Feature ArticleExtended Range Distributed Serial-Scan Laser Imaging In Turbid Coastal Conditions
By Dr. Fraser Dalgleish
One pressing need in the drive to better secure the coastal environment and associated natural and man-made assets is the ability to rapidly identify suspicious undersea objects. Unfortunately, murky harbor and coastal waters typically render this task nearly impossible, even with the most sophisticated underwater camera technologies. However, relatively simple system architectures that can extend operational ranges and rapidly transmit high-quality imagery and other information to remote locations can be realized if serially scanned laser illuminators and single-element detectors are operated in a distributed configuration, possibly among multiple undersea robotic platforms.
To better understand the performance of this distributed undersea imaging technique in natural waters, and to determine if precise alignment is required between laser illuminator and detector subsystems during such operations, the Ocean Visibility and Optics Laboratory at Florida Atlantic University’s Harbor Branch Oceanographic Institute tested a recently developed prototype distributed laser imager in a range of turbid estuarine and coastal conditions off the east coast of Florida.
The distributed laser serial imaging system concept, which attempts to reduce the capture of non-image resolution element photons by employing large separation distances between the source and receiver, was introduced in the early 1970s at Scripps Institution of Oceanography and was known as time-varying intensity (TVI).
This system concept also limits development of small-angle blur/glow/forward scatter by reducing the distance between the laser source and the target, but with the drawback that image swath and, hence, coverage rates are reduced.
In a test tank environment, the bistatic serial-scan laser imaging approach has been shown to produce quality images through a wide range of environmental conditions with low-cost, low-power laser transmitters. Furthermore, in an attempt to mitigate the reduced image swath, multistatic distributed architectures that use frequency division multiple access have been shown to allow multiple, nearby target regions to be imaged simultaneously. To produce photorealistic 3D imagery with this serial-scan imaging technique, a light-field rendering technique has also recently been applied.
Recent Field Results
The approach by Harbor Branch and program partner Metron Inc. (Reston, Virginia) has the objectives of developing a time-resolved radiative transfer model for accurately predicting physical layer channel frequency response and also an image performance prediction tool, both of which can be applied through a wide range of geometries and optical environments for bistatic and multistatic serial-scan laser imaging scenarios. The experimental validation of these new models involved developing a prototype serial-scan laser system apparatus that included interchangeable two-axis scanned continuous-wave (CW) and pulsed laser transmitters in separate 19-centimeter-diameter housings and a four-channel, high-speed receiver that is compatible with both the CW and pulsed modes of operation and is housed within a 32-centimeter-diameter glass sphere.
The receiver was configured such that there was an angle of 35 degrees between the center axis of the downward-staring photomultiplier tube (PMT) assembly and the center axis of the three outward-staring PMT assemblies, which are separated radially by 120 degrees. Each PMT assembly has a flat-top angular response over a total field of view of 12 degrees. Therefore, each photon collector is staring into a unique solid angle with no direct overlap between adjacent collector apertures. This was determined to be a useful way for generating at-sea data to demonstrate relaxation in required pointing accuracy of the receiver and to validate the newly developed simulation tools. To continue this article please click here.
Dr. Fraser Dalgleish is an associate research professor with Harbor Branch Oceanographic Institute at Florida Atlantic University. In 2006 he established the Ocean Visibility and Optics Laboratory, an engineering research lab specializing in new laser imaging, communications and lidar instrumentation for undersea applications.