Def Sci J. Vol 34, No 1, January 1984, pp 1- 18 Inherent Optical Properties of Natural Seawater SHUBHA SATHYENDRANATH National Institute of Oceanography, Dona Paula, Goa-403004. Received 16 January 1984 Abstract. The inherent optical properties of pure seawater and of optically active substances generally present in seawater are briefly reviewed. Inter - relationship between inherent properties and some of the important apparent properties are also presented, so that apparent properties can be calculated quite simply, if the inherent properties are known, or vice versa. I. Introduction Optical devices are being designed increasingly for underwater applications such as ranging, sounding and communications. Passive and active remote sensing techni- ques, which make use of the visible part of the electromagnetic spectrum, have been developed recently, for studying oceanic parameters such as phytoplankton concentration, suspended sediment load, and water clarity, as well as for monitor- ing pollution from hydrocarbons. Energy transfer from sun to sea, and primary productivity of the oceans are dependent on the behaviour of light in the sea. Underwater photography is now being increasingly used, for studying the depths of the oceans. In all such studies the parameters of common interest are the optical properties of the sea. Studies on the optical properties of the sea are complicated because of their high variability with time and space, and difficulties in developing suitable instruments for direct measurements of these properties. This paper reviews briefly the present knowledge on the inherent optical properties of pure seawater and on optically active substances that are generally present in seawater. 2. Terminology, and Symbols When treating a complex science, it is desirable to work with a concrete founda- tion of notations, units, symbols and definitions for the parameters that are to be