OPTICAL OCEANOGRAPHY GLOSSARY
Prepared for the Comprehensive Dictionary of Physics,
Dipak Basu (Editor-in-Chief)
to be published by CRC Press LLC (Boca Raton, FL)
For inclusion in the Dictionary of Geophysics, Astrophysics, and Astronomy,
Richard Matzner (Editor)
Prepared by N. J. McCormick* and C. D. Mobley#
*University of Washington, Mechanical Engineering, Seattle, Washington 98195-2600
#Sequoia Scientific, Inc., 9725 S.E. 36th St., Mercer Island, Washington 98040
absorbance The logarithm to the base 10 of the ratio of the radiant power at a given wavelength incident on a volume to the sum of the scattered and directly transmitted radiant powers emerging from the volume [--]; also called optical density.
absorptance The fraction of the incident power at a given wavelength that is absorbed within a volume [--].
coefficient The absorptance per unit distance of photon travel in a medium, i.e., the limit of the ratio of the spectral absorptance to the distance of photon travel as that distance becomes vanishingly small [m-1].
diffuse absorption coefficient for downwelling (upwelling) irradiance The ratio of the absorption coefficient to the mean cosine of the downward (upward) radiance.
specific absorption coefficient The absorption coefficient [m-1] per unit mass of material, e.g., per unit chlorophyll a concentration [mg chl a m-3]; in [m2 mg chl a-1].
efficiency see efficiency factor
for a body of water The ratio of plane irradiance leaving a water body to the plane irradiance incident on it [--]; i.e., the upward irradiance to the downward irradiance just above the surface.
for single scattering The probability of a photon surviving an interaction [--]; it equals the ratio of the scattering coefficient to the beam attenuation coefficient.
AOP see apparent optical property
aphotic zone That portion of the ocean where light is insufficient for plants to carry on photosynthesis.
apparent optical property A ratio of radiometric quantities that depends both on the IOP's and on the directional nature of the ambient light field and which is spatially and temporally stable enough to describe a water body; examples include the average cosine of the light field, the irradiance reflectance, the remote sensing reflectance, and the diffuse attenuation coefficients.
asymmetry factor see scattering, mean cosine of scattering angle
asymptotic radiance distribution The (normalized) angular shape of the radiance distribution at depths far from the boundary of a homogeneous medium; the directional and depth dependencies of the asymptotic radiance distribution decouple and all radiometric variables (e.g., irradiances) decrease spatially at the same rate as the radiance, as governed by only the inherent optical properties; see diffuse attenuation coefficient
asymptotic regime see water
attenuation coefficient see beam attenuation coefficient and diffuse attenuation coefficient
average cosine see mean cosine of radiance or scattering
coefficient The integral over the hemisphere of backward directions of the volume scattering function.
fraction The ratio of the backscattering coefficient to the scattering coefficient.
beam attenuation coefficient The limit of the ratio of the spectral absorptance plus spectral scatterance to the distance of photon travel as that distance becomes vanishingly small [m-1]; equal to the sum of the absorption and scattering coefficients.
beam spread function The irradiance distribution on the inner surface of a sphere as generated by an initially collimated, narrow beam and normalized to the beam power [m-2]; numerically equals the point spread function.
beam transmissometer An instrument for measuring the fraction of a collimated beam lost by absorption and scattering per unit distance of photon travel; measures the beam attenuation coefficient.
Beer's law Refers to any situation in which there is exponential attenuation of light in a specified direction.
benthic The portion of the marine environment inhabited by marine organisms that live in or on the bottom of the ocean.
benthos Bottom-dwelling marine organisms.
bidirectional reflection distribution function (BRDF) The ratio of the reflected radiance from a surface into a specific localized direction to the incident plane irradiance of a collimated beam in a specific direction [sr-1].
bioluminescence Light produced by organisms as a result of conversion of chemical energy to radiant energy.
biomass The amount of living matter per unit of water surface or water volume (i.e., in the water column) [kg m-2 or kg m-3].
