Glossary of Terms

Often researchers contact Biospherical Instruments with more or less general questions about how to select the best sensors or instruments for their work.  The purpose of this lexicon is to provide a ready reference to the common terms used daily at here at BSI  -- an operational aid to finding common ground for problem solving.  Note that often these terms are used in discussions about aquatic research, so you may perceive a special focus (bias?) rather than a rigorous technical definition.

Those interested in pursuing The Truth may consult a number of excellent references, such as Curt Mobley's Light and water (1994), Academic Press, New York; J.T.O. Kirk's Light and photosynthesis in aquatic ecosystems, 2nd ed. (1994), University Press, Cambridge; or the Optical Society of America's Handbook of optics, 2nd ed. (1995), Michael Bass, editor in chief,  McGraw Hill Inc., New York. 

• Attenuation coefficient, diffuse.  The diffuse attenuation coefficient (abbreviated k(λ)) quantifies the rate of decrease of sunlight underwater in a narrow spectral band.  Often calculated from vertical profiles of irradiance, this rate of  decrease is typically logarithmic.  

• Cast.  A cast is a vertical profile of data collected at a single location.  It is often divided up into segments such as the down cast, the up cast, and the dark segment.  In addition, multiple casts per station my be made.

• Cosine collector.  The fore-optics of a cosine irradiance sensor, a cosine collector has a directional response that varies in proportion with the cosine of the relative zenith angle of incidence.  This means the signal will have a maximum responsivity to light from directly overhead (local zenith) and should have no responsivity to photons coming directly from the side or below.  Cosine collectors often look like diffusing flat plates with sloping sides.

• Dark voltage or dark segment.  The “Dark Offset” or “Dark Voltage” or “Dark Current” or “The Dark” is the baseline reading from the instrument when there is no light.  Dark voltages are almost always a function of instrument temperature.  Dark voltages are almost always small.  Darks voltages may be the opposite polarity from the reading in light.  A dark segment is a interval of readings in a profile that indicate the values to use as darks when applying calibrations values to the profile.

• Einstein.  A mole (6.02x10²³) of quanta.  

• Filter-photodetector Radiometer  is a light sensor that measures a selective waveband of broadband sunlight where the spectral selection is performed using a combination of optical filter and photodetector.  The waveband may be either narrowband (e.g. 10 nm FWHM) or broadband (e.g. PAR).  Filter-photodetectors may be designed with combinations of filters to achieve very high resistance to spectral leakage.

• FWHM.  Abbreviation for full-width at half-maximum.  An expression of the spectral width of a detector or filter.  The width is reported as the difference between the lower and upper wavelengths of the  spectrum where the value of the response is equal to one-half the maximum value. 

• Irradiance detector is a sensor which measures the flux of light incident on a surface.  Spectral irradiance is abbreviated E(λ) where λ is the center wavelength of the detector.  Calibration units common for our irradiance sensors are µW^.cm^-2.nm^-1 for narrow bandwidth  detectors, or either Q^.m^-2.s^-1 (quanta) or µE^.m^-2.s^-1 (microEinsteins) for broadband sensors such as PAR (400-700 nm).   The SI unit for spectral irradiance is W^.m^-2.nm^-1.

• Irradiance detector, scalar  is an irradiance sensor with equal sensitivity to light from all directions.  Scalar spectral irradiance is abbreviated E_o(λ).  Scalar irradiance collectors often look like spheres.  Chloroplasts frequently resemble small scalar collectors, and researchers concerned with photosynthesis or phytoplankton ecology often measure irradiance with scalar sensors.  An above-water irradiance reference sensor equipped with a scalar collector often features a plate-like cutoff to help eliminate surface reflection.  These sensors are called "hemispherical scalar collectors." 

• Irradiance detector, cosine  is a plane (vector) irradiance sensor with a directional response proportional to the cosine of the relative zenith angle of incidence.  Cosine spectral irradiance is abbreviated E_d(λ) or E_u(λ) for downwelling or upwelling orientation.  

• NIST.  United State's National Institute of Standards and Technology.  This is the national agency in the United States responsible for maintaining national standards, such as calibration references and procedures.

• PAR.  Photosynthetically Active Radiation (also sometimes called photosynthetically available radiation).    A PAR sensor is typically an irradiance sensor that is equally sensitive to light between 400 and 700 nm and insensitive to light outside this region.  In terms of collector geometry, a PAR sensor is usually equipped with either a scalar or cosine collector.  At BSI, the most common units of calibration for broadband PAR are or either Q^.m^-2.s^-1 (quanta) or µE^.m^-2.s^-1 (microEinsteins).  PAR may also be calculated by integrating the weighted visible channels from a multiwavelength instrument such as a PRR. 

• Radiance detector  is a sensor which measures the flux of light flowing in a specified direction per unit solid angle.  Spectral radiance is abbreviated L(λ) where λ is the center wavelength of the detector.  Submersible radiance sensors are normally oriented to measure the nadir (upwelling) radiance, abbreviated L_u(λ).  The calibration unit of radiance used in our radiance sensors is µW^.cm^-2.nm^-1.sr^-1 although nE^.m^-2.s^-1 (nanoEinsteins) is used to measure natural fluorescence (L_u(chl)).  Note that the SI unit for spectral radiance is W^.m^-2.nm^-1.sr^-1.

• Reflectance, radiance.  For the purposes of our instrumentation, the spectral radiance reflectance is the ratio of the upwelling radiance to the downwelling irradiance:  L_u(λ)/E_d(λ).

• Reflectance, irradiance.    For the purposes of our instrumentation, the spectral irradiance reflectance is the ratio of the upwelling irradiance to the downwelling irradiance:   E_u(λ)/E_d(λ).

• Response, spectral.  The spectral response of a sensor is the magnitude of the signal from a sensor as a function of wavelength of the exciting irradiance.  

• RMA.  Abbreviation for Return Material Authorization.  An RMA number is assigned to each return to aid the manufacturing team at Biospherical in coordinating and scheduling repairs. This RMA number must be clearly marked on the outside of each package for instrument systems being returned to the factory for repair.

• Spectral leakage.  Spectral leakage is the erroneous reporting of light energy within a band that actually originates from wavelengths outside the band.  

• Steradians (or "square radians").  The unit used to quantify a solid angle in three dimensional space.  It is analogous to the use of "radian" to quantify an angle in two dimensional space.  A sphere subtends 4 pi steradians, a hemisphere subtends 2 pi steradians, etc.  

• Vertical profile.  A vertical profile is obtained by moving an instrument vertically through the water while recording data as a function of depth.  For measurements of light, it is not generally relevant whether or not the data are recorded when the instrument is raised (the "up-cast") or lowered (the "down-cast").