Frequently Answered Questions

Often researchers contact Biospherical Instruments with more or less general questions about how to select the best sensors or instruments for their work.  To the left is a list of general topics culled from our support team's list of Frequently Asked Questions. 

The purpose of the lexicon below is to help provide a ready reference to the common terms used daily at here at BSI.  Note that often these terms are used in discussions about optics in aquatic research, so you may perceive a specialized focus to terms rather than a broad 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.

• Dynamic Range.  The ratio between the largest and smallest measurable value is called the dynamic range.  Because it is an expression in power, those with an engineering bent often express dynamic range in decibels (db), although it is also convenient to use the resolution of the associated analog-to-digital electronics.  Thus, you may see dynamic range expressed in bits (e.g. 16 or 20 bits).  To be truly useful, wide dynamic range requires more than just extra bits -- high-quality, low-noise electronics are also needed to ensure that any extra bit depth is well used.

• 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.

• Directional response.  The sensitivity of a light sensor as a function of the angle of incidence of the light.  For instance, the fore-optics of a cosine irradiance sensor, a cosine collector has a directional response that should vary in proportion with the cosine of the 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.

• Einstein.  A mole (6.02x10²³) of quanta.   See also Units, below.

• 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.    Cosine collectors often look like diffusing flat plates with sloping sides.

• 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.  

• Response Time.  The amount of time needed for a sensor and associated circuitry to respond to a change in condition.  This time constant is one of the items used in optimizing the design elements of a sensor.  At BSI, 0.1 seconds is the typical value.  If we abbreviate the response time constant as "t" then  a full step change in input signal will change the output to 63% of its final value in t seconds, and will settle to within 1% of the final value in 5*t seconds, 0.1% of the final value in 7*t seconds, and 0.01% of the final value in 9*t seconds.  Also, a step change in input signal will change from 10% to 90% of its starting and ending values in 2.2*t seconds.   As a practical matter in the field,  sensors are not called upon to respond to large step-like changes and thus field data is not normally limited by the response time constant. 

• 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. 

• Units, Radiometric.  Units used to describe electromagnetic radiation.  Watts, Joules, Quanta, Einsteins -- sometimes you want to interconvert from one unit to another -- if so, give this Excel spreadsheet a try.   "For marine atmospheres with sun altitudes above 22°, the quanta/watt ratio for the region 400-700nm is 2.77 x 1018 quanta/sec/watt to an accuracy of plus or minus a few percent."  This quote and a further discussion of the relationship of quanta to watts in the water column can be found in Smith and Morel (1974) Limnol. Oceanogr. 19(4):591-600. 

• Units, Photometric.   Visible radiation is often described with units like lumens, lux, phots, stibs, nits, footcandles, candelas, and lamberts.  These all refer to a detector with a spectral response which mimics the visual response of the "Standard Observer."   When expressed in radiometric terms, this means:  1) a wavelength of maximum response at 550nm; and 2) a sharply sloping spectral response (0.41% response at 700 nm relative to the peak, 0.12% at 400nm).  It is possible to convert from quantum units to photometric units only if the complete spectral distribution is known.

• 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").