Let us begin by reminding ourselves just what we mean by “the inherent optical properties” and “the apparent optical properties” of surface waters. The inherent optical properties are those that belong to the aquatic medium itself: properties that belong to a small sample of the aquatic medium taken out of the water body just as much as they belong to a great mass of the medium existing within the water body itself. The properties of particular concern to us are the absorption coefficient, a, the scattering coefficient, b, and the volume scattering function, β(θ). The absorption coefficient at a given wavelength is a measure of the intensity with which the medium absorbs light from a parallel beam per unit pathlength of medium (see Eq. 1.18). The scattering coefficient at a given wavelength is a measure of the intensity with which the medium scatters light from a parallel beam per unit pathlength of medium (see Eq. 1.17). Both a and b have the units, m-1. The normalized volume scattering function specifies the angular (θ) distribution of single-event scattering around the direction of a parallel incident beam. It is often normalized to total scattering and referred to as the scattering phase function, P(θ) (see Eq. 1.21). Since these properties belong, as I have already said, to a small sample of the medium, just as much as they do to a great slab of ocean, they can be measured in the laboratory. The absorption coefficients at various wavelengths can be measured with a suitable spectrophotometer: the scattering coefficient and the volume scattering function can be measured with a light scattering photometer. The apparent optical properties are not properties of the aquatic medium as such although they are closely dependent on the nature of the aquatic medium. In reality they are properties of the light field that, under the incident solar radiation stream, is established within the water body.