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Bloodstain pattern analysis
|Subject: Bloodstain pattern analysis Mon Feb 27, 2012 1:06 am|| |
Bloodstain pattern analysis (BPA) is one of several specialties in the field of forensic science. The use of bloodstains as evidence is not new; however, the application of modern science has brought it to a higher level. New technologies, especially advances in DNA analysis, are available for detectives and criminologists to use in solving crimes and apprehending offenders.
The science of bloodstain pattern analysis applies scientific knowledge from other fields to solve practical problems. Bloodstain pattern analysis draws on the scientific disciplines of biology, chemistry, mathematics and physics. If an analyst follows a scientific process, this applied science can produce strong, solid evidence, making it an effective tool for investigators.
Results of BPA
Not every result of BPA will qualify as incontrovertible evidence, but the following are some things a bloodstain pattern analyst may be able to determine conclusively and state as fact:
A basic understanding of blood spatter analysis allows first responding officers and investigators alike to assist in correctly collecting and preserving bloodstain data at the scene. If they know what they have at the scene, then based on their department policy they should know what they need to do next. Bloodstain pattern analysis requires sufficient education and training to be an effective investigative technique, which not all law enforcement officers attending a crime scene will necessarily have.
- Movement and direction of persons or objects while they were shedding blood.
- Position of persons or objects during bloodshed.
- Movement of persons or objects after bloodshed.
- The mechanism or object used to create a specific pattern.
- The direction a stain was traveling when it was deposited.
- The area of origin of an impact pattern.
- The minimum number of impacts during an incident.
- The sequence of events.
Bloodstain analysts receive specialized training. The foundation course in bloodstain pattern analysis is the Basic Bloodstain Pattern Analysis Course. This is taught at many government and private institutions. The course criteria were developed by the International Association of Bloodstain Pattern Analysts (IABPA) with the following stated purpose:
Beyond this basic course are conferences, seminars, and courses such as the Maths and Physics for BPA and The Advance Bloodstain Pattern Analysis Course, both of which are provided by the Ontario Police College (OPC) and the Royal Canadian Mounted Police (RCMP). These two institutions also have Bloodstain Analyst Understudy programs. The International Association for Identification (IAI) provides its own certification in bloodstain pattern analysis.
A course of instruction designed for investigators, crime scene technicians, forensic technicians, and others involved in criminal and medical-legal investigations and crime scene analysis. The course is intended to develop a fundamental knowledge of the discipline of bloodstain pattern analysis. The course should illustrate to the student basic principals of bloodstain pattern analysis and the practical application of the discipline to actual casework. The course syllabus is not intended to create an "instant" expert.
In addition to formal study of the subject, practical experience and experimentation is paramount in the development of a skilled bloodstain pattern analyst.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:13 am|| |
Bloodstain pattern categories
There are several different thoughts on how to classify and define bloodstain patterns. The following is one accepted way of categorizing them based on the mechanism that created the stain. The three stain groups are: Passive, Projected, and Transfer/Contact. The definitions used below are from the suggested IABPA terminology list:
Passive bloodstains are those stains created by the force of gravity.
- Passive Drops - Bloodstain drop(s) created or formed by the force of gravity acting alone.
- Drip Pattern - A bloodstain pattern which results from blood dripping into blood.
- Flow Pattern - A change in the shape and direction of a bloodstain due to the influence of gravity or movement of the object.
- Pool Pattern - A bloodstain pattern formed when a source of blood is stationary for a period of time.
A projected stain occurs when some form of energy has been transferred to a blood source.
- Low Velocity Impact Spatter (LVIS) - A bloodstain pattern that is caused by a low velocity impact\force to a blood source.
- Medium Velocity Impact Spatter (MVIS) - A bloodstain pattern caused by a medium velocity impact\force to a blood source. A beating typically causes this type of spatter.
- High Velocity Impact Spatter (HVIS) - A bloodstain pattern caused by a high velocity impact/force to a blood source such as that produced by gunshot or high-speed machinery.
- Cast-Off Pattern - A bloodstain pattern created when blood is released or thrown from a blood-bearing object in motion.
- Arterial Spurting (OR Gushing) Pattern - Bloodstain pattern(s) resulting from blood exiting the body under pressure from a breached artery.
- Back Spatter - Blood directed back towards the source of energy or force that caused the spatter.
- Expiratory Blood - Blood that is blown out of the nose, mouth, or a wound as a result of air pressure and/or air flow which is the propelling force.
A transfer or contact stain is produced when an object with blood comes in contact with an object or surface that does not have blood. It may be possible to discern the object that left the blood impression.
