In any water body some of the light which penetrates the water is caused, by scattering within the water, to pass up through the surface again. Of this emergent flux, 90% originates within the depth (equal to l/Kd) in which downward irradiance falls to 37% (l/e) of the subsurface value. It can be regarded as a sample derived from the underwater light field, and so by studying it with appropriate detection instruments above the surface, information about that field and therefore about the optically significant components of the medium, can be obtained. There is not much point in having detection instruments just above the surface: they would be more useful below. If, however, this emerging radiant flux can be studied by remote sensing instruments, located a considerable distance above the surface, in an aeroplane or space satellite, then the considerable advantage is gained that information about the underwater environment over a large area can be obtained in a short time. This makes it possible to acquire a synoptic view of a large aquatic ecosystem, with a fraction of the time and effort that would be involved in carrying out measurements over the same area from a surface vessel.

It will readily be appreciated, however, that a price must be paid. Measurements of the emergent flux, from a great distance, cannot be as accurate, or yield as much information, as measurements carried out within the water itself. We shall now consider the kinds of measurements that can be made, the correction procedures that must be carried out, and the nature of the information that may be obtained.