45
inversely proportional to its wavelength. That means shorter
wavelengths possess more energy than the longer (Watson
and Zielinski 2013).
How Is Radiation Measured?
In ocean (and also in freshwater ecosystems) optics, radiant
energy is measured using two classes of light detectors: thermal and quantum. Thermal detectors (thermometers, thermocouples, bolometers, and pyranometers) absorb radiant
energy and convert it into heat energy, wherein the detector
responds to consequent changes in temperature of the absorbing medium. Quantum detectors (photographic films, photovoltaic, photoconductive, and photoemissive) react directly
to the number of incident photons and not on the cumulative
energy of the photons (Mobley 1994; Cunningham and
McKee 2013).
Spectral radiance (unit: W sr
−1
m
−2
nm
−1
) is the fundamental radiometric quantity of interest in aquatic optics. It is
the radiant flux emitted, reflected, transmitted, or absorbed
by a given surface, per unit solid angle per unit projected
area. It describes the spatial, temporal, directional, and spectral structure of light. However, full radiance distributions
are difficult to measure and assimilate. Therefore, quantities
such as total scalar irradiance (W m
−2
nm
−1
), downward and
upward (planar and scalar) irradiances are obtained by integrating radiances over defined intervals of solid angle.
Profiling radiometer assemblies enable precise descriptions
of radiative transfer in natural waters (Moore et al. 2009).
Reflectance (Fig. 2), an important AOP fundamental to
remote sensing of the oceans, is computed from the above
mentioned radiance and irradiance measurements. Earlier,
ocean color remote sensing scientists used irradiance reflectance (the ratio of upwelling irradiance to downwelling irradiance) to develop algorithms for IOPs and other ocean
parameter retrievals (Morel and Prieur 1977). However,
recently, remote sensing reflectance (ratio of upwelling radiance to downwelling irradiance, measured just above the
water surface) is more preferred by optical oceanographers
(O’Reilly et al. 1998), as it is less sensitive to conditions
such as sun angle and sky conditions. Radiative transfer
studies relate water AOPs to IOPs.
Optical Tools
Optical oceanography relies strongly on field observations. Although the use of optics in the study of oceans
dates back to ancient times, advances in optical technology have played a crucial role in improving our understanding and exploration of the aquatic environments via
means of imaging, vision, and sensing. Some of the earliest ocean color measurements were those of Secchi disc
depth using a Secchi disk (Fig. 3b) named after the nineteenth century priest and astronomer Pietro Angelo Secchi
aboard the papal yacht L’Immacolata Concezione to
determine water transparency (Wernand 2010). These
measurements were made using white discs of 0.4–3.75 m
diameter to measure ocean clarity. Observations of light
penetration depth were also made during Britain’s 1872–
1876 HMS Challenger expedition (Wernand 2013). The
depth is determined by lowering the disc in water until it
disappears from view.
In 1887, Francois Alphonse Forel introduced his ocean
color comparator scale ranging from blue to green for identification of ocean color, later extended by Willie Ule from
green to brown. Referred to as the Forel-Ule scale, it is well
known and most commonly used in oceanography and limnology to determine color of natural waters. Wernand and
van der Woerd (2010) proposed a reintroduction of the scale
to expand the historical datasets and facilitate correlation
with recent satellite ocean color observations. The scale is
well characterized and stable ensuring coherent and wellcalibrated datasets. Such simple methods have enabled participation from citizens through a number of citizen science
projects such as the citclops (http://www.citclops.eu/) and
eye on water (http://eyeonwater.org/) across Europe and
beyond (Busch et al. 2016).
Optical sensors measure interaction of light (via absorption and scattering) with water constituents and thereby
enable an assessment of the variability in water optical properties in relation to the observed OAC concentrations
(Zielinski et al. 2009; Busch et al. 2013). Such observations
are fundamental in the establishments of bio-optical models
that relate OACs to their optical properties. Via methods of
bio-optical inversion, these models enable determination of
bio-geo-chemical parameters form remotely sensed signals
(see section “Why do we use satellite measurements?”).
Commonly used measurements of ocean color parameters
include those of light transmission, absorption, scattering,
fluorescence, and radiance distribution via methods of spectrophotometry, fluorometry, and radiometry respectively
(Dickey et al. 2011). Sensors with selective membranes have
enabled additional in situ monitoring of parameters like
nutrients, dissolved oxygen, and carbon dioxide (Moore
et al. 2009). However, field observations are limited in space
and time and thereby lack regular or repeated global coverage. Therefore, satellite missions, which began monitoring
the Earth in the 1960s, play an essential role by remotely
Marine Optics and Ocean Color Remote Sensing
inversely proportional to its wavelength. That means shorter
wavelengths possess more energy than the longer (Watson
and Zielinski 2013).
