Underwater Optical Imaging: Status and Prospects
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Underwater Optical Imaging: Status and Prospects Jules S. Jaffe, Kad D. Moore Scripps Institution of Oceanography. La Jolla, California USA John McLean Arete Associates • Tucson, Arizona USA Michael R Strand Naval Surface Warfare Center. Panama City, Florida USA Introduction As any backyard stargazer knows, one simply has vision, and photography". Subsequent to that, there to look up at the sky on a cloudless night to see light have been several books which have summarized a whose origin was quite a long time ago. Here, due to technical understanding of underwater imaging such as the fact that the mean scattering and absorption Merten's book entitled "In Water Photography" lengths are greater in size than the observable uni- (Mertens, 1970) and a monograph edited by Ferris verse, one can record light from stars whose origin Smith (Smith, 1984). An interesting conference which occurred around the time of the big bang. took place in 1970 resulted in the publication of several Unfortunately for oceanographers, the opacity of sea papers on optics of the sea, including the air water inter- water to light far exceeds these intergalactic limits, face and the in water transmission and imaging of making the job of collecting optical images in the ocean objects (Agard, 1973). In this decade, several books by a difficult task. Russian authors have appeared which treat these prob- Although recent advances in optical imaging tech- lems either in the context of underwater vision theory nology have permitted researchers working in this area (Dolin and Levin, 1991) or imaging through scattering to accomplish projects which could only be dreamed of media (Zege et al., 1991). Recent years have also seen in years past, it makes sense to have a humble attitude the development of many new types of imaging sys- and to realize that it is likely that the most sophisticat- tems. The desire to image underwater objects is a goal ed imaging systems in the sea are those of the animals, shared (among others) by pelagic and benthic ecolo- who depend upon their visual receptors in order to gists, geomorphologists and marine resources manage- find prey, to mate, and to escape harm. Nevertheless, ment personnel. the recent decade has witnessed a large increase in our abilities to image objects in the sea. This is due to the Propagation of Light in the Sea current revolution in electronics and sensing technolo- Certainly the basic physics of the propagation of gy coupled with advances in signal and image pro- light in the sea influences the overall performance of cessing. underwater optical imaging systems as, for example, In this article, we intend to provide a brief history the transparency of the intergalactic medium to light of underwater optical imaging and a brief summary of creates opportunities for astronomers to view distant its relationship to other fields of ocean optics. However, objects. In the sea, the inherent optical properties (IOPs) our major task is to inform the reader about advances in are the parameters that govern the propagation of light. underwater imaging that have occurred in the last So, for example, absorption and scattering must be decade. Without doubt, the bulk of our roots trace back taken into account in order to predict the performance to the work of Seibert Q. Duntley who was first at the of underwater imaging systems in various situations. Massachusetts Institute of Technology (MIT) and then For purposes of system modeling and simulation, accu- at the Scripps Institution of Oceanograph~ where the rate data are needed for attenuation, in order to esti- Visibility Laboratory of the University of California San mate imaging distances, forward scatter, which leads to Diego was established. Duntley's classic article, "Light image blur, and backward light scatter, which general- in the Sea" (Duntley, 1963), created a baseline of under- ly limits the contrast of underwater images by creating standing which drew upon 20 years of studying light a veiling glow. Fortunatel~ recent advances in optical propagation for many uses including "photosynthesis, instrumentation for measuring these parameters (see Oceanography • VoL 14 • No. 3/2001 64 article this issue by Maffione) promise to increase our systems can be broadly classified into two areas: pas- knowledge of these properties. sive and active. Passive systems utilize light in order to Since it is not always possible to perform measure- image objects that have been illuminated by some ments of the IOPs and, moreover, what is of interest in source other than that associated with the imaging sys- many oceanographic situations are the path averaged tem. Examples are imaging of objects using sunlight or quantities, various researchers have examined issues other sources of illumination such as bioluminescence. related to underwater imaging on a more empirical Passive imaging is especially attractive for covert oper- basis. Since the major source of blurring in optical ations such as fish seeking prey or the