As indicated above, there are other terms currently used in BPA and different ways of classifying bloodstain patterns. For example there is a debate over the misnomer of the LVIS, MVIS, and HVIS as it relates to the physical term "velocity". A sub-committee of the SWGSTAIN has been tasked with addressing the terminology issues and develop a taxonomy for bloodstain patterns.
- Wipe Pattern - A bloodstain pattern created when an object moves through an existing stain, removing and/or altering its appearance.
- Swipe Pattern - The transfer of blood from a moving source onto an unstained surface. Direction of travel may be determined by the feathered edge.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:16 am|| |
"Velocity" impact stains
Contrary to what the name states, the terms low-, medium-, and high-velocity impact spatter do not describe the velocity of the blood droplets as they fly through the air. The variation in the "velocity" is meant to describe the amount of energy transferred to a blood source in order to create the stains. Velocity is a speed (m/s) with a direction. Often the terms force and energy are quoted in conjunction with the unit ft/s or m/s which is incorrect. Force is related to velocity and mass (N or 1 kg ·m·s−2). Energy (work) is related to the force exerted on an object (J or N·m or kg·m2·s−2). As indicated above, there has been great debate over these terms and their definitions. Below is one method of differentiating low-, medium-, and high-velocity impact spatter. The term "impact" should not be confused with the mechanism by which the spatter was created. It actually refers to the blood impacting onto an object or area with some degree of force.
Low velocity impact spatter
Low velocity impact spatter (LVIS) is generally produced when objects traveling less than 1.5 m/s come in contact with a blood source. The preponderance of stains is generally larger than 8 mm in diameter.
Medium velocity impact spatter
Medium velocity impact spatter (MVIS) is generally produced when objects traveling between 1.5 m/s and 7.5 m/s come in contact with a blood source. The preponderance of stains is generally between 1.5 mm and 4 mm in diameter. Mechanisms that could produce this type of pattern include blunt force trauma or cutting/stabbing actions.
High velocity impact spatter
High velocity impact spatter (HVIS) is generally produced when objects traveling greater than 30 m/s come in contact with a blood source. The preponderance of stains is generally smaller than 1.5 mm in diameter. This pattern often has a mist-like appearance. High velocity patterns may be created by gunshots or explosives, but may also be caused by industrial machinery, coughing, or sneezing.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:21 am|| |
Blood is a tissue that is circulated within the body to assist other parts of the body. This connective tissue has specialized cells that allow it to carry out its complex functions. For a healthy person, approximately 8% of their total weight is blood. For a 70-kg (154-lb.) individual, this equates to 5.6 L (12 US pints).
Blood contains three components suspended within plasma. The three components are erythrocytes, leukocytes, and platelets.
- Erythrocytes - also known as red blood cells, are transporters. The role of erythrocytes is to transport oxygen. To do this it produces great quantities of hemoglobin, which gives it the distinct red color. Blood that has passed through the heart and been oxygenated (in the arteries) tends to have a brighter shade of red as opposed to blood that is returning to the heart (in the veins). There are about 30 trillion erythrocytes circulating in the human blood at any given time.
- Leukocytes - also known as white blood cells, are defenders. The role of leukocytes is to defend the body against harmful bacteria and microorganisms. There are five different types of leukocytes all having different sizes, shapes, structures, and functions. Leukocytes fight infection and disease. There are about 430 billion leukocytes circulating in the human blood at any given time (~1 per 700 erythrocytes).
Plasma is a yellowish fluid that carries the suspended erythrocytes, leukocytes, and platelets. It is composed of water (92%), proteins (7%), and other materials such as salts, waste, and hormones, among others. Plasma makes up about 55% of blood. The remaining 45% is blood cells and platelets. Because plasma is lighter than the blood cells and platelets, it can be easily separated. Plasma does not separate from blood cells in the body because it is in a constant state of agitation.
- Platelets - are pieces of larger cells that have broken off in the bone marrow. These bits of cytoplasm are enclosed by a membrane and do not have a nucleus. They play a major role in homeostasis (control of bleeding) by plugging up a breach in a vessel.
In physics there are two continuous physical states of matter, solid and fluid. Once blood has left the body it behaves as a fluid and all physical laws apply.
- Gravity - is acting on blood (without the body's influence) as soon as it exits the body. Given the right circumstances blood can act according to ballistic theory.
- Viscosity - is the amount of internal friction in the fluid. It describes the resistance of a liquid to flow.
- Surface tension - is the force that gives the ability to blood to maintain its shape. When two fluids are in contact with each other (blood and air) there are forces attracting all molecules to each other.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:28 am|| |
Blood spatter flight characteristics
Experiments with blood have shown that a drop of blood tends to form into a sphere in flight rather than the artistic teardrop shape. This is what one would expect of a fluid in freefall. The formation of the sphere is a result of surface tension that binds the molecules together.
