Bio-optical Calibration and Measurements Protocols for U.S.
JGOFS Equatorial Pacific Cruises
Charles C. Trees and Curtiss O. Davis
The accurate determination of in-water upwelled (E) and
downwelled (E) spectral irradiances and upwelled spectral radiance
(L) require adherence to strict calibration and measurement
protocols. NASA sponsored a workshop (8--12 April, 1991) to establish
calibration standards, protocols, and sampling strategies for ocean
optical measurements to be used for SeaWiFS baseline algorithm development
and system validation. Calibration and measurement protocols for JGOFS
EqPac cruises will follow those protocols. This document gives a brief
overview; for a more detailed review refer to the proceedings (Mueller and
Austin, 1991) from that workshop. There are two major sources of
measurement errors in determining E, E and L; (1) the
perturbation of the in-water radiant energy field caused by the ship and
(2) the estimation of quantitative corrections for atmospheric variability
in the radiant energy incident on the surface during the in-water
measurements. To correct for these potential problems the following
protocols will be followed during the transect and time series
Spring: Charles Trees (CHORS) Fall: Daniel Sullivan (CHORS)
James Aiken (PML, U.K.) Ian Bellan (PML, U.K.)
Time Series Cruises:
Spring: Curtiss Davis (JPL) Fall: Joseph Rhea (JPL)
Joseph Rhea (JPL) Michael Hamilton (JPL)
- The bio-optical profiling systems for the transect and time series
cruises are similar in that they include a Biospherical spectroradiometer
(MER 1032 and 1048), a Sea Tech transmissometer (660 nm) and a Sea Tech
fluorometer. Both MERs have also been adapted to measure solar stimulated
fluorescence [(L(683 nm)]. The two surface incident irradiance
detectors are different in that one was built by Biospherical (Dr. Davis')
whereas we built out own. The MERs and surface incident irradiance
detectors will be calibrated before and after each cruise. The
calibration of these optical instruments will be performed at the CHORS
Calibration Facility, using standard sources traceable to the National
Institute of Standards and Technology (NIST). In addition, both MERs will
be optically characterized to define their performance characteristics.
This characterization includes linearity, cosine response of the
irradiance detectors, field of view of radiance detectors, irradiance and
radiance immersion factors, spectral response, wavelength calibration and
instrument stability and precision checks. The CHORS MER has already been
characterized and will go through a second characterization prior to the
first transect cruise in Jan. 1992. Dr. Davis' MER will be shipped back
to CHORS after his first time series cruise (June, 1992) for a post-cruise
calibration and its first characterization. Characterization needs to be
performed every 3--5 years.
Dr. James Aiken will deploy his towed multi-sensor Undulating
Oceanographic Re-corder (UOR), which measures downwelled and upwelled
spectral irradiance (cosine collectors), fluorescence and beam
transmission (660 nm) during its undulations from near surface to
ca 70 meters. The UOR irradiance sensors will be calibrated at CHORS
before and after each cruise. In addition, a partial characterization of
the sensors will be performed to document their cosine responses,
immersion factors, linearity and spectral responses.
QA/QC: CHORS Calibration Facility has a complete
capability for characterizing and calibrating environmental radiometers,
following the procedures described in the NASA report (Mueller and Austin,
1991). These procedures have been routinely applied to our own equipment,
as well as characterizing and calibrating ocean radiometer systems for
NOARL, the Naval Oceanographic Office and Dalhousie University (Dr.
Marlon Lewis' expendable L meter). We have cross-checked MER
instrument calibrations with Dr. Ken Voss at the University of Miami and
with Biospherical Instruments, Inc. and realized agreements in irradiance
calibrations within <5 % with both, and also agree with the University
of Miami for radiance within <5 %. In 1989, we compared a Multispectral
Airborne Radiometer System radiance calibration using our facilities, with
a calibration performed at Goddard Space Flight Center, and obtained
agreement within 5 %.
- Between the laboratory calibrations of the MER's, secondary checks of
the instruments performance will be performed using stable lamps in rugged
fixed geometric configurations. These portable secondary calibrations in
the field will provide a record of the instruments stability and will
assist in evaluating possible corrections to the optical data if
inconsistencies appear later. More importantly these sources would give
advance warnings of potential problems, which could be corrected in the
field. The CHORS Portable Secondary Standard (PSS) will be taken on the
transect cruises. Intercalibration using the standard will be performed
on Dr. Davis' MER prior to the departure of the time series legs. Because
of prior commitments, the PSS has to be shipped back to CHORS after each
- The profiling spectroradiometers (MER 1032 and 1048) will be
deployed from the stern using the starboard crane fully extended (65 ft).