Bouguer-Lambert law see Beer's law
Bouguer's law see Beer's law
BRDF see bidirectional reflectance distribution function
BSF see beam spread function
capillary wave see water wave, capillary
CDOM see colored dissolved organic matter
chlorophyll Chemical compounds occurring in plants that enable radiant energy to be converted to chemical energy in the process of photosynthesis; there are several types (e.g., denoted as chl a, b, etc.) with chl a typically the most present.
closure see optical closure
coastal ocean see water
Coastal Zone Color Scanner (CZCS) A multi-spectral line scanner operational from October 1978 to June 1986 which had four channels devoted to ocean color, each of 20 nm bandwidth and centered at 443, 520, 550, and 670 nm.
color see ocean color
colored dissolved organic matter (CDOM) High-molecular-weight organic compounds (humic and fluvic acids) formed from the decomposition of plant tissue; they strongly absorb light at the blue end of the spectrum and can give water a yellowish color at high concentrations.
conversion efficiency see energy conversion efficiency
cosine collector A radiant energy detector whose effective light collection area is proportional to the cosine of the angle between the incident light and the normal to the detector surface; used to measure plane irradiances.
Coulter counterâ One of a class of instruments that measures particle size distribution from the change in electrical conductivity as particles flow through a small orifice; originally developed by Coulter Electronics.
absorption The cross sectional area of a beam containing power equal to the power absorbed by a particle in the beam [m2].
scattering The cross sectional area of a beam containing power equal to the power scattered by a particle in the beam [m2].
CZCS see coastal zone color scanner
critical depth The depth in seawater below which respiratory carbon loss by phytoplankton exceeds photosynthetic carbon gain; no net phytoplankton production occurs.
euphotic depth The depth to which significant phytoplankton photosynthesis can take place; typically taken to be the depth at which PAR falls to 1% of its value just below the surface [m].
optical depth The optical path length in the vertically downward direction.
detritus The particulate decomposition or disintegration products of plankton, including dead cells, cell fragments, fecal pellets, shells, and skeletons, and sometimes mineral particles in coastal waters.
diel Pertains to occurrences on a 24 hour cycle; any periodic diurnal or nocturnal (i.e., day or night) cycle.
diffuse absorption coefficient see absorption
diffuse attenuation coefficient
asymptotic The value of the diffuse attenuation coefficient in the asymptotic regime; it depends only on the inherent optical properties.
downward (upward) irradiance The negative of the derivative with respect to depth of the natural logarithm of the downward (upward) plane irradiance [m-1].
downward (upward) scalar irradiance The negative of the derivative with respect to depth of the natural logarithm of the downward (upward) scalar irradiance [m-1].
PAR The negative of the derivative with respect to depth of the natural logarithm of the photosynthetically available radiation [m-1].
diffuse scattering coefficient for downwelling (upwelling) irradiance The ratio of the scattering coefficient to the mean cosine of the downward (upward) radiance.
dissolved organic matter see colored dissolved organic matter
diurnal Pertains to occurrences during the day, as opposed to nocturnal occurrences at night; sometimes incorrectly used for diel occurrences.
divergence law for irradiance see Gershun's law
einstein One einstein is one mole of photons ($6.023 ´ 1023 photons).
absorption The ratio of the absorption cross section to the geometrical cross section of the particle.
attenuation The sum of the absorption plus scattering efficiency factors.
scattering The ratio of the scattering cross section to the geometrical cross section of the particle.
energy conversion efficiency The rate of chemical energy accumulation per unit volume divided by the rate of absorption of light energy by phytoplankton per unit volume; it is linearly related to quantum yield.
euphotic zone The water layer in which significant phytoplankton photosynthesis can take place; typically taken to be the layer down to which PAR falls to 1% of its value just below the sea surface.
euphotic zone midpoint Typically taken to be the layer at which PAR falls to 10% of its value just below the sea surface.
fluorescence The inelastic scattering process in which a photon is absorbed by a molecule and shortly thereafter (10-11 to 10-8 s) another photon of greater wavelengh is emitted; the emitted radiance is unpolarized.
