Ocean optics

Knowledge of ocean optics is needed in aquatic remote sensing research in order to understand what information can be extracted from the color of the water as it appears from satellite sensors in space.

Where waters are “optically deep,” the bottom does not reflect incoming sunlight, and the seafloor cannot be seen by humans or satellites.

Based on what color they appear to sensors, researchers can map habitat types, including macroalgae, corals, seagrass beds, and more.

[6] Apparent optical properties (AOPs) depend on what is in the water (IOPs) and what is going on with the incoming light from the Sun.

Because Rrs is a ratio, it is slightly less sensitive to what is going on with the light field (such as the angle of the sun or atmospheric haziness).

Models refer to anything that is not explicitly measured in the water, including satellite-derived variables that are estimated using empirical relationships (for example, satellite-derived chlorophyll-a concentration is estimated from the ratios between green and blue remote sensing reflectance using an empirical relationship).

Scientists study individual tiny objects such as plankton and detritus particles using flow cytometry and in situ camera systems.

Using satellite data, researchers estimate things like particle size, carbon, water quality, water clarity, and bottom type based on the color profile as seen by satellite; all of these estimations (aka models) must be validated by comparing them to optical measurements made in situ.

Oceanographers, physicists, and other scientists who have made major contributions to the field of ocean optics include (incomplete list): David Antoine, Marcel Babin, Paula Bontempi, Emmanuel Boss, Annick Bricaud, Kendall Carder, Ivona Cetinic, Edward Fry, Heidi Dierssen, David Doxaran, Gene Carl Feldman, Howard Gordon, Chuanmin Hu, Nils Gunnar Jerlov, George Kattawar, John Kirk, ZhongPing Lee, Hubert Loisel, Stephane Maritorena, Michael Mishchenko, Curtis Mobley, Bruce Monger, Andre Morel, Michael Morris, Norm Nelson, Mary Jane Perry, Rudolph Preisendorfer, Louis Prieur, Chandrasekhara Raman, Collin Roesler, Rüdiger Röttgers, David Siegel, Raymond Smith, Heidi Sosik, Dariusz Stramski, Michael Twardowski, Talbot Waterman, Jeremy Werdell, Ken Voss, Charles Yentsch, and Ronald Zaneveld.

While ocean optics is an interdisciplinary field of study applies to a wide range of topics, it is not often taught as a course in graduate programs for marine science and oceanography.

First, there is a summer lecture series operated by the International Ocean Colour Coordinating Group (IOCCG) which usually takes place in France.

[32] For independent learning, Curt Mobley, Collin Roesler, and Emmanuel Boss wrote the Ocean Optics Web Book as an open-access online guide.

The properties of particles, such as this single particle of detritus, determine how they absorb and scatter light.
MODIS-Aqua satellite image of the Black Sea captured on June 20, 2006. All the water that we can see at the scale of this image is optically deep, because the seafloor is not visible to the satellite sensor.
Many oceanographic buoys and weather buoys at sea are located in optically deep waters, like the one being recovered in this photo 60 nautical miles north of Oahu, Hawaii.
Where light reaches the bottom, the water is known as optically shallow, such as in this pool. The pattern of light on the bottom is caused by light refraction at the surface when ripples and small waves bend the water surface.
The water at many tropical beaches, such as this beach on the Kure Atoll , is optically shallow. Light reflects off white-colored sand, creating a turquoise color.
Sentinel-2 satellite image of the confluence of the Rio Negro and the Solimões River in Brazil. The dark colored water of the Rio Negro is rich in dissolved substances (high absorption ), while the brighter brown colored water of the Solimões River is rich in sediments (high scattering ). The properties of these two water types can be studied with methods central to the field of ocean optics.
Scattering involves how light is bounced into many directions by objects such as very small particles in the ocean. Measuring light scattering involved measuring light coming from different angles.
A scientist measures K d (PAR) from a boat in the Chesapeake Bay. This is a measure of downwelling light attenuation using a flat-topped light sensor (small brown metal cylinder at left), called a cosine collector, to measure the light coming down onto a flat surface from above.
Divers set up an equipment package including a sensor to measure PAR at the seafloor. This is a measure of scalar light using a round shaped light sensor (white lightbulb-like object at left), called a spherical quanta sensor, to measure light coming from all directions spherically.
A conductivity-temperature-depth rosette (CTD-rosette) sampler instrument package ready for deployment. PAR sensors are often attached to the top circular rung of the equipment package. Optical sensors like fluorometers and transmissometers are often attached to the bottom section of the equipment package, below the Niskin bottles , on the same level as the CTD sensor (light green cylinder just visible at the bottom of this image).
Ocean optics studies dissolved and particulate substances in the water, spanning a wide range of sizes. Many of these things are very small in size, from less than 0.1 nm to organisms at the centimeter scale. A single human hair is ~100 microns in width, for reference.
Particle size in the ocean follows a logarithmic pattern with concentration: there are exponentially more small particles than large particles. This plot shows concentration (number of particles per volume of water) on the y axis vs. particle size on the x axis. [ 25 ]
Researchers prepare an Imaging FlowCytoBot (IFCB) for water sampling.
Scientists stand next to an Imaging FlowCytoBot (IFCB).
Ocean color remote sensing involves learning about the ocean based on its color as viewed by satellite sensors. The light reaching the satellite sensor starts as incoming light from the Sun, then gets scattered and absorbed by Earth's atmosphere and surface, including water on the surface. Accurate ocean color measurements depend on accurate measurements of the optical properties of the water.
A researcher prepares a filtration rig aboard a research vessel. Some optical properties of water, like absorption by particles, are measured by filtering water and measuring the color signature of the material on the filter.
Visualization of satellite-derived global plant life, both oceanic (mg m −3 chlorophyll-a) and terrestrial ( normalized difference land vegetation index ), provided by the SeaWiFS Project, NASA Goddard Space Flight Center. The field of ocean optics includes methods that help researchers estimate ocean chlorophyll-a concentrations.
Schematic of processes that need to be measured to fully understand ocean productivity and carbon sequestration. Many of these topics involve optical measurements.