CCTV and the Law
Scanning for evidence
By Elliott Goldstein
Forensic investigators now have a new tool to use to document crime, accident, and fire scenes — the 3D laser scanner.
By Elliott Goldstein
FARO, a manufacturer of such devices, states on its website (www.faro.com), “3D laser scanning is the process of capturing millions of data points of a real-world environment (a 3D point cloud), allowing you to view that environment virtually. These point clouds can be used to produce accurate, realistic, 3D computer graphical models for use in a variety of applications including investigations of crime, accident and fire scenes.”
Laser Scanners use various types of technology to measure objects in a scene.
“Time-of-flight” scanners emit pulses of laser light, which bounce off an object. The scanner measures the time for the reflected laser light to return to the distance sensor.
“By timing the round-trip flight,” says FARO, “the distance between the surface of an object and the scanner can be calculated.” Often used for long-range scans, they can capture as many as 50,000 points per second.
“Phase shift” laser scanners, which are faster and provide higher resolution, emit an infrared laser that is reflected back to the system. “The distance between the object and the scanner is then calculated by analyzing the phase shifts in the wavelength of the return beam compared to the emitted light,” says the company.
Traditionally, investigators at a crime, accident or fire scene must decide what to photograph, videotape, measure and collect and then do so. They don’t have much time and are often working in cramped, sometimes dangerous areas. These investigators use tape measures, measuring wheels and cameras to capture images, collect and record measurements, and document scenes and objects within them.
The advantage of laser scanners is the sheer volume of data that can be captured — an estimated 10 million points in only five minutes. As such, explains FARO, information can be retained and documented by investigators objectively thanks to the accuracy obtained by the laser technology and later analyzed, using purpose-built software, “long after the crime scene is gone.”
One of the many benefits of laser scanning is the ability to use its data to generate photorealistic 3D models of the scanned scene. These models can then be used in courtroom presentations to interactively and accurately bring the trier of fact (judge or jury) to the scene. For example, Crown Attorneys could create a view of the crime scene to show the paths of people allegedly involved in the crime, providing a more compelling argument over photographs or videos. By generating models of the scene using scan data, the scene can be revisited and re-examined at a later date for additional information.
Laser scanners can be used both indoors and out, thereby permitting the accurate capture of the detailed dimensions of rooms and environments.
In addition to providing room visualizations, laser scanners can be used to determine bullet trajectories, analyse blood stain patterns, and demonstrate what was visible from a particular viewpoint.
The Forensic Identification Services (FIS) at the Toronto Police Service currently uses a FARO laser scanner for scene documentation and analysis. FIS can now create a “fly-through rendering” in order to present a reconstruction of a crime scene. More information is available at the Toronto Police website, including a demonstration video (www.torontopolice.on.ca/forensics)
Laser scanning is an exciting new tool in the arsenal of scene of crime officers!
Elliott Goldstein, B.A., J.D. is a Woodbridge, Ont.-based lawyer (firstname.lastname@example.org).