Models of Particle Optical Properties
Marine particles are extremely complex and varied in their composition, pigmentation, shape, internal structure, and packaging. In order to understand their interaction with light some idealization have to be made. Since it is impractical to study each individual particle (there are more than a billion bacteria in one ml of seawater), particles are lumped into groups of particles having similar properties.
In order to model the optical properties of particles (that is to derive analytical or numerical descriptions of these properties) we need empirical data providing the necessary inputs (size, shape, index of refraction, internal structure and packaging). Some of these data can be obtained from microscopy while others need to be deduced from other measurements; for example to obtain information on the index of refraction of a sediment grain one could immerse it in oils of diﬀerent indexes and microscopically observe when the least optical contrast (scattering) is observed. Most often an inverse optical approach is observed; that is an optical model (such as Mie theory) is used to ﬁt empirical data and the values of the index of refraction that provide the best match with the observations are chosen to be those of the suspension. Aas (1996), Table 1, provides a compilation for data on the real part of index of refraction of marine particles and the diﬀerent methodologies used to obtain them. He also shows how the estimates for phytoplankton are consistent with a model of the index of refraction that is based on their composition.
The speciﬁc models for the diﬀerent types particles are provided in their own chapters. They vary in methodology used to obtain them; some are based strictly on observations, either in the lab or in the ﬁeld. Other are based on numerical calculations (e.g. using Mie theory) with observations based inputs. The latter are particularly useful when laboratory observations are lacking due to the complexity associated with the measurements (e.g. the volume scattering function). It is particularly satisfying when independent observations and models agree (referred to as optical closure) as it provide mutual validation for the approaches.