One of the newest tools on the market is the laser line scanner, a device that works exceptionally well from a towed or moving platform for underwater search and/or surveys (see figure). The Laser Line Scanner (LLS), in its simplest form, is a sensor that takes advantage of a laser to concentrate intense light over a small area in order to illuminate distant targets and extend underwater imagery beyond that offered by more conventional means. The LLS builds up an optical image from a rapidly acquired series of spots on the seafloor, each sequentially illuminated by a pencil sized diameter laser beam that scans the bottom perpendicular to the direction of the sensor support platform. This technique minimizes the effects of forward scattered and back-scattered light. The resultant data are displayed as a continuous "waterfall" image that can be recorded on a standard video cassette recorder. Distinct frames can also be captured and used to generate mosaic images of seafloor features. The optical sensor consists of subassemblies for the imbedded sensor control electronics, the laser, a scanner and a detector. These four subassemblies are integrated into a single physical unit and installed inside a watertight pressure housing.

The scanner subassembly is composed of two rotating, four-faceted mirrors, rigidly attached to a common rotating shaft. The illumination laser is oriented such that its output beam is incident on the smaller of the mirrors, deflecting the beam downward to the seafloor. The receiver views the seafloor reflected light, incident on the larger mirror such that it is actually tracking the output laser spot as it scans. The unscattered, unabsorbed light that manages to reach the bottom illuminates a small, localized area that is called the primary scan spot. As the mirrors rotate, the scan spot traces a continuous line across the bottom. When there is relative motion between the scanner and the target, perpendicular to the scan direction, then sequential scan lines will be displaced slightly and the target will be scanned in two dimensions. By synchronizing the scan rate with the forward velocity of the sensor, it is possible to control the spacing between the scan lines, ensuring that true image aspect ratios are preserved.

By sampling the output of the photo-multiplier tube within the receiver with mirror rotation, it is possible to build up a 2-dimensional reflectance map of the scanned area. When each new scan line is introduced at the top of the operator console display screen, it automatically displaces the last one at the bottom and a "waterfall" display is created. This gives the operator a realistic downward view of passing over the bottom in real time. A computer system executes the functional algorithms that control the process. Typical size for a LLS system is about 13.8 in (35 cm) diameter by 5.1 ft (1.6 m) long, with an in-air weight of 300 lb (136 kg). By using folded optics, it may be possible to shorten the length with a larger diameter to achieve better form factor for certain applications.

The niche where the LLS system seems to fit best is between sidescan sonar and video camera for search or survey operations. The good optical resolution offered by these systems makes them an ideal tool for applications such as limited area search, corridor surveys (e.g., pipelines and cable routes), and high resolution environmental surveys. As such, they have been historically transported on tow bodies such as the Science Applications International Corporation, San Diego, California system shown in the concept. Towed systems have inherent long line stability with high data rate tethers to a topside processing and display console.

Perhaps one of the best-known uses of the Laser Scanner was the search for wreckage of the ill-fated TWA Flight 800 off Long Island. The extent of the Boeing 747 debris field was enormous, with most of the pieces relatively small. Visibility was marginal and, although a great number of divers were used to recover such wreckage, their time on bottom under a no-decompression schedule was extremely limited. Sidescan sonar was of limited use, due to the soft bottom and large number of small aluminum pieces.

An example of the imaging ability is provided in the image of a seat section located during the TWA search (see photo).

As with most tools, the Laser Line Scanner has distinct limitations as well as specific advantages. For example, one drawback to LLS is that it typically requires motion in order to generate an image. This precludes stationary imagery. Efforts to "dither' the scan while the sensor is stationary have been experimented with, but such a procedure is not yet available for common use. Finally, the cost of LLS systems is substantial. Depending on a host of factors, a price tag of about $700,000 would not be uncommon. Day rates will vary, but a typical at-sea cost might be in the area of $2,500 to $3,000 per day, including two operators.

The LLS systems can be readily integrated with ROVs and, subject to the requirements for stable flight, used to good advantage to augment common video cameras. Maximum depths are currently limited to about 6,562 ft (2,000 m), primarily due to laser window structural limitations. Future trends include color imagery (Ocean News & Technology, June/July 1997) and improved resolution.