ocean optics

Where waters are “optically deep,” the bottom does not reflect incoming sunlight, and the seafloor cannot be seen by humans or satellites.{{cite journal |last1=Armstrong |first1=R. A. |last2=Singh |first2=H. |last3=Rivero |first3=S. |last4=Gilbes |first4=F. |title=Monitoring coral reefs in optically-deep waters |journal=Proceedings of the 11th International Coral Reef Symposium, Ft. Lauderdale, Florida. Session Number 17 |date=2008 |citeseerx=10.1.1.545.7429 }} The vast majority of the world's oceans by area are optically deep. Optically deep water can still be relatively shallow water in terms of total physical depth, as long as the water is very turbid, such as in estuaries.

Where waters are “optically shallow,” the bottom reflects light and often can be seen by humans and satellites.{{cite journal |last1=McKinna |first1=Lachlan I. W. |last2=Werdell |first2=P. Jeremy |title=An approach for identifying optically shallow pixels when processing ocean-color imagery |journal=Optics Express |date=2019 |volume=26 |issue=22 |pages=A915–A928 |pmid=30469992| doi=10.1364/OE.26.00A915|pmc=6506854 }} Here, ocean optics can also be used to study what is under the water. Based on what color they appear to sensors, researchers can map habitat types, including macroalgae, corals, seagrass beds, and more. Mapping shallow-water environments requires knowledge of ocean optics because the color of the water must be accounted for when looking at the color of the seabed environment below.

File:Negro-Amazon confluence and Manaus (Brazil) from space (cropped).jpg), 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.]]

Inherent optical properties (IOPs) depend on what is in the water. These properties stay the same no matter what the incoming light is doing (daytime or nighttime, low sun angle or high sun angle).{{cite web |last1=Mobley |first1=Curtis |title=Inherent Optical Properties. Ocean Optics Web Book. |url=https://www.oceanopticsbook.info/view/inherent-and-apparent-optical-properties/inherent-optical-properties |access-date=23 August 2021 |date=19 May 2021}}

Water with large amounts of dissolved substances, such as lakes with large amounts of colored dissolved organic matter (CDOM), experience high light absorption. Phytoplankton and other particles also absorb light.{{cite web |last1=Roesler |first1=Collin |title=Absorption Overview. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/absorption/absorption-overview |access-date=24 August 2021 |date=6 April 2021}}

File: Haze-Reflection-2.png

Areas with sea ice, estuaries with large amounts of suspended sediments, and lakes with large amounts of glacial flour are examples of water bodies with high light scattering. All particles scatter light to some extent, including plankton, minerals, and detritus. Particle size effects how much scattering happens at different colors; for example, very small particles scatter light exponentially more in the blue colors than other colors, which is why the ocean and the sky are generally blue (called Rayleigh scattering). Without scattering, light would not “go” anywhere (outside of a direct beam from the sun or other source) and we would not be able to see the world around us.{{cite web |last1=Mobley |first1=Curtis |last2=Boss |first2=Emmanuel |title=Scattering Overview. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/scattering/scattering-overview |access-date=24 August 2021 |date=18 May 2021}}

Attenuation in water, also called beam attenuation or the beam attenuation coefficient, is the sum of all absorption and scattering. Attenuation of a light beam in one specific direction can be measured with an instrument called a transmissometer.{{cite web |last1=Mobley |first1=Curtis |title=Measuring IOPs. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/inherent-and-apparent-optical-properties/measuring-iops |access-date=24 August 2021 |date=19 May 2021}}

Apparent optical properties (AOPs) depend on what is in the water (IOPs) and what is going on with the incoming light from the Sun. AOPs depend most strongly on IOPs and only depend somewhat on incoming light aka the “light field.” Characteristics of the light field that can affect AOP measurements include the angle at which light hits the water surface (high in the sky vs. low in the sky, and from which compass direction) and the weather and sky conditions (clouds, atmospheric haze, fog, or sea state aka roughness of the surface of the water).{{cite web |last1=Mobley |first1=Curtis |title=Apparent Optical Properties. Ocean Optics Web Book. |url=https://www.oceanopticsbook.info/view/inherent-and-apparent-optical-properties/apparent-optical-properties |access-date=24 August 2021 |date=27 October 2020}}

