28
2.2.7 Light in the Sea
Light in the ocean is attenuated through absorption by water molecules and dissolved constituents and through scattering by individual molecules and particulate
matter. Absorption is least in the violet/blue region of the spectrum (380–450 nm)
and increases at lower and higher wavelengths such that absorption is strong in the
ultraviolet (UV) and infrared (IR) bands. Absorption at individual wavelengths is
characterized by exponential decay such that
I I
e
al
/ 0 =
−
(2.1)
where I 0 is the incident radiation, I is the radiation measured after passage of light
through a sample over a distance l, e is the base of the natural logarithm (2.718), and
a is the absorption coefficient. Across a homogeneous medium, a perfect hyperbola
is observed and the absorption coefficient is derived through logarithmic transformation such that
ln /
I I
al
0
(
)= −
(2.2)
and
2 303
0
.
log /
/
I I
l
a
(
) = −
(2.3)
Light absorption by pure water (Pope and Fry 1997) interferes with the optical
detection of dissolved constituents in seawater so that the corresponding absorption
of water must be subtracted from the seawater absorption coefficient to obtain the
absorption coefficient of these constituents. Since the irradiance ratio is unitless,
the resulting coefficient is given as inverse distance. If l is expressed in meters, then
the absorption coefficient is expressed as m
−1
. Normally these measurements are
performed upon filtered samples to avoid scattering by particulate matter.
Light attenuation in the ocean medium (c) is given by the sum of absorption (a)
and scattering (b). These inherent optical properties are mostly measured in the
laboratory using spectrophotometers and scatterometers or by means of submersible research instruments available to measure these parameters. Nevertheless, measurement of apparent optical properties, dependent on prevailing irradiance, is more
suited