According to NIST, the method may be useful in diverse fields including precision machining and assembly, plus forensics.
NIST’s 3D mapping system is said to combine a form of laser detection and ranging (LADAR), which is sensitive enough to detect weak reflected light, with the ranging accuracy made possible by frequency combs. The frequency comb, a tool for precisely measuring different frequencies of light, is used to continuously calibrate the laser in the imaging system.
Operating with laser power of nine milliwatts, NIST’s 3D mapping system scans a target object point by point across a grid, measuring the distance to each point. The system uses the distance data to make a 3D image of about one million pixels in under 8.5 minutes at the current scanning rate. Distances to points on a rough surface that reflects light in many directions can be determined to within 10 micrometres in half a millisecond, with an accuracy that is traceable to a frequency standard.
The system is also claimed to have wide dynamic range, enabling precise 3D mapping of targets with varied surface types and reflective properties. NIST researchers demonstrated the range by scanning footprints in soil, vegetation such as cactus (imaging individual spines), and complex mechanical devices such as a piston for a motorcycle.
The new NIST method offers a unique set of capabilities compared to conventional 3D mapping techniques. The NIST system is similar to optical coherence tomography, for example, but can operate much farther away from the target and is inherently accurate because of the frequency comb. The NIST system does not need a reference artefact to be placed next to the target, something typically required for interferometry-based systems.
LADAR typically measures distance based on the round-trip flight time of laser light, which reflects off the target and is detected by a sensor. In the NIST LADAR system, the laser sweeps continuously across a band of frequencies.
The initial laser output is combined with the reflected light and the resulting ‘beat’ signals are converted to voltage and analysed by digital signal processing to generate time delay data, which is used to calculate the distance.
This basic technique is well established but by including a frequency comb to continuously calibrate the swept laser, the NIST system can operate much more rapidly, yielding one measurement point every half a millisecond and simultaneously maintain sub-micrometre accuracy traceable to a frequency standard. Finally, the system uses real-time, fast processing digital electronics to produce fully calibrated, 3D megapixel images.
In use, NIST’s 3D mapping system could be used to make virtual casts of forensic evidence such as footprints in dirt. Conventional plaster casts that record impression evidence normally require a lot of effort to make and are difficult to compare to each other or to shoes. Furthermore, conventional analysis can destroy the evidence. By contrast, a remotely created 3D image of a footprint can non-destructively reveal more details than a photograph, such as exact measurements of shoe tread. The tread may show individual wear marks from a bicycle pedal, for example, a type of detail that could link a specific shoe to a crime scene.