Remote sensing reflectance (Rrs) is a measure of light radiating out from beneath the ocean surface at all colors, normalized by incoming sunlight at all colors. 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).{{cite web |title=Remote Sensing Reflectance (Rrs) |url=https://oceancolor.gsfc.nasa.gov/atbd/rrs/ |website=NASA Ocean Color |access-date=24 August 2021}}

Rrs is measured using two paired spectroradiometers that simultaneously measure light coming in from the sky and light coming up from the water below at many wavelengths. Since it is a measurement of a light-to-light ratio, the energy units cancel out, and Rrs has the units of per steradian (sr-1) due to the angular nature of the measurement (upwelling light is measured at a specific angle, and incoming light is measured on a flat plane from a half-hemispherical area above the water surface).{{cite web |last1=Mobley |first1=Curtis |title=Reflectances. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/inherent-and-apparent-optical-properties/reflectances |access-date=24 August 2021 |date=28 October 2020}}

K_d is the diffuse (or downwelling) coefficient of light attenuation (K_d), also called simply light attenuation, the vertical extinction coefficient, or the extinction coefficient.{{cite web |title=Light |url=https://www.waterontheweb.org/under/lakeecology/04_light.html |website=Understanding: Lake Ecology Primer |publisher=Water on the Web |access-date=24 August 2021 |date=31 January 2011}} K_d describes the rate of decrease of light with depth in water, in units of per meter (m⁻¹). The “d” stands for downwelling light, which is light coming from above the sensor in a half-hemispherical shape (aka half of a basketball). Scientists sometimes use K_d to describe the decrease in the total visible light available for plants in terms of photosynthetically active radiation (PAR) – called “K_d(PAR).” In other cases, Kd can describe the decrease in light with depth over a spectrum of colors or wavelengths, usually written as “K_d(λ).” At one color (one wavelength) Kd can describe the decrease in light with depth of one color, such as the decrease in blue light at the wavelength 490 nm, written as “K_d(490).”

In general, K_d is calculated using Beer's Law and a series of light measurements collected from just under the water surface down through the water at many depths.{{cite web |last1=Schulz |first1=K. L. |title=Light in Lakes |url=https://www.esf.edu/efb/schulz/Limnology/Light.html |website=Limnology Lecture |access-date=24 August 2021}}{{cite book |last1=Kirk |first1=John T. O. |title=Light and Photosynthesis in Aquatic Ecosystems |date=1994 |publisher=Cambridge University Press |isbn=0521459664 |edition=3rd}}

{{See also|Closure (atmospheric science)}}

“Closure” refers to how optical oceanographers measure the consistency of models and measurements. 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). Closure includes measurement closure, model closure, model-data closure, and scale closure. Where model-data closure experiments show misalignment between data and models, the cause of the misalignment may be due to measurement error, issues with the model, both, or some other external factor.{{cite web |last1=McCormick |first1=Norman |last2=Mobley |first2=Curtis |title=Brief Definitions. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/references/brief-definitions |access-date=24 August 2021 |date=22 March 2021}}{{cite journal |last1=Chang |first1=Grace C. |last2=Dickey |first2=Tommy D. |last3=Mobley |first3=Curtis D. |last4=Boss |first4=Emmanuel |last5=Pegau |first5=W. Scott |title=Toward closure of upwelling radiance in coastal waters |journal=Applied Optics |date=2003 |volume=42 |issue=9 |pages=1574–1582 |doi=10.1364/AO.42.001574|pmid=12665088 |bibcode=2003ApOpt..42.1574C |url=https://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1151&context=sms_facpub |url-access=subscription }}

