There are many different factors to consider when analyzing blood stain patterns. The first thing that an investigator wants to determine is what kind of pattern is being presented. Blood stain patterns can be presented as: • Drip Stains/Patterns – Blood Dripping into Blood – Splashed (Spilled) Blood – Projected Blood (with a syringe) • Transfer Stains/Patterns • Blood Spatter – Castoff – Impact – Projected • Shadowing/ Ghosting • Swipes and Wipes • Expiratory Blood When an investigator is analyzing drip stains/ patterns, blood spatter, shadowing/ghosting, and expiratory blood there are different factors they have to look at, these factors include: – Whether the velocity of the spatter is low, medium or high A low velocity spatter is usually four to eight millimeters in size and is often a result of dripping blood after a victim sustains an injury such as a stab or in some cases a punch. For example, if a victim is stabbed and then walks around bleeding, the drops of blood that are left behind are low velocity. The low velocity drops in this example are passive spatters. Low velocity spatter can also result from pools of blood around the body and transfers. A medium velocity spatter is a result of a force anywhere from five to a hundred feet per second .This type of splatter can be caused by a blunt force such as a baseball bat or an intense beating. This type of spatter is usually no more than four millimeters. This type of spatter can also be a result of a stabbing. This is because arteries can be hit if they are close to the skin and blood can spurt from these wounds. This is classified as projected blood. A High velocity spatter is generally caused by a gunshot wound but can be from a wound from another type of weapon if enough force is used. Once the type of velocity is determined it’s important to determine the angle of impact. These two factors are important to find so that it’s possible for a point of origin to be determined. A general observation that can be made by investigators about the angle without any calculations being involved is that the sharper the angle, the longer the “tail” of the drop is. The angle of impact is determined by dividing the width by the length of the drop. Once the angle is determined investigators then take the arcsine (inverse sine function)of that number and then use stringing ( the use of strings to chart the trajectories of all the blood droplets in the air) to determine the point of origin ( where the stings converge). Back to Crime Library
Often found at the scenes of violent crimes, the analysis of bloodstains can provide vital clues as to the occurrence of events. Though bloodstain pattern analysis (BPA) can be a subjective area of study at times and often reliant on the experience of the investigator, the idea that blood will obey certain laws of physics enables the examination of blood at an incident scene and on items of evidence to offer at least an insight into what was likely to have occurred. The successful interpretation of bloodstain patterns may provide clues as to the nature of the offence, the possible sequence of events, any disturbance to the scene that may have occurred, and even the position of individuals and objects during the incident. It may prove beneficial in refuting or corroborating eyewitness accounts. Types Single Drop Impact Spatter Cast-Off Stain Transfer Bloodstains Projected Pattern/Arterial Damage Stain Pool Stains Insect Stains Expiration Stains Examination of Bloodstain Patterns Bloodstains at an incident scene may not always be visible to the naked eye, either due to low amounts of blood present or an individual cleaning in attempts to remove signs of bloodshed. Despite the use of cleaning reagents or even attempting to cover the stains with paint, detectable traces will generally remain, which can be visualised using various chemicals or specialised light. Although blood will not fluoresce under UV light like some bodily fluids, it will significantly darken, thus enhancing its visibility. Furthermore, certain chemical reagents can be used to visualise latent bloodstains. These tests, such as luminol and phenolphthalein, generally work by reacting with a constituent of blood to produce some kind of chemiluminescence. However it should always be remembered that these chemical reagent tests are often presumptive, meaning that they can only indicate that the stain is possibly blood. In reality, other substances may react with the reagent in the same way. A lack of a bloodstain can be just as revealing. The absence of blood in a continuous bloodstain is known as a void, and may suggest that something or someone was present in that area when the bloodstain was caused. This could indicate an object present at the time of the incident has been removed from the scene, or an individual (or even multiple individuals) were present in specific locations when blood was shed. It can easily be incorrectly assumed that blood found at an incident scene belongs to a victim, however it must be taken into account that some bloodstains may have resulted from the perpetrator being injured at some point. Either way, the information available from the presence of bloodstains is not limited to bloodstain pattern analysis, but also DNA analysis. See the DNA analysis page for more information. Point of Origin – Directionality and Angle of Impact Although it may be possible to estimate area of origin purely through visual observation of bloodstain patterns, in some instances trigonometry may be utilised to determine a more precise point of origin. Depending on the type of bloodstain pattern, it may be possible to establish the angle at which a blood droplet hit a target, referred to as the angle of impact. By measuring the ratio of the width of the bloodstain to the length, it can be possible to calculate the angle of impact. If the angle of impact of multiple bloodstains is established, it may be possible to determine the area of convergence (the point where lines of travel from multiple stains meet) through stringing techniques and establish the area of origin. Documentation and Collection Jackson, A. R. W, Jackson, J. M., 2011. Forensic Science. Essex: Pearson Education Limited. Scientific Working Group on Bloodstain Pattern Analysis. [online] Available at: [http://www.swgstain.org] White, P. C., 2004. Crime Scene to Court: The Essentials of Forensic Science. Cambridge: The Royal Society of Chemistry. |