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Optical measurements in coastal waters

This article gives an introduction of the instruments available (and the application of these instruments) to measure optical properties in coastal waters. Attention is paid to optical measurements in general, regional characteristics and implications for remote sensing.

Optical measuments using satellite and airborne sensors is a powerful, operational tool for monitoring coastal zones. This technology can provide accurate, large-scale, synoptic environmental information essential for understanding and managing marine ecosystems. Optical sensor data allows the assessment of in-water properties, such as suspended matter or phytoplankton concentration, benthic substrate type, vegetation composition, and bathymetry. Optical sampling methods enable for example the continuous and contactless measurement of sediment concentrations, which is an important advantage compared to the mechanical sampling methods.

Figure 1: Downward irradiance spectra measured using a TRIOS Ramses sensor in a shelf sea (Irish Sea, 6th July 2005, 16:07 GMT, 54° 08.115 N, 4° 27.814 W )

Optical characteristics of a light-transmitting medium can be specified in terms of its inherent optical properties (IOPs). The main IOPs are:

• the absorption coefficient (a) (m^-1)
• the scattering coefficient (b) (m^-1)
• the attenuation coefficient (c), with c = a + b (m^-1)
• the scattering phase function (β) (sr^-1)