Three-dimensional (3D) scanners used at crime scenes for forensic investigations are not just the stuff of prime time television. Investigators and crime laboratories are using 3D laser scanning measurement systems to measure and model, in 3D simulations, the critical aspects of crime scenes.
A 3D laser scanning measurement system has a motorized swiveling head that sweeps a 632 nm wavelength laser beam over an entire room or outdoor scene, capturing up to a million measurements per second. A panoramic image is output with a 3D data visualization that reproduces the dimensions of the area. These data are then used to identify bullet trajectories, victim positioning and orientation, witness viewpoints, etc., with high accuracy and speed.
In the typical use of a 3D scanner, investigators will position a length artifact in the scene to be scanned along with everything else. The length of the artifact is a known value, and the measured value is compared with the known value in the field to test if the instrument is working correctly before critical measurements are performed.
"They take a complete 3D scan so that later they can go back and query this data to see, for example, how far this wall is from where this person is standing over here," describes Christopher Blackburn, a key member of the PML team involved in dimensional measurements. "There are points in space, and each point is an X, Y, Z coordinate. So you can query the distance from any combination of different points. You have the whole thing captured."
Without established length standards to assess the scanner performance, the investigators may not be able to defend the method as having an acceptably high level of certainty and the results might not be admissible in court.
The 2009 National Academy of Sciences (NAS) report "Strengthening Forensic Science in the United States: A Path Forward" questioned the reliability of some forensic sciences including the use of this 3D scanning technique. Furthermore, pressure began building in the forensics community to have crime laboratories and/or stand-alone crime scene units in the United States adhere to specific standards in their services, which require traceability to the SI.
This challenge was brought to PML thanks to the efforts of Robert Thompson from NIST's Law Enforcement Standards Office. Aware of the research in the Dimensional Metrology Group of PML's Semiconductor and Dimensional Metrology Division, Thompson arranged for the Group to get in touch with Leica Geosystems, a major manufacturer of 3D laser scanning instruments.
A new measurement system was developed by PML's Blackburn, along with Vincent D. Lee, Craig Shakarji, and Daniel Sawyer, in close collaboration with Tony Grissim of Leica Geosystems. This system calibrates the company's length artifact, a twin-target pole. To complete the calibration, the scientists attached a spherically mounted retroreflector to the top of the pole and aligned the top target of the pole at a specific point under a fixed microscope. They measured the distance to the retroreflector using a laser tracker, which uses laser interferometry to measure distance. The pole was then moved carefully to align the bottom target of the pole under the fixed microscope. The laser tracker was engaged again, recording the distance between the top and bottom targets of the pole with an uncertainly of ≈120 µm.
"This artifact can now be put in the crime scene so the investigators can get a length reference to test the instrument before measurements are performed," explains Blackburn. "This helps establish traceability back to the SI for the evidence they'll be presenting in court."
These NIST-calibrated artifacts, which help provide traceability, have been a tremendous boost for Leica Geosystems, which has increased sales and their workforce:
"From a job creation standpoint, the Leica Public Safety Group has grown from one to five employees in the past few years," explains Tony Grissim of Leica Geosystems. "Sales of our ScanStation laser scanners are at an all-time high and part of the reason is due to the successful collaboration with NIST to create the tools needed for quality control as required by ISO 17025. The economic benefit also trickles down to Leica's U.S.-based suppliers of the accessories which make up our total solution."
More recently, Leica Geosystems has approached PML for help in establishing measurement services for an entirely different type of artifact for a new laser scanning system they have developed to support the forensics and law enforcement community. Research under this new collaboration is expected to be underway later this summer.
The service that PML is providing has been so well received that even individual law enforcement agencies are sending their length artifacts directly to NIST for calibration—ensuring that the instruments are performing as specified before critical crime scene measurements are performed in the field.
"It's a good feeling to know that the measurements we provide are increasing the reliability of forensic evidence, creating jobs and helping U.S. industry grow and meet customer needs," summarizes Daniel Sawyer, the team's Project Leader. "What more could you ask for?"