Navy inspecting images is due to forward scatter, the nature of the near- objects without being detected. Active systems take forward Volume Scattering Function (VSF) has been advantage of a user generated source of light. A simple the subject of investigations during the past quarter of example is an underwater camera system which uses a century. Some of the earliest work in this area was either strobe or continuous artificial illumination. accomplished by Mertens and Replogle (1977) who These types of systems offer substantial benefits for measured how light within the ocean propagated from underwater imaging in that the incident light can be a point source, expressed as the Point Spread Function collimated into very narrow beams, be monochromatic (PSF), compared with how a beam of light propagated (lasers), with the option of very short duration (strobe, across the same distance, expressed as a Beam Stread pulsed lasers). In this article, we will be mostly con- Function (BSF). More recently experimental measure- cerned with active systems. These systems typically ments of the PSF (McLean and Voss, 1991; Voss, 1991), allow imaging at greater ranges with higher contrast have provided validation of a small angle scattering than passive systems that use sunlight. Moreover, with theory and also experimental observations of PSFs short pulses of lasers, they can be operated even in with a parameterization. Additional Monte Carlo sim- moderate levels of ambient light. ulations (Jaffe, 1995) have confirmed that there is an A basic classification for underwater imaging sys- invertible relationship between the small angle scatter tems was formulated many years ago by Duntley and of the VSF and the PSF for optical depths less than 5 coworkers (1960s) and is summarized in Figure 1, total attenuation lengths. Unlike near-forward angles, adapted from Jaffe (1990). If only short ranges are instrumentation to routinely measure the details of the desired, a simple system with a controlled light beam VSF at larger angles has yet to be developed. A single and a good camera can yield excellent pictures as set of VSFs (Petzold, 1972) measured in San Diego Bay underwater photographers know. If longer ranges are and environs in the 1960s have formed the basis for desired, the separation of lights and cameras presents much examination and speculation about the imaging substantial advantages in that backscatter can be properties of water. The underwater imaging commu- reduced greatly. This approach was employed by the nity eagerly awaits the development of modern VSF team that imaged the sunken luxury liner Titanic by meters and their deployment in a variety of different using the two Russian MIR submarines. environments. Better performance than this requires a bit more From the point of view of the environmental physi- exotic technology. Two examples shown here are range cist, the ultimate property of underwater light field is gated imaging (where the backscatter is simply time- radiance (neglecting the polarization properties as gated out) and synchronous scan imaging, which takes embodied by the Stokes vector). Therefore, no optical advantage of the concurrent scanning of a highly colli- system built or envisioned needs to measure anything mated source and a receiver with a narrow field of more. However, since radiance is both a time and space view. Although there seems to be continuing debate in varying quantity, its measurement can be complex. the oceanographic community about which of these Sampling theorems related to both spatial and tempo- types of systems will yield better images (the authors ral measurements that would be necessary in order to here represent different views), it is certainly true that provide unaliased views of the radiant field lead to in order to obtain extended range imaging (better than some pessimistic conclusions. Since the spatial vari- three total attenuation lengths) from platforms which ability is ultimately related to the wavelength of light permit only limited camera light separation, some type and temporal variability its bandwidth, spatial sam- of sophistication is needed beyond simply rearranging pling may need to be quite high and the temporal cameras and lights. changes in the light field can be quite rapid. Nevertheless, the latest generation of underwater Current Underwater Imaging Technology imaging systems take advantage of advances in light Imaging for Biological Observations generation, sensing and data processing to obtain In this section we consider underwater imaging underwater images that were only dreamed about systems that have been used mostly for observing ani- years ago. mals. Moreover, the systems described here are more sophisticated than the kind of system pictured in the Principles of Underwater Imaging left side of Figure 1. A system that uses a sheet of light As in sonar systems, underwater optical imaging to illuminate its subject has substantial advantages for Oceanography • VoL 14 • No. 3/2001 65 Camera-light close Camera-light separated Range Gated Synchronous Scan Light rr~_~a Light Pulse I | Total No.