If you follow the yellow brick road of evidence left by prints, impressions, and markings, it can lead you to the answer you’ve been looking for all along. These three are the most common types of evidence found at crimes scenes, and both crime scene investigators (CSI) and forensic laboratory technicians must know how to properly document, collect, and analyze them.
This type of physical evidence is more generally referred to as impression evidence or impression and patterns evidence. Patterns are what appears within the impression, and they are used to establish a match between the impression and the actual object or person.
With new technologies that have allowed us to examine DNA evidence, impressions have become the underdog of valued evidence. It is true that DNA is considered to be more reliable, but impression evidence still holds weight in a criminal case either by itself or as supplemental evidence to DNA.
Types of Impression Evidence
Edmond Locard, a pioneer in the field of forensic science, once said, “It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.” Basically no matter how careful a criminal thinks they are, Locard theorized that they will leave something physical behind. Some impression evidence must then be examined by a specialist. Take a look at this list of impression evidence, and see if you can figure out which ones would require a specialist to analyze.
- Tire Treads
- Foot/Shoe Prints
- Bite Marks
- Bullet Markings
- Tool Markings
While all of these types of evidence require a certain degree of education to be properly assessed, typically bite marks would be sent to a forensic odontologist and bullet markings would be analyzed by a ballistics expert. Tool marks at a crime scene may also require a specialist. This would depend on where the mark was found (on a body vs. on a wall) and what the tool is presumed to have left the mark (bullet vs. knife).
Photographing the Evidence
Before you can do anything with the evidence, you must first secure the crime scene and take pictures. Depending on the size of the investigation team, they may have a designated forensic photographer, but it is not entirely uncommon for CSI to take over this responsibility. Since impression evidence is fragile, it is especially important to correctly document them through pictures. Check out these tips that will help you capture evidence the right way every time.
- The right equipment makes it easier for you to capture the evidence in the best possible way.
- A DSLR (digital single-lens reflex) camera will take photographs with better clarity than a basic digital camera.
- Have an adjustable tripod and a variety of lenses with you to obtain different perspectives.
- Filters, electronic flashes, light meters, and gray cards all ensure that your pictures look consistent throughout a crime scene.
- Always have a measuring device and place it next to the evidence as a means for size comparison.
- Bring a cover or shield for your camera to protect it from the weather while you are taking pictures.
- The right angles should provide an exact representation of the evidence.
- Close-ups are used to capture the tiniest of details on the evidence, such as the pattern on a two-dimensional impression.
- Mid-range pictures are great for illustrating distance between two pieces of evidence like a gash on a wall and blood splatter.
- Long-range or overview shots are necessary to capture as much of the crime scene as possible, which allows you to see all of the evidence in its proper context.
It would be advantageous for you to log the details of and number every photograph that is taken. Doing so allows you to organize your collection of evidence, and makes it easier for you and anyone else to analyze them when needed.
Preserving the Evidence
Fingerprints, tire treads, and foot/shoe prints tend to be the most delicate pieces of evidence. They could easily be destroyed by weather, or once they are collected, are not handled extremely carefully. The following are the two primary techniques used preserve impression evidence:
- Latent impression recovery is the process used to remove two-dimensional prints from a solid surface.
- Prints can be discovered using a special light, often referred to as oblique lighting.
- Relies on the electrostatic charge left behind from when the two surfaces made contact.
- The charge attracts the powder, revealing the print, and is then removed with tape or a lifting device.
- You can recover fingerprints, shoe prints, and tire tracks with this method.
- It is used when the print is found on a hard surface, like a linoleum floor.
- Casting is used to retrieve a 3-D image of the impression found in soil or mud.
- You can use this to retrieve shoe prints and tire treads.
- The CSI will pour dental stone into the impression to create the cast.
- Dental stone is used at a dentist’s office, and is a common choice because of its precision and durability.
- Casting is used to assist the forensic technician in recreating the crime scene.
- It is also needed in order to preserve the evidence for legal purposes.
Collecting this evidence to solve the mystery may not be as easy as clicking your heels to go home, but the fun lies in the challenge. There are no dull moments in the lives of CSI and forensic science technicians. Feel free to check out the rest of our site to learn more about what it takes to work in this exciting field.