Published hydro-optical theory pertinent to this study is critically reviewed; the optical quality of the instrumentation used is assessed and a method is proposed for judging the angular response of a diffuse collector. Consideration is given to the possibility of self shading, by an Undulating Oceanographic Recorder (UOR), of upwelling irradiance measurements; this would adversely affect the calculation of reflectance, an important optical parameter in remote sensor calibration work. A selection of optical measurements made at estuarine, coastal and deep sea locations are analysed and empirical relationships between optical coefficients are presented. A sample set of data obtained by means of a UOR during a tow across the Arctic Front is considered in detail, and a simple analysis of the contributing components of the diffuse attenuation coefficient is carried out. Since underwater visibility is limited by the beam and diffuse attenuation coefficients, diver observations of targets of known optical quality should provide a simple means of estimating these. Two methods suggested in the literature are considered but rejected on both theoretical and practical grounds; new methods are proposed, tested, found satisfactory and recommended for diver use. Relationships are established between the turbidity and the penetration of light in an estuarine environment. The proportion of light reaching the bed of an estuary depends upon the depth and turbidity, and this may be represented by the diffuse optical depth of the bed, Jh. Theoretical considerations show that Jh(t ) varies at twice the tidal frequency and this is confirmed by field observations . Calculations indicate that the same phenomenon must occur in coastal waters where tidal ranges and tidal variations in turbidity are significant. Certain aspects of hydro-optical research that require further work are identified, and viable programmes of investigation are proposed.

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