Underwater Optical Imaging: Status and Prospects

Introduction As any backyard stargazer knows, one simply has to look up at the sky on a cloudless night to see light whose origin was quite a long time ago. Here, due to the fact that the mean scattering and absorption lengths are greater in size than the observable universe, one can record light from stars whose origin occurred around the time of the big bang. Unfortunately for oceanographers, the opacity of sea water to light far exceeds these intergalactic limits, making the job of collecting optical images in the ocean a difficult task. Although recent advances in optical imaging technology have permitted researchers working in this area to accomplish projects which could only be dreamed of in years past, it makes sense to have a humble attitude and to realize that it is likely that the most sophisticated imaging systems in the sea are those of the animals, who depend upon their visual receptors in order to find prey, to mate, and to escape harm. Nevertheless, the recent decade has witnessed a large increase in our abilities to image objects in the sea. This is due to the current revolution in electronics and sensing technology coupled with advances in signal and image processing. In this article, we intend to provide a brief history of underwater optical imaging and a brief summary of its relationship to other fields of ocean optics. However, our major task is to inform the reader about advances in underwater imaging that have occurred in the last decade. Without doubt, the bulk of our roots trace back to the work of Seibert Q. Duntley who was first at the Massachusetts Institute of Technology (MIT) and then at the Scripps Institution of Oceanograph~ where the Visibility Laboratory of the University of California San Diego was established. Duntley's classic article, "Light in the Sea" (Duntley, 1963), created a baseline of understanding which drew upon 20 years of studying light propagation for many uses including "photosynthesis, vision, and photography". Subsequent to that, there have been several books which have summarized a technical understanding of underwater imaging such as Merten's book entitled "In Water Photography" (Mertens, 1970) and a monograph edited by Ferris Smith (Smith, 1984). An interesting conference which took place in 1970 resulted in the publication of several papers on optics of the sea, including the air water interface and the in water transmission and imaging of objects (Agard, 1973). In this decade, several books by Russian authors have appeared which treat these problems either in the context of underwater vision theory (Dolin and Levin, 1991) or imaging through scattering media (Zege et al., 1991). Recent years have also seen the development of many new types of imaging systems. The desire to image underwater objects is a goal shared (among others) by pelagic and benthic ecologists, geomorphologists and marine resources management personnel.