How Is Radiation Measured?
In ocean (and also in freshwater ecosystems) optics, radiant
energy is measured using two classes of light detectors: thermal and quantum. Thermal detectors (thermometers, thermocouples, bolometers, and pyranometers) absorb radiant
energy and convert it into heat energy, wherein the detector
responds to consequent changes in temperature of the absorbing medium. Quantum detectors (photographic films, photovoltaic, photoconductive, and photoemissive) react directly
to the number of incident photons and not on the cumulative
energy of the photons (Mobley 1994; Cunningham and
McKee 2013).
Spectral radiance (unit: W sr
−1
m
−2
nm
−1
) is the fundamental radiometric quantity of interest in aquatic optics. It is
the radiant flux emitted, reflected, transmitted, or absorbed
by a given surface, per unit solid angle per unit projected
area. It describes the spatial, temporal, directional, and spectral structure of light. However, full radiance distributions
are difficult to measure and assimilate. Therefore, quantities
such as total scalar irradiance (W m
−2
nm
−1
), downward and
upward (planar and scalar) irradiances are obtained by integrating radiances over defined intervals of solid angle.
Profiling radiometer assemblies enable precise descriptions
of radiative transfer in natural waters (Moore et al. 2009).
Reflectance (Fig. 2), an important AOP fundamental to
remote sensing of the oceans, is computed from the above
mentioned radiance and irradiance measurements. Earlier,
ocean color remote sensing scientists used irradiance reflectance (the ratio of upwelling irradiance to downwelling irradiance) to develop algorithms for IOPs and other ocean
parameter retrievals (Morel and Prieur 1977). However,
recently, remote sensing reflectance (ratio of upwelling radiance to downwelling irradiance, measured just above the
water surface) is more preferred by optical oceanographers
(O’Reilly et al. 1998), as it is less sensitive to conditions
such as sun angle and sky conditions. Radiative transfer
studies relate water AOPs to IOPs.
Optical Tools
Optical oceanography relies strongly on field observations. Although the use of optics in the study of oceans
dates back to ancient times, advances in optical technology have played a crucial role in improving our understanding and exploration of the aquatic environments via
means of imaging, vision, and sensing. Some of the earliest ocean color measurements were those of Secchi disc
depth using a Secchi disk (Fig. 3b) named after the nineteenth century priest and astronomer Pietro Angelo Secchi
aboard the papal yacht L’Immacolata Concezione to
determine water transparency (Wernand 2010). These
measurements were made using white discs of 0.4–3.75 m
diameter to measure ocean clarity. Observations of light
penetration depth were also made during Britain’s 1872–
1876 HMS Challenger expedition (Wernand 2013). The
depth is determined by lowering the disc in water until it
disappears from view.
In 1887, Francois Alphonse Forel introduced his ocean
color comparator scale ranging from blue to green for identification of ocean color, later extended by Willie Ule from
green to brown. Referred to as the Forel-Ule scale, it is well
known and most commonly used in oceanography and limnology to determine color of natural waters. Wernand and
van der Woerd (2010) proposed a reintroduction of the scale
to expand the historical datasets and facilitate correlation
with recent satellite ocean color observations. The scale is
well characterized and stable ensuring coherent and wellcalibrated datasets. Such simple methods have enabled participation from citizens through a number of citizen science
projects such as the citclops (http://www.citclops.eu/) and
eye on water (http://eyeonwater.org/) across Europe and
beyond (Busch et al. 2016).
Optical sensors measure interaction of light (via absorption and scattering) with water constituents and thereby
enable an assessment of the variability in water optical properties in relation to the observed OAC concentrations
(Zielinski et al. 2009; Busch et al. 2013). Such observations
are fundamental in the establishments of bio-optical models
that relate OACs to their optical properties. Via methods of
bio-optical inversion, these models enable determination of
bio-geo-chemical parameters form remotely sensed signals
(see section “Why do we use satellite measurements?”).
Commonly used measurements of ocean color parameters
include those of light transmission, absorption, scattering,
fluorescence, and radiance distribution via methods of spectrophotometry, fluorometry, and radiometry respectively
(Dickey et al. 2011). Sensors with selective membranes have
enabled additional in situ monitoring of parameters like
nutrients, dissolved oxygen, and carbon dioxide (Moore
et al. 2009). However, field observations are limited in space
and time and thereby lack regular or repeated global coverage. Therefore, satellite missions, which began monitoring
the Earth in the 1960s, play an essential role by remotely
Marine Optics and Ocean Color Remote Sensing