As sunlight enters the ocean, it interacts with the particulates and the dissolved materials within the water. When light interacts with particulates, the direction of propagation of the light can be changed through the scattering process, and part of the light may be absorbed by the particles and changed into other forms or wavelengths of energy. Similarly, dissolved materials may absorb light energy and convert it into other forms of energy.
When light is absorbed at one wavelength, and part of it is re-radiated in another, the process is called fluorescence. All these processes change the intensity of the light as a function of direction, the light field. Since these processes are a function of the wavelength of light, scattering and absorption change the spectrum of the light field. We thus see that particulates and dissolved materials have spectral scattering and absorption characteristics that change the spectral light field.
The scattering and absorption characteristics are called the Inherent Optical
Properties (IOP) of the natural water body. The light field can be thought
of as the intensity of light as a function of direction, depth and wavelength.
Properties that are directly derived from the light field are called the
Apparent Optical Properties (AOP). Instantaneously changing the light field
that enters the ocean, for example, by a cloud moving in front of the sun
immediately changes the AOP, but not the IOP.
If one knows all the IOPs, i.e. all the scattering, absorption, and fluorescence characteristics of the water itself and the particulates and dissolved materials within it, and if one knows the light field immediately above the ocean, one could then, in theory, predict the light field anywhere in the ocean. This is the field of radiative transfer.
Since particulates and dissolved materials have associated with them certain scattering, absorption, and fluorescence characteristics, it is natural to try and use the IOP to determine the nature of the particulates and the dissolved materials. This is IOP inversion.
WET Labs produces many instruments that measure IOPs, both for radiative transfer studies and for inversion. For example, the ac-9 can be used to measure the spectral absorption and scattering characteristics of particulates and dissolved materials. It's data is therefore widely used as an input to radiative transfer models. The measured absorption and scattering characteristics can also be used to estimate the concentration of particulate pigments, particle index of refraction, particle size distribution, and the concentration of dissolved organic matter, for example. Also see our table of biogeochemical properties and related IOPs. Read more about IOPs (pdf file).
NASA protocols
Under the SIMBIOS project NASA carefully described, amongst other
items, the protocols for the measurement of the IOP. In addition, the IOP
themselves are defined and described. View the pertinent sections of the NASA/SIMBIOS protocols for IOP.
Further reading: Article by E. Boss and J.R.V. Zaneveld: The effect of bottom substrate on the inherent optical properties: Evidence of bigeochemical processes
IOP Links
absorption • beam attenuation • scattering • volume scattering function • fluorescence • turbidity
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