Fresnel reflectance The fraction of radiant energy in a narrow beam that is reflected from a surface at which there is an index of refraction mismatch.
fulvic substance High molecular weight organic compounds resulting from plant decay, especially phytoplankton. see colored dissolved organic matter
gelbstoff see colored dissolved organic matter
Gershun's law The conservation of energy equation obtained by integrating the monochromatic radiative transfer equation (for a medium with no inelastic scattering or other sources) over all directions; it states that the depth derivative of the net plane irradiance equals the negative of the absorption coefficient multiplied by the scalar irradiance.
Gershun tube A tube used to limit the field of view of a radiometer to a small solid angle; used in measuring radiances.
gilvin see colored dissolved organic matter
gravity wave see water wave, gravity
humic substance High molecular weight organic compounds resulting from plant decay, especially terrestrial plants; water-soluble soil humic substance imparts a yellow color to water; see also colored dissolved organic matter.
Hydrolightâ A widely used, commercially available software package for computing radiances and irradiances as a function of depth and wavelength for a variety of water types, surface illuminations, and bottom conditions.
index of refraction see refractive index
inelastic scattering Scattering in which the wavelength changes because radiant energy is transferred to the scattering molecule.
inherent optical property Any optical quantity that depends only on the properties of the water and is independent of the ambient light field; examples include the absorption coefficient, the scattering coefficient, and the beam attenuation coefficient; apparent optical properties become inherent optical properties if the radiance distribution is asymptotic.
intensity The radiant power in a given direction per unit solid angle per unit wavelength interval [W sr-1 nm-1].
internal wave see water wave, internal
inverse problems A general classification for problems in which (ir)radiance measurements are used to infer inherent optical properties.
medium characterization Determination of the absorption and scattering inherent optical properties.
source Determination of the sum of the inelastic scattering and true emission (i.e., the source function) at the wavelength of interest.
IOP see inherent optical property
irradiance The radiant power per unit area per unit wavelength interval [W m-2 nm-1].
downward see downward plane irradiance
downward (upward) plane The downward (upward) directed radiant power per unit area onto an upward (downward) facing horizontal surface [W m-2 nm-1].
downward (upward) scalar The downward (upward) directed radiant power per unit area onto a spherical collecting surface [W m-2 nm-1].
net see net plane irradiance
net plane The downward plane irradiance minus the upward plane irradiance [W m-2 nm-1].
ratio The ratio of the upward plane irradiance to the downward plane irradiance [--].
reflectance see irradiance ratio
scalar The power per unit area incident from all directions onto a spherical collecting surface [W m-2 nm-1]; it equals the downward scalar irradiance plus the upward scalar irradiance.
upward see upward plane irradiance
Jerlov water type see water
Junge particle distribution A power-law distribution function [particles m-3 m m-1] often used to describe the particle concentration [particles m-3] per unit size interval [m m] versus equivalent spherical diameter of particles; the number density for particles of equivalent diameter x is proportional to x-k, with k typically 3 to 5 in natural waters.
K-function see diffuse attenuation coefficient
Lambertian surface A surface for which radiance is reflected or emitted equally in all directions over the hemisphere.
light-harvesting complex A pigment-protein complex containing chlorophyll that supplies additional energy in photosynthesis.
marine snow Particles of organic detritus and living forms whose downward drift, in a dense concentration, appears similar to snowfall.
downward (upward) radiance The average cosine of the nadir (zenith) angle of all downward (upward) traveling photons; it equals the ratio of the plane irradiance to the scalar irradiance for the downward (upward) direction [--].
scattering angle see scattering
total radiance The average cosine of the polar angle of all photons; it equals the ratio of the net plane irradiance to the total scalar irradiance [--].
mean free path The average distance between photon-matter interactions; it equals the inverse of the beam attenuation coefficient [m].
