurate measurement of many blood spots, determining their length and width using a metric ruler or caliper. This measurement of the stain on the wall can be used to mathematically determine the angle that the blood droplet impacted the wall. These days, there are several computer programs readily available to make these mathematical (trigonometry--sine function) determinations for the technician. Placing the AOI pattern over the POC pattern allows a three-dimensional POC to be determined. This is the position of the victim in relationship to the wall or ceiling when the bloodstain pattern was produced.
Bloodstain analysis equipment
In the past, bloodstain analysis was done using colored string to represent the path of blood droplets as they left the weapon. These would be taped to the wall and floor, creating a three-dimensional pattern representing the blood as released from the attack. Some departments also use colored plastic trajectory rods placed in a similar manner. These methods are time-consuming and labor-intensive. The introduction of laser equipment capable of being placed on a bloodstain and projecting a laser bean in the direction of the source, or placed on a POC and projected to specific bloodstains on a wall, has increased the speed and accuracy of determining bloodstain patterns. The angle of impact for a large number of blood droplets can be determined rapidly.
Bloodstain pattern analysis provides an important piece of the evidence puzzle at a crime scene. The goal of this analysis is to link a specific pattern of bloodstains with a specific source event (attacked victim). In cases where multiple blunt trauma blows are delivered by one or more assailants and the scene is very complex, it is not always possible for this link to be made accurately.