Glossary

12-bit Resolution
The capability of a measuring device to distinguish one in 4096 parts of a quantity. For example, a 12-bit system with a range of 2.5 V should be able to resolve 0.6 mV steps in a signal.

A

Absorption
The ability of matter to absorb light energy impingent on it. This light energy is converted into heat, or in the case of plant matter, it is converted into photosynthetic energy or secondary energy processes such as fluorescence emission. The net light lost from an incident beam of light is referred to by the absorption coefficient or absorbance. The amount of light absorbed by materials in the water depends on the materials themselves and the wavelength of light incident upon them. By varying the wavelength of light, one can deduce information relating to specific material in the water. This is a basic principle behind spectrophotometry.

Absorption Coefficient
A numeric measure of the amount of light energy that water itself and the dissolved and suspended substance within the water take up, which results in less energy or less light penetrating down into a water body. It can be measured for some or all of the wavelengths of a light ray and is reported by meter. It is an inherent optical property.

Apparent Optical Properties
Water properties that depend on ambient light and inherent optical properties. They depend on the angle of the sun, whether it is day or night, and the amount of cloud cover. AOPs include:
• irradiance attenuation coefficient
• irradiance reflection coefficient
• Secchi disk depth

Attenuation
The total light lost from a beam of light propagating through water. These losses are due to scattering from suspended materials and the molecular scattering of the water itself, and to particulate and dissolved materials and the water itself absorbing the light. The amount of attenuation is primarily dependent upon the wavelength of the propagated light, the concentration of suspended materials and the concentration and composition of both particulate and dissolved absorbing materials.

Backscattering
Light scattered from particles scattered at angles greater than 90 degrees with respect to the incident direction of the source as it impinges on particles in the water. Backscattering is particularly useful in application that tie remote ocean color images from satellites and planes to in-water optical processes.

Beam attenuation
A decrease in light energy from a collimated beam that is passing through a water sample with a specific pathlength. It is an inherent optical property.

Benthic
That portion of the marine environment inhabited by organisms living at or near the bottom of the ocean.

Bioluminescence
Light produced by organisms from conversion of chemical to radiant energy.

Calibration
A procedure for comparing the signal from an instrument with known or standard values for turbidity, temperature, pressure, salinity, etc.

Chlorophyll
A compound in plants that converts radiant energy to chemical energy through the process of photosynthesis.

Chlorophyll Fluorescence
Fluorescence attributed to chlorophyll-containing organisms. In the ocean these organisms are phytoplankton, which represent the first level of the ocean food chain. They are also of particular interest because they are the chief determinates of how much carbon dioxide is removed from the atmosphere by the ocean.

Coccolith
A small round body, found in chalk formations.

Colored Dissolved Organic Matter (CDOM)
Organic compounds such as humic and fluvic acids that form from decomposing plant matter. CDOM absorbs light at the blue end of the spectrum and can give water a yellowish color at high concentrations.

CDOM Fluorescence
Fluorescence attributed to colored dissolved organic matter in the ocean. In coastal areas this can provide an indication of how much organic carbon material is getting washed into the ocean. In the ocean, it provides an indication of carbon cycling into the oceanic pool from decaying phytoplankton and other life forms.

Clarity
A qualitative measurement of the ability of water to transmit light. Clarity can be assessed using transmissometers and turbidity sensors.

Detector
An electronic device that responds in a predictable way to electromagnetic or mechanical parameter such as light, current, or voltage.

Diatom
A major group of phytoplankton whose cells are enclosed in shells.

E

Euphotic depth, Euphotic zone
The depth to which significant phytoplankton photosynthesis can occur. Typically the depth at which PAR falls to 1% of its value just below the surface.

Euphotic zone midpoint
Generally, the layer at which PAR falls to 10% of its value just below the sea surface.

Fluorescence
An optical phenomenon that occurs when light absorbed by a material creates a molecular excitation that causes the material to re-emit light as a different wavelength.

Fluorometer
An instrument that provides an indication of the concentration of a given material by measuring the amount of fluorescence attributed to the material. For example, a fluorometer provides an excitation beam at a wavelength that is known to cause fluorescent emission from chlorophyll and measures light at a wavelength that matches the chlorophyll emission. As a result, the amount of chlorophyll-containing biomass can be estimated.

Gain
The value of a system output divided by the value at its input. For example, if the voltage of the output terminal of an amplifier is 10 V, and the voltage at its input is 1 V, the gain is 10.

H

Humic substance
Organic compounds with a high molecular weight resulting from plant decay, especially terrestrial plants. See CDOM.

Inherent Optical Properties
Water properties that depend only on the content of the water, regardless of light conditions. IOPs include absorption, attenuation, scattering, fluorescence, and volume scattering function.

Irradiance
Light energy per unit area per unit wavelength interval (W m-² nm-¹) from all directions. See Radiance.

Irradiance Attenuation Coefficient
A measurement that estimates the amount of light attenuated, which is absorbed or scattered, over a water column. It is an apparent optical property.

Irradiance Reflection Coefficient
An indicator of scattering in the water relating to the water's brightness. It is an apparent optical property.

Mie Scattering
A form of atmospheric scatter that occurs when radiation interacts with atmospheric particles whose diameter is approximately equal to the wavelength of the radiation.

Mixed Layer
An isothermal surface layer of water, approximately 40–150 m deep caused by wind and convection.

Particulate Organic Carbon
The suspended particle load that controls the chemistry of the oceans. The physical and chemical properties of the particles control how rapidly a chemical species is removed from solution and incorporated in sediment.

Photosynthetically Available Radiation (PAR)
Wavelengths—roughly 400–700 nanometers—of incoming sunlight that can be absorbed by plants for photosynthesis.

Photosynthesis
The manufacture of carbohydrates from carbon dioxide and water in the presence of chlorophyll, using radiant energy and releasing oxygen.

Phycocyanin
The bluish pigment used by cyanobacteria to capture light for photosynthesis.

Phycoerythrin
The reddish pigment used by cyanobacteria to capture light for photosynthesis

Phytoplankton
Free-floating flora that convert inorganic compounds into complex organic compounds. This process of primary productivity supports the pelagic food-chain. Phytoplankton vary in size from less than 1 to serveral hundred µm.

Primary Production
The amount of organic matter produced from inorganic matter by photosynthesis: for a water column, measured in g C m^-2-2 h^-1 (C is carbon).

Radiance
The amount of light energy or photons measured in a specified time period in a unit area from a given direction.

Reflectance
In radiation transfer, the fraction of incoming radiation that is reflected from a medium. The sum of this, the transmittance, and the absorbance must equal unity.

Rhodamine
A fluorescent dye sometimes introduced to water so in-water circulation can be investigated through tracking its fluorescence signal.

Scattering Coefficient
Numeric measure of the amount of light diffused from the downward direction in water, by the water itself and its contents. This is an IOP. Measured in inverse meters (m^-1).

Scattering
The amount of light that is reflected and refracted away from a particle. The measurement is widely used to estimate overall concentrations of particles suspended in water, and can also be used to look at specific material properties.

Secchi disk
A white disk of 20–30 cm diameter used to measure clarity of water. The Secchi depth is the depth at which the disk disappears from view.

Spectrofluorometer
A fluorometer that uses multiple excitation wavelengths and multiple emission wavelengths to isolate and determine relative concentrations of different materials in the water.

Spectrometer
A instrument that measures absorption and attenuation as a function of different wavelengths of light.

Turbidity
A qualitative term that refers to the concentration of suspended particles in the water.

Turbidity Sensor
A scattering sensor used to determine the concentration of suspended particles in the water.

Volume Scattering Function (VSF)
The amount of light scattered into a given angle by a given material relates to the size and shape of the material and its molecular composition. The function that describes this scattering through different angles is called the VSF. By measuring part of the VSF one can specify types of materials, size distributions and other material-specific parameters other than just concentrations.