Mie scattering see scattering
mixed layer Near-surface waters subject to mixing by wind and waves; there is little variation in salinity or temperature below the mixed layer.
nadir angle The angle between a given light ray and the downward vertical direction.
nocturnal Pertains to occurrences during the night, as opposed to diurnal occurrences during the day.
ocean color A generic term referring to the spectral dependence of the radiance leaving a water body.
ocean color sensor Any instrument for sensing of ocean color, usually from aircraft or satellites.
oceanic optics see optical oceanography
open ocean see water
optical constants The nonnegative real and imaginary parts of the refractive index; the constants often strongly depend on frequency.
measurement closure Refers to making consistent measurements; examples include independently verifying that the sum of the absorption coefficient plus the scattering coefficient equal the beam attenuation coefficient and that the integral over all directions of the volume scattering function equals the scattering coefficient.
model closure Refers to developing consistent analytical and/or numerical models for computing underwater light fields that make predictions consistent with observations.
scale closure Refers to making the transition from small scale (e.g., single particle) data to bulk scale data in a manner consistent with observations.
optical density see absorbance
optical depth see depth
optical oceanography The subdiscipline of oceanography concerned with the propagation and interaction of radiation, typically at wavelengths between » 350 and » 750 nm, with seawater.
optical path length The integral of the dimensionless product of the beam attenuation coefficient [m-1] multiplied by an infinitesimal unit of distance [m] along the direction of travel.
package effect The discrepancy between the spectral absorption coefficient of a particle suspension, consisting of a spatially nonuniform distribution of pigment molecules in cells, and the corresponding coefficient of a homogeneous solution containing the same amount of pigment.
PAR see photosynthetically available radiation
Petzold see volume scattering function
phase function Describes the angular distribution of scattered radiation, as given by the ratio of the volume scattering function to the scattering coefficient [sr-1]; the integral of the phase function over all directions is unity.
pheophytin A phytoplankton pigment that is inert and does not contribute to photosynthesis.
photoinhibition The decrease in photosynthetic rate with increasing irradiance that is caused by surpassing the photosynthetic capacity.
photosynthesis The manufacture of carbohydrates from carbon dioxide and water in the presence of chlorophyll, by utilizing radiant energy and releasing oxygen; the chemical change induced in chlorophyll by the absorption of a quantum of radiant energy.
photosynthetically available radiation (PAR) The integral over visible wavelengths (350-700 nm or sometimes 400-700 nm) of the number of photons available for photosynthesis [photons s-1 m-2]; it can be computed by integrating the spectrally-dependent scalar irradiance divided by the photon energy at each wavelength.
photosynthetic pigment Molecules whose structures efficiently absorb light within the 400-700 nm range.
photosynthetic capacity The maximum photosynthetic rate per unit of biomass.
gross The total rate of carbon dioxide fixation with no allowance for the CO2 simultaneously lost in respiration, e.g., in m moles CO2 (or O2) (mg chl)-1 h-1 or mg C (carbon) (mg chl)-1h-1.
maximum see photosynthetic capacity
net The total rate of photosynthetic CO2 fixation minus the rate of loss of CO2 in respiration.
specific Photosynthetic rate, net or gross, per unit biomass or per unit volume, e.g., [m moles CO2 (or O2) (mg chl)-1 h-1].
phytoplankton see plankton
plankton Passively drifting or weakly swimming organisms.
plankton bloom An unusually high concentration of phytoplankton usually producing a discoloration of the water body.
phytoplankton Plant forms of plankton, generally with sizes from less than 1 to several hundred m m.
zooplankton Animal forms of plankton.
