Using an enhanced LADAR (Laser Detection And Ranging) system, researchers at the National Institute of Standards and Technology (NIST) have created a long-range, laser-based imaging device that can generate high-definition 3D maps of objects at distances of up to 10.5 m (35 ft). The technology could find applications in precision machining and assembly, as well as in forensics where it could map evidence non-destructively.
In normal operation, LADAR (usually called LIDAR), measures distance by measuring the minute time differences between the transmission and reception of single-band laser signals that are beamed at an object. These signals are then digitally processed and analyzed to produce time delay data that is used to calculate the distance and produce high-resolution maps.
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The NIST LADAR device, on the other hand, produces a range of wavelengths by utilizing what is known as a frequency comb (an "optical ruler" if you will) as a method of measuring signals to reveal the difference between the frequency measured and the comb frequencies produced.
In other words, the NIST system receives the initial reflected laser light from the object scanned and then combines that with the light from the comb frequency light produced by a secondary laser. This produces a third signal that is a combination of the two that, in general terms, helps the system accurately define with great precision the position of each part of the object scanned.
In using this type of technology, the NIST system is claimed to operate much faster than ordinary scanning systems, producing two measurement points every millisecond whilst ensuring sub-micrometer accuracy due to the precise nature of the frequency comb in defining the spatial coordinates.
In operation, the system scans an object using these measurement points across a grid and then calculates the distance between each point. Using this distance data, the system then generates a 3D image of approximately 1 million pixels in around 8.5 minutes, with distances on aspects of textured surfaces reflecting light in numerous directions accurately measured to within 10 micrometers.
The diversity, range, and accuracy of the system was demonstrated by producing a number of highly-detailed 3D scans, including footprints in soil and of individual leaves in vegetation, and in the capture of complex mechanical devices where – most impressively – even the stamped identification number on a motorcycle piston scanned from a distance was rendered fully legible.
One of the mooted applications for this 3D mapping technology is in the collection of forensic evidence, such as making virtual casts of compression footprints in soil. Orthodox casts in plaster take time and effort to make, are problematic to compare to one another and – without the utmost care – can destroy evidence. A 3D image of a footprint produced at a distance in a non-destructive manner, however, not only preserves evidence, but with the micro-millimeter accuracy of the NIST system, can reveal far more detail than a conventional photograph.
Unlike 3D scanning systems used in the likes of targeting systems in military drones, the NIST system uses laser power of just 9 milliwatts, but is still sensitive enough to detect weak reflected light and interpret this into images at a distance. And, with the ranging accuracy made possible by frequency combs, the subsequent detail of the NIST system is far more granular than that achievable with conventional LADAR 3D imaging devices.
Claiming a unique set of capabilities when compared with conventional 3D mapping technologies, the developers of the NIST system draw parallels with optical coherence tomography (like that used in medical imaging equipment) in the level of detail captured, citing its inherent accuracy through the use of frequency comb technology. They also point out that their system captures images at a much greater distances than coherence tomography does.
According to the NIST team, several manufacturers have expressed interest in the device. Presently around desktop size, the team claims that there is potential to develop a much smaller, chip-scale instrument.
The research was funded by NIST and DARPA, with the results published in the journal Optics Express.
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