Having described the nature of the incident radiation flux presented to the aquatic biosphere and the influences to which the light is subject within the water, we shall now consider the kind of light field that results. We shall begin, in this chapter, by examining the ways in which the properties of this light field are studied. The physical definitions of these properties are given in Chapter 1.
The most frequently and easily measured property of the underwater light field is irradiance. Knowledge of this parameter is valuable, not only because it provides information about how much light is available for photosynthesis, but also because irradiance plays a central role in the theory of radiation transfer in water. An irradiance meter, since it is meant to measure the radiant flux per unit area, must respond equally to all photons that impinge upon its collector, regardless of the angle at which they do so. With any given meter this can be tested by observing the way in which its response to a parallel light stream varies with its angle to that light stream.
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