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As 3D scanners become more common for capturing and preserving evidence, police, forensic technicians, attorneys, and others involved in the legal industry need to be aware of the applications, benefits, and limitations of how this technology is applied in recording data and subsequently how it can be used to aid in the analysis of evidence and crime scenes. The following article provides an introduction into the ever growing application of 3D scanners in the forensics industry.
The start of any forensic investigation must begin with the collection, review, and analysis of evidence. As a general rule of thumb, the better the quality of evidence, the better the analysis and likelihood of solving the crime. This is why there is a regular stream of new products, equipment, and software to aid the forensic technician in the collection, organization, and analysis of evidence. Many products or technologies are specialized in the collection of evidence while others' sole purpose is to keep evidence organized. However, even fewer technologies can collect, organize, and provide the analysis tools all in one package. This is the main reason why 3D scanning for forensics is an ever growing and useful application of laser based measurement technologies in fighting crimes and reconstructing events.
Often referred to as High Definition Surveying (HDS), 3D scanning became popular in the late 1990s for surveying buildings, terrain, and other architectural features in a very rapid and detailed manner. However, it wasn't long before many in the industry realized that the quick capture of vast amounts of point data was invaluable over the more traditional total stations. 3D Scanners can obtain tens of thousands of point measurements per second, while most total stations capture one measurement every few seconds (at best). Therefore, the time savings and possibilities available with a 3D scanner would be practically impossible with more traditional methods. The end result is that accident and crime scenes can be released in a fraction of the time with more measurements than were ever possible before.
The term "3D Scanners" makes up a broad range of devices and technologies all aimed at taking a large quantity of measurements of an object's surface or environment. The choice and application of 3D scanning often depends on the size of the object (or environment) being scanned along with an understanding of what will be done with the data once it is captured. These two factors often drive the type of technology that can be used to obtain measurements and in some cases, what type of software will be used to edit the data.
Some scanning technologies are based on optical methods whereby photographs are used to collect and match points in corresponding photographs (i.e. Photogrammetry and stereo matching) while the lesser known CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) allow the interior structures of objects to be "scanned" and examined. The more common 3D Lidar (Light Detection and Ranging) scanners emit a beam of light and measure the part of the beam that is reflected back to the instrument. These are the most common types of scanners—often featured on "CSI" type shows as being able to collect and preserve data from very large crime scenes.
The two most common types of Lidar scanners are “phase-based” and “pulse-based” scanners, which refer to the method for determining the distance to any surface that has been scanned. Phase based scanners use a method where a continuous transmitted laser beam sent out by the unit strikes the object being measured and a small part of the signal is reflected back along the identical path to the instrument. Once detected by the receiver, the weakened reflected signal is then compared with the signal of the original transmitted beam (or reference signal). The difference in phase between the two signals is then measured and the differences are a direct derivative of the distance the laser light has travelled.
Pulse based scanners work on the principle that the speed of light is a constant number. Therefore, a small pulse of light can be sent out to the surface of the object being measured. As the light gets reflected back to the scanner, the total time for the pulse is measured. The distance to the object's surface is 1/2 the total distance traveled by the laser pulse. Both pulse based and phase based Lidar scanners make up the majority of the most common types of scanners used to document crime scenes, roadways and areas spanning hundreds of meters.
There are many ways to classify 3D Scanners, but it is often useful to think about scanning in terms of the size of the object being scanned or the practical scanning range of the instrument:
1. Microscanning: Objects up to a few centimeters in size. This is especially practical for fingerprints, tool marks, bullet casings, and other small sized evidence that would otherwise be difficult to measure by other techniques. These methods require "scaled down" scanners or specially designed microscopes that capture a number of images that when combined, can be processed to provide 3D surface data.
3D Surface of a hair follicle created by a digital 3D Microscope. Image courtesy of Keyence Inc.
Microscanning of fingerprints can provide even greater details for comparison and matching purposes. *Image courtesy of Gunter Weber.
2. Close Range Scanning: Objects in the range up to 3 meters. Tire prints, footprints, shoe tread, and bones can be scanned in place (i.e. as they are found without having to be handled) with a portable 3D scanner. However, in many cases, the evidence is removed and subsequently scanned in a lab environment for further comparative analysis.
3D close range scan of a tire tread done with a structured laser light scanner. (Right side shows a close up of the resultant digitized mesh). The tread pattern can be accurately measured and then compared to a print found at a crime scene.
Although most people are familiar with CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) scanners for use in a medical environment, they are also used as accurate close range scanners for Medical Examiners.
A prime example of forensic work being done with CT/MRI scanners is at the Institute of Forensic Medicine at the University of Bern, Switzerland, where doctors have pioneered a "virtual digital autopsy" through the combination of technologies such as CT, MRI, and photogrammetry. The procedure is now called Virtopsy and is a non-invasive technique of gathering valuable information about internal bleeding, bullet paths, hidden fractures, and other aspects of the body without the use of scalpels.
This can be an especially useful method of investigating suspicious death cases where bodies have been found either in a mummified state or after a long time of decay.
A 3D digital model created from CT scans. The surgical repairs done to a fractured hip (left side of image) quickly become apparent when shown in 3D.
3. Mid Range Scanning: Distances approximately in the range of 30-120 meters. This is perhaps the most commonly advertised type of scanning (often seen on "CSI" type shows). These types of scanners are well suited for indoor crime scenes and for reasonably sized outdoor scenes. Larger areas require multiple setups and scans. The resulting data can be automatically "stitched" by use of reference targets included in each scan or by overlapping portions of each scan that can be recognized as matching areas by the software. "Registration" is the term often used for combining and accurately matching several independent or overlapping scans into one larger point cloud.
Sample crime scene scanned with the Faro Photon 120 Lidar Scanner. Image courtesy of Faro.
4. Long Range Scanning: Evidence in the range of up to 1,500 meters. In many cases, a crime or accident occurs over distances greater than a few meters. Therefore, these types of scanners can be called upon to document airplane and traffic accidents or even natural disasters. An example is Optech's Ilris 3D scanner that can achieve a scanning range up to 1,500 meters. The obvious advantage is that there are fewer setups required while still being able to capture a large amount of data in a short period of time.
5. Extra Long Range and Kinetic Scanners: There are also extra long range scanners (that can scan several kilometers at once) and even kinetic scanners that allow scanning of objects while mounted on a moving vehicle. These are less frequently used in forensics and are geared more toward terrestrial surveying. However, their use could be called upon for pre-operative and post-analysis of military campaigns along with the documentation of large scale natural disasters such as earthquakes or hurricanes.
There is a wide variety of options depending on the type of evidence you wish to capture with a 3D scanner. Depending on the size of the object, the type of data you wish to gather and the type of analysis you wish to perform you may choose a very different product.
Look for more on the benefits and limitations of 3D scanners in Part 2 of this article.
Eugene Liscio, P. Eng, is the owner of AI2-3D. A company that specializes in 3D Forensic Measurement and Visualizations. He is a forensic animator and photogrammetry specialist. AI2-3D, Toronto, Ontario, 416-704-2695, firstname.lastname@example.org, www.ai2-3d.com.