Ocean optics has been applied to study topics like primary production, phytoplankton, zooplankton,{{cite journal |last1=Behrenfeld |first1=Michael J. |display-authors=etal |title=Global satellite-observed daily vertical migrations of ocean animals |journal=Nature |date=2019 |volume=576 |issue=7786 |pages=257–261 |doi=10.1038/s41586-019-1796-9|pmid=31776517 |s2cid=208329129 |url=https://archimer.ifremer.fr/doc/00593/70556/ }}{{cite journal |last1=Picheral |first1=Marc |last2=Guidi |first2=Lionel |last3=Stemmann |first3=Lars |last4=Karl |first4=David M. |last5=Iddaoud |first5=Ghizlaine |last6=Gorsky |first6=Gabriel |title=The Underwater Vision Profiler 5: An advanced instrument for high spatial resolution studies of particle size spectra and zooplankton |journal=Limnology and Oceanography: Methods |date=2010 |volume=8 |issue=9 |pages=462–473 |doi=10.4319/lom.2010.8.462|s2cid=13671970 |url=https://archimer.ifremer.fr/doc/00133/24413/ |doi-access=free |bibcode=2010LimOM...8..462P }} shallow-water habitats like seagrass beds and coral reefs,{{cite book |last1=Zimmerman |first1=R. C. |last2=Dekker |first2=A. G. |title=Aquatic Optics: Basic Concepts for Understanding How Light Affects Seagrasses and Makes them Measurable from Space. In: Seagrasses: Biology, Ecology and Conservation |date=2007 |publisher=Springer |location=Dordrecht |doi=10.1007/978-1-4020-2983-7_12 |url=https://doi.org/10.1007/978-1-4020-2983-7_12}}{{cite journal |last1=Hochberg |first1=E. J. |last2=Atkinson |first2=M. J. |title=Spectral discrimination of coral reef benthic communities |journal=Coral Reefs |date=2000 |volume=19 |issue=2 |pages=164–171 |doi=10.1007/s003380000087|s2cid=34304562 }} marine biogeochemistry,{{cite journal |last1=Organelli |first1=Emanuele |display-authors=etal |title=Two databases derived from BGC-Argo float measurements for marine biogeochemical and bio-optical applications |journal=Earth System Science Data |date=2017 |volume=9 |issue=2 |pages=861–880 |doi=10.5194/essd-9-861-2017|bibcode=2017ESSD....9..861O |s2cid=21668439 |doi-access=free }} heating of the upper ocean,{{cite journal |last1=Oschlies |first1=Andreas |title=Feedbacks of biotically induced radiative heating on upper-ocean heat budget, circulation, and biological production in a coupled ecosystem-circulation model |journal=Journal of Geophysical Research: Oceans |date=2004 |volume=109 |issue=C12 |doi=10.1029/2004JC002430|bibcode=2004JGRC..10912031O |url=http://oceanrep.geomar.de/7965/1/557_Oschlies_2004_FeedbacksOfBioticallyInducedRadiative_Artzeit_pubid5978.pdf }} and carbon export to deep waters by way of the ocean biological pump.{{cite journal |last1=Stemmann |first1=L. |last2=Boss |first2=Emmanuel |title=Plankton and Particle Size and Packaging: From Determining Optical Properties to Driving the Biological Pump |journal=Annual Review of Marine Science |date=2012 |volume=4 |pages=263–290 |doi=10.1146/annurev-marine-120710-100853|pmid=22457976 |bibcode=2012ARMS....4..263S }} The portion of the electromagnetic spectrum usually involved in ocean optics is ultraviolet through infrared, about 300 nm to less than 2000 nm wavelengths.{{cite web |last1=Mobley |first1=Curtis |title=Ocean Color. Ocean Optics Web Book |url=https://www.oceanopticsbook.info/view/remote-sensing/ocean-color |access-date=21 August 2021 |date=2021}}

File:CTDondeck.jpg 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).]]

The most widely used optical oceanographic sensors are PAR sensors, chlorophyll-a fluorescence sensors (fluorometers), and transmissometers. These three instruments are frequently mounted on CTD(conductivity-temperature-depth)-rosette samplers. These instruments have been used for many years on CTD-rosettes in global repeat oceanographic surveys like the CLIVAR GO-SHIP campaign.{{cite journal |last1=Dickey |first1=Tommy D. |title=The emergence of concurrent high-resolution physical and bio-optical measurements in the upper ocean and their applications |journal=Reviews of Geophysics |date=1991 |volume=29 |issue=3 |pages=383–413 |doi=10.1029/91rg00578 |bibcode=1991RvGeo..29..383D }}{{cite journal |last1=Boss |first1=Emmanuel |display-authors=etal |title=Optical techniques for remote and in-situ characterization of particles pertinent to GEOTRACES |journal=Progress in Oceanography |date=2015 |volume=133 |pages=43–54 |doi=10.1016/j.pocean.2014.09.007|bibcode=2015PrOce.133...43B |s2cid=51780423 |url=https://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1228&context=sms_facpub }}

Optical instruments are often used to measure the size spectrum of particles in the ocean. For example, phytoplankton organisms can range in size from a few microns (micrometers, μm) to hundreds of microns. The size of particles is often measured to estimate how quickly particles will sink, and therefore how efficiently plants can sequester carbon in the ocean's biological pump.

File:Prepping the Imaging Flow Cytobot.jpg

File:Ready for a View of Tiny Things.jpg

Scientists study individual tiny objects such as plankton and detritus particles using flow cytometry and in situ camera systems. Flow cytometers measure sizes and take photographs of individual particles flowing through a tube system; one such instrument is the Imaging FlowCytoBot (IFCB).{{cite journal |last1=Olson |first1=Robert J. |last2=Sosik |first2=Heidi M. |title=A submersible imaging-in-flow instrument to analyze nano-and microplankton: Imaging FlowCytobot |journal=Limnology and Oceanography: Methods |date=2007 |volume=5 |issue=6 |pages=195–203 |doi=10.4319/lom.2007.5.195|s2cid=83979355 |doi-access= |bibcode=2007LimOM...5..195O }} In situ camera systems are deployed over the side of a research vessel, alone or attached to other equipment, and they capture photographs of the water itself to image the particles present in the water; one such instrument is the Underwater Vision Profiler (UVP).{{cite journal |last1=Picheral |first1=Marc |last2=Guidi |first2=Lionel |last3=Stemmann |first3=Lars |last4=Karl |first4=David M. |last5=Iddaoud |first5=Ghizlaine |last6=Gorsky |first6=Gabriel |title=The Underwater Vision Profiler 5: An advanced instrument for high spatial resolution studies of particle size spectra and zooplankton |journal=Limnology and Oceanography: Methods |date=2010 |volume=8 |issue=9 |pages=462–473 |doi=10.4319/lom.2010.8.462|s2cid=13671970 |url=https://archimer.ifremer.fr/doc/00133/24413/ |doi-access=free |bibcode=2010LimOM...8..462P }} Other imaging technologies used in the ocean include holography{{cite journal |last1=Nayak |first1=Aditya R. |display-authors=etal |title=A Review of Holography in the Aquatic Sciences: In situ Characterization of Particles, Plankton, and Small Scale Biophysical Interactions |journal=Frontiers in Marine Science |date=2021 |volume=7 |doi=10.3389/fmars.2020.572147|doi-access=free |bibcode=2021FrMaS...772147N }} and particle imaging velocimetry (PIV), which uses 3D video footage to track the movement of underwater particles.{{cite web |title=Deep particle image velocimetry |url=https://www.mbari.org/technology/emerging-current-tools/instruments/technology-deeppiv/ |website=Monterey Bay Aquarium Research Institute (MBARI) |access-date=24 August 2021 |date=2017}}

{{See also|Ocean color}}

File:The path covered by light from the sun through the water body to the remote sensing sensor.jpg

Ocean optics research done “in situ” (from research vessels, small boats, or on docks and piers) supports research that uses satellite data. In situ optical measurements provide a way to: 1) calibrate satellite sensors when they are just beginning to collect data, 2) develop algorithms to derive products or variables like chlorophyll-a concentration, and 3) validate data products derived from satellites. 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.{{cite journal |last1=Mascarenhas |first1=Veloisa |last2=Keck |first2=Therese |editor1-last=Jungblut |editor1-first=S. |editor2-last=Liebich |editor2-first=V. |editor3-last=Bode |editor3-first=M. |title=Marine Optics and Ocean Color Remote Sensing (Conference paper) |journal=YOUMARES 8 – Oceans Across Boundaries: Learning from Each Other |date=2018 |pages=41–54 |doi=10.1007/978-3-319-93284-2_4|doi-access=free }} In situ data are often available from publicly accessible data libraries like the SeaBASS data archive.

File:Preparing to Gather Water Samples.jpg

File:Seawifs global biosphere 2002.png), 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.]]

File:Monitoring processes in the ocean from remote sensing.jpg

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. Two summer-term courses have been developed for graduate students from many different institutions. First, there is a summer lecture series operated by the International Ocean Colour Coordinating Group (IOCCG) which usually takes place in France.{{cite web |title=2022 IOCCG Summer Lecture Series |url=https://ioccg.org/what-we-do/training-and-education/ioccg-sls-2022/ |website=Frontiers in Ocean Optics and Ocean Colour Science |access-date=24 August 2021 |date=2021}} Second, there is an ongoing course in the United States called the “Optical Oceanography Class” or “Ocean Optics Class” in Washington State and later in Maine, which has been running continuously since 1985.{{cite journal |last1=Perry |first1=Mary Jane |title=SIDEBAR. The Optical Oceanography Class Turned 30 in Summer 2015 |journal=Oceanography |date=2016 |volume=29 |issue=1 |pages=32–33 |doi=10.5670/oceanog.2016.07|doi-access=free }}

For independent learning, Curt Mobley, Collin Roesler, and Emmanuel Boss wrote the [https://www.oceanopticsbook.info/ Ocean Optics Web Book] as an open-access online guide.

{{portal|Oceans}}

Related fields and topics:

• Atmospheric optics
• Color of water
• Electromagnetic spectrum
• History of optics
• Oceanography
• Ocean color
• Optical depth
• Spectral color
• Transparency and translucency
• Visible spectrum
• Water clarity
• Water remote sensing
• Water quality

Inherent and apparent optical properties and in-water methods:

• Absorption (electromagnetic radiation)
• Argo (oceanography)
• Attenuation coefficient
• Beer-Lambert Law
• Marine optical buoy
• Scattering
• Secchi disk

Remote sensing and radiometric methods:

• Albedo
• Atmospheric correction
• NASA Earth Science
• Spectralon

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{{reflist}}

• [https://www.oceanopticsbook.info/ Ocean Optics Web Book]{{cite book |last1=Kirk |first1=John T. O. |title=Light and Photosynthesis in Aquatic Ecosystems |date=1994 |publisher=Cambridge University Press |isbn=0521459664 |edition=3rd}}{{cite book |last1=Preisendorfer |first1=Rudolph W. |title=Hydrologic Optics (6 Volumes) |date=1976 |publisher=U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Environmental Research Laboratories, Pacific Marine Environmental Laboratory |url=http://misclab.umeoce.maine.edu/education/HydroOptics/D:/START.pdf}}

Category:Oceanography

Category:Applied and interdisciplinary physics

Category:Scattering, absorption and radiative transfer (optics)

Category:Optics

Category:Marine biology

Category:Aquatic ecology

Category:Biological oceanography

Category:Water

Category:Earth sciences

Category:Earth observation in-situ sensors