Practical Crime Scene Processing and Investigation - Chapter 11

Why are close-up detail and close-up photographs so important in documenting bloodstain patterns?

Close-up detail is a critical factor in bloodstain pattern photography. Photographs taken of millimeter-sized stains from several feet away are unlikely to allow for any type of analysis and are, for the most part, a waste of effort. Investigators must ensure that good close-up photographs are included in any bloodstain photography.

What is meant by a complex pattern?

Complex patterns are often encountered, patterns that exhibit characteristics of more than one of the six mechanisms. Even in the instance of a complex pattern, when the basic pattern types are discernable within the complex pattern, the bloodstain pattern analyst can still eliminate certain actions as a source for the pattern.

Describe the basic steps used to conduct a bloodstain pattern analysis.

a. Become familiar with the entire scene b. Identify the discrete patterns among the various bloodstained surfaces c. Classify these patterns based on their physical characteristics using some form of established taxonomy d. Evaluate aspects of directionality and motion in the stain or pattern e. Evaluate angles of impact, points of convergence, and areas of origin f. Evaluate interrelationships among stains, patterns, and other evidence g. Evaluate viable source events to explain the pattern h. Validate the analysis through some form of peer review

What are the three key elements considered when viewing and evaluating a luminol reaction?

a. Color b. Duration c. Intensity

What is the general rule regarding the relationship of a bloodstain's shape and impact angle?

The shape of certain bloodstains provides indicators as to the direction of deposition as well as to the spatial origin of the blood. The collapse of a free flight droplet on a surface produces a stain with a circular or elliptical shape. The ratio between the length of the long and short axes of the resulting stain has an empirical relationship to the angle at which the droplet struck the target.

In your own words, describe the concept behind the pattern diversity principle.

The variations in combinations of blood volumes and forces acting on those volumes lead to recognizable classes of patterns. When a blood mass comes under external force, recurring patterns are produced. Different mechanisms product different patterns with different class characteristics.

What is meant by stain directionality, and how is it determined?

An examination of the individual spatter stains within any pattern will likely establish a direction the droplets were traveling at the moment they struck a surface. This is referred to as directionality, the directional angle or the gamma angle of the stain. This determination is based on the collapse of the fluid droplet, the resulting long axis of the stain, and the creation of scallops, tails, and satellite stains, these scallops and satellites will appear on the side opposite the initial contact point of the stain. In the more spherically shaped stains, the scallops and tails will appear in a heavier concentration on the side opposite the initial contact point.

How are point of convergence and area of origin used to evaluate an impact spatter pattern?

If the analyst can establish both the directional angle and the impact angle for well-formed stain and do so for a number of such stains associated to the same impact event, the vectors defined by these angles will usually converge in three-dimensional space. The area were the stain vectors converge provides indications of where the corresponding droplets originated their flight. Information derived from directionality, point of convergence, and an area of origin analysis can assist the crime scene technician in understanding the direction and origin of the event that created the impact pattern and can effectively place individuals in the scene at discrete moments in time during the crime.

How is infrared photography helpful in documenting bloodstain patterns on some surfaces?

Infrared photography represents a significant tool for the crime scene processor, particularly when evaluating patent bloodstains on dark or multicolored clothing. With the majority of the visible light eliminated, the fabric appears white, while the blood, with its high IR absorbance, appears black.

What are the two primary categories in the bloodstain pattern taxonomy, and what distinguishes one from the other?

a. Spatter stains b. Nonspatter stains c. Spatter occurs when a blood mass is broken up into small droplets and put into free flight by some mechanism. When these droplets strike a surface, they produce circular or elliptical-shaped stains. In spatter patterns the primary stains are made up of circular-elliptical-shaped stains. The nonspatter category is made up of stains in which the primary stain is not a circular or elliptical shape, includes pattern transfers, flows, pools, saturations, smears, gush patterns, and blood into blood.