This spherical shape of blood in flight is important for the calculation of the angle of impact (incidence) of blood spatter when it hits a surface. That angle will be used to determine the point from which the blood originated which is called the Point of Origin or more appropriately the Area of Origin.
A single spatter of blood is not enough to determine the Area of Origin at a crime scene. The determination of the angles of impact and placement of the Area of Origin should be based on the consideration of a number of stains and preferably stains from opposite sides of the pattern to create the means to triangulate.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:51 am|| |
Point and area of convergence
To determine the point/area of convergence an analyst has to determine the path the blood droplets travelled. The tangential flight path of individual droplets can be determined by using the angle of impact and the offset angle of the resulting bloodstain. “Stringing” stains is a method of visualizing this. For the purpose of the point of convergence, only the top view of the flight paths is required. Note that this is a two-dimensional (2D) and not a three-dimensional (3D) intersection.
- The point of convergence is the intersection of two bloodstain paths, where the stains come from opposite sides of the impact pattern. (see figure 3)
In the past, some analysts have drawn lines along the major axes of the stains and brought them to an area of convergence on the wall. Instead of using a top-down view, they used a front view. This provides a false point/area of convergence.
- The area of convergence is the box formed by the intersection of several stains from opposite sides of the impact pattern. (see figure 4)
Fig. 3 Point of convergence
Fig. 4 Area of convergence
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:52 am|| |
Area of origin
The area of origin is the area in three-dimensional space where the blood source was located at the time of the bloodletting incident. The area of origin includes the area of convergence with a third dimension in the z direction. Since the z-axis is perpendicular to the floor, the area of origin has three dimensions and is a volume.
The term point of origin has also been accepted to mean the same thing. However it has been argued, there are problems associated to this term. First, a blood source is not a point source. To produce a point source the mechanism would have to be fixed in three-dimensional space and have an aperture where only a single blood droplet is released at a time, with enough energy to create a pattern. This does not seem likely. Second, bodies are dynamic. Aside from the victim physically moving, skin is elastic and bones break. Once a force is applied to the body there will be an equal and opposite reaction to the force applied by the aggressor (Newton's third law of motion). Part of the force will move the blood source, even a millimetre, and change the origin while it is still producing blood. So the source becomes contained in a three-dimensional volume, or region.
As with the area of convergence, the area of origin is easily calculated by using BPA software. There are other longer, mathematical methods of determining the area or origin, one of which is the tangential method.
- Point (Area) of Origin - The common point (area) in three-dimensional space to which the trajectories of several blood drops can be retraced. (see figure 5)
Fig. 5 Area of origin (blue area representing a volume in 3D space)
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:53 am|| |
Main article: Forensic photographyCrime scene photography has some unique requirements. In the event there is a bloodletting scene, the basics are still required but special attention must be given to the bloodstains. The current means of documenting the scene include, 35 mm (B&W, colour, and specialty film), digital cameras, and video (Hi-8, DV, and other formats). Each method has its pros and cons. Often the scene is documented using multiple methods. (Videography has been included here because it follows the same principles and provides crime scene images.)
There are three types of crime scene photos:
- Overall – wide-angle images (28–35 mm range) that capture the scene as it is. This type of image provides anyone who has not been in the scene a good overall layout.
- Mid-range – images taken with a normal lens (45–55 mm range) give greater detail than the overall shots. In the case of a bloodletting scene, the mid-range image could capture a single bloodstain pattern.
Many times an analyst cannot attend a bloodletting scene. Therefore, the analyst may have to do all his work based on the crime scene images and notes of the person who attended. An appropriate sized scale should be in overall, mid-range, and close-up images. For overall images the scales should be parallel and perpendicular to the floor. This provides the analyst, and anyone else who looks at the images, a proper perspective on what they are observing. (Note: in some cases overall and mid-range images are taken with and without a scale.)
- Close-up – images taken with a macro lens giving the greatest amount of detail. For example, a medium velocity impact pattern can contain thousands of individual stains where there is a preponderance of small stains (1–3 mm in diameter) some of which require individual images.
|Subject: Re: Bloodstain pattern analysis Mon Feb 27, 2012 1:53 am|| |
In popular culture
- Serial killer Dexter Morgan of the Dexter novels and Showtime series is a blood spatter analyst for the ficticious Miami Metro Police Department.
- Catherine Willows is a blood spatter analyst on the CBS series CSI: Crime Scene Investigation.
|Subject: Re: Bloodstain pattern analysis || |