point spread function The apparent radiance due to an unresolved Lambertian (cosine-emitting) point source, as normalized to the source intensity in the direction of maximum emission [m-2]; numerically equal to the beam spread function.
polarization state The description of a light field using four components related to the oscillation of the elecric field vector of the photons; see Stokes parameters.
primary production The amount of organic matter produced from inorganic matter by photosynthesis, e.g., in [g C (Carbon) m-3] or, for a water column, in [g C (Carbon) m-2].
primary productivity The rate of production of organic matter from inorganic matter by photosynthesis, e.g., in [g C (Carbon) m-3 h-1] or, for a water column, in [g C (Carbon) m-2 h-1].
productivity see primary productivity
PSF see point spread function
Q factor see efficiency factor
quanta meter An instrument to measure the number of photons (e.g., photosynthetically available radiation), as opposed to energy.
quantum yield The number of CO2 molecules fixed in biomass per quantum of light absorbed by a plant; it is linearly related to energy conversion efficiency.
quasi-single-scattering approximation A radiative transfer model that accounts for only single scattering of photons with the assumption that forward-scattered light is treated as unscattered.
radiance The radiant power in a beam per unit solid angle per unit area perpendicular to the beam per unit wavelength interval [W m-2 sr-1 nm-1].
radiative transfer equation The linear integrodifferential equation that describes the rate of change with distance of the radiance in a collimated beam at a specified location, direction, and wavelength; the equation accounts for all losses (e.g., due to absorption and scattering out of the beam) and gains (e.g., by emission or scattering into the beam).
radiometer An instrument used to measure radiant energy, as opposed to the number of photons.
radiometry The science of the measurement of radiant energy.
Raman scattering see scattering
Rayleigh scattering see scattering
Fresnel see Fresnel reflectance
irradiance see irradiance ratio
remote sensing see remote sensing reflectance
refraction The change in direction of a photon when crossing an interface between two media that have different real indices of refraction; see index of refraction.
refractive index The complex number for which the real part (typically about 1.34 for seawater) governs scattering at interfaces and the imaginary part governs absorption; the real part is the ratio of the velocity of light in vacuum to the phase velocity in the medium.
remote sensing reflectance The ratio of the "water-leaving" radiance in air to the downward plane irradiance incident onto the sea surface.
RTE see radiative transfer equation
scatterance The fraction of the incident power at a prescribed wavelength that is scattered within a volume [--].
albedo see albedo for single scattering
angle The angle between the directions of the incident and scattered photons.
anisotropic Scattering that is not spherically symmetric.
asymmetry parameter see mean cosine of scattering angle
back Scattering through angles greater than 90o; see backscattering
coefficient The limit of the ratio of the incident power at a prescribed wavelength that is scattered within a small volume to the distance of photon travel as that distance becomes vanishingly small [m-1].
efficiency see efficiency factor
Henyey-Greenstein phase function An approximate phase function that is parameterized by only the mean cosine of the scattering angle.
mean cosine of scattering angle The integral over all directions of the volume scattering function multiplied by the cosine of the scattering angle, divided by the integral over all directions of the volume scattering function; also called the single-scattering asymmetry factor.
Mie The exact solution for scattering of electromagnetic waves by homogeneous spheres of any size; the computations require knowledge of the complex index of refraction of the sphere relative to the surrounding medium and the ratio of the sphere circumference to the wavelength of the radiation.
Mie size parameter The ratio of a sphere's circumference to the wavelength [--].
Petzold data see volume scattering function
Raman scattering Inelastic photon scattering in which the energy of the scattered photon equals the energy of the incident photon plus or minus energy determined by the vibrational and rotational frequencies of the molecule; it is characterized by a volume scattering function that has forward-backward directional symmetry.
Rayleigh scattering A mathematical model that describes scattering off very small particles (e.g., molecules); it is characterized by a volume scattering function that depends inversely with the fourth power of the wavelength and has forward-backward directional symmetry.