In addition the ship will be positioned with the sun off the stern and
will maintain that orientation throughout the optical cast (approximately
30 minutes). These two measurement protocols will help to minimize the
errors in the determination of E, E, and L caused by
ship shadow. At least one bio-optical profile should be made daily with
the cast being close to local apparent noon (LAN) as long as the solar
zenith angles are greater than 10 degrees. At angles less than this
errors can be introduced in measurements of E and L by the
instrument's own shadow. There will be times during these cruises when
measurements will have to be made a few hours before of or after LAN to
minimize this self-shadowing effect.
- The dark current of optical sensors is frequently temperature
dependent and as a consequence, collection of accurate optical data
requires careful attention to dark current variability. We will make dark
current measurements before and after each cast and we will ordinarily use
the post-cast dark readings, when the instrument temperature is closer to
ambient conditions. When there is a large temperature difference between
the instrument on the deck and the water temperature, the instrument will
be allowed to equilibrate with the water temperature at the beginning of
- Surface incident spectral irradiance will be measured during the
optical cast to correct for atmospheric variability in radiant energy
incident on the sea surface. This instrument will be mounted as high up
on the ship as possible and away from any structures that might cause a
shadow on or a reflection into the instrument.
- The windows on the Sea Tech transmissometers will be cleaned with lens
cleaner and a tissue, then rinsed with distilled water, and finally rinsed
with isopropyl alcohol and wiped dry. Cleaning of the windows and reading
of these ``on-deck air calibrations'' will be performed before each cast to
verify that the windows are clean. An air calibration to check for
temporal degradation in the transmissometer's source and detector will be
performed in the laboratory before and after each cruise. In addition a
characterization of the A-to-D system in the MER's will be performed so
that a known input voltage generates a predicted output voltage. For
transmissometers used on the CTD, an end-to-end check of the instrument is
required unless the CTD A-to-D channels are also calibrated.
Transmissometer data collected with the CTD will be corrected for
temperature, salinity and pressure using the algorithm provided by Sea
Tech. In addition, the data will be evaluated for temperature hysteresis
effects which, if deemed significant, will be corrected for using a
modified algorithm developed by Bishop (1986).
- The windows on the fluorometers will be cleaned with lens cleaner
and rinsed with distilled water. Since the measurements of fluorescence
are in relative units no additional calibration is planned, except that
the voltage measuring A-to-D system recording the fluorescence must be
characterized as described for the transmissometers.
- The MERs have scalar PAR (photosynthetically available radiation)
collectors for measuring broad banded radiation from 400--700 nm. Because
of the difficulty in calibrating and accurately measuring radiant energy
over this broad spectral band (Gordon and McCluney, 1975; Kirk, 1983), PAR
will be calculated (Morel and Smith, 1974) from the downwelling spectral
irradiance data (E).
- Sky radiance will be estimated by occulting the sun's image on the
deck cell measuring incident spectral irradiance. This measurement is
useful for estimating the mean cosine at the surface and can be used with
profile measurements of E, E, and c to estimate by (Gordon,
1990). The ability to exploit this and similar relationships will greatly
enhance both the development and verification of bio-optical algorithms.
- All-sky photographs will be taken before each optical cast to
document the cloud cover. This information is important for identifying
measurements made under questionable environmental conditions.
- Bishop, J.K.B. (1986).
- The correction and suspended particulate matter
calibration of the Sea Tech transmissometer data. Deep-Sea Research,
- Gordon, H.R. (1975).
- Estimation of the depth of sunlight
penetration in the sea for remote sensing. Applied Optics, 14(2):413--416.
- Gordon, H.R. (1990).
- Estimation of inherent optical properties from
irradiance measurements: Monte Carlo simulations, pp. 49--54, In:
Ocean Optics X, Proc. SPIE 1302, R.W. Spinrad (ed.).
- Gordon, H.R. and W.R. McCluney (1975).
- Estimation of the depth of
sunlight penetration in the sea for remote sensing. Appl. Opt.,
- Kirk, J.T.O. (1983).
- Light and Photosynthesis in Aquatic
Ecosystems, Cambridge University Press, Cambridge, 401 pp.
- Morel, A. and R.C. Smith (1974).
- Relation between total quanta and
total energy for aquatic photosynthesis. Limnology and Oceanography, 19(4):591--600.
- Mueller, J.L. and R.W. Austin (1992).
- Ocean Optical Protocols for
SeaWiFS Validation. SeaWiFS Technical Report Series, Volume 5,
NASA Technical Memorandum 1044566.