SeaWiFS The Sea-viewing Wide Field-of-view Sensor launched in 1998 that measures radiance in wavelengths centered at 412, 443, 490, 510, 555, and 670 nm (all with 20 nm bandwidth) and 765 and 865 nm (both with 40 nm bandwidth).
Secchi depth The depth at which a Secchi disk disappears from view as it is lowered in water [m].
Secchi disk A white disk of diameter 20-30 cm, used as a qualitative way of measuring water clarity.
seston The total animate and inanimate particulate matter in natural waters.
Snell's law The law that describes the refraction of light at an interface between two media that have different real indices of refraction.
source functions The terms in the radiative transfer equation that describe the inelastic scattering and true emission contribution to a beam of radiation; some authors also include elastic scattering in the source term.
specific Adjective used to express a quantity per unit mass.
specific absorption coefficient see absorption
spectral Adjective used to denote either wavelength dependence or a radiometric quantity per unit wavelength interval.
spectroradiometer A radiometer that measures radiant energy as a function of wavelength.
Stokes parameters] Four parameters used to describe polarized radiation; one component describes the radiance, two components describe the states of linear polarization, and one component describes the state of circular polarization.
tripton The inanimate particulate matter in natural waters.
two-flow equations The two coupled differential equations for irradiances obtained by integrating the radiative transfer equation over the hemispheres of downward and upward directions.
two-stream approximation see two-flow equations
visible wavelengths Approximately 400 to 700 nm.
volume scattering function (VSF) The ratio of the scattered intensity [W sr-1] to the incident irradiance [W m-2] per unit volume [m3], given in [m-1 sr-1]; the integral of the volume scattering function over all directions and all final wavelengths is the scattering coefficient so the VSF can be written as the product of the phase function [sr-1] and the scattering coefficient [m-1].
Petzold data A widely used data set containing volume scattering functions for various waters ranging from very clear to very turbid; the scattering phase function is highly peaked at small scattering angles and is reasonably independent of water type.
VSF see volume scattering function
asymptotic regime Depths at which the rate of decay with depth of all radiometric variables, given by the asymptotic diffuse attenuation coefficient, depends only on the inherent optical properties.
case 1 type Water whose optical properties are determined primarily by phytoplankton and co-varying CDOM and detritus; not a synonym for open ocean waters.
case 2 type Water whose optical properties are significantly influenced by CDOM, detritus, mineral particles, bubbles, or other substances whose concentrations do not co-vary with the phytoplankton concentration; not a synonym for coastal waters.
coastal Shallow water generally situated over continental shelves; often, but not always, case 2 waters.
eutrophic Waters with high phytoplankton biomass; chlorophyll a concentrations exceed 10 mg m-3.
Jerlov type A water clarity classification scheme based on the downward diffuse attenuation coefficient just below the sea surface; the types are numbered I, IA, IB, II and III (ranging from clear to turbid open ocean waters) and 1 through 9 (ranging from clear to turbid coastal waters).
mesotrophic Waters with moderate concentrations of phytoplankton biomass; chlorophyll a concentrations range between approximately 0.5 to 10 mg m-3.
oligotrophic Waters with low phytoplankton biomass, typical in many open ocean regions; chlorophyll a concentrations are below 0.5 mg m-3.
open ocean Water seaward of the approximate edges of the continental shelves; often, but not always, case 1 waters.
capillary A wave in which the primary restoring force is surface tension; waves with wavelength less than about 1.7 cm are considered capillary waves.
gravity A wave in which the primary restoring force is gravity; waves with wavelength greater than about 1.7 cm are considered gravity waves.
internal A wave that occurs within seawater whose density changes with depth either gradually or abruptly at an interface discontinuity.
white disk see Secchi disk
yellow substance see colored dissolved organic matter
zenith angle The angle between a given light ray and the upward direction.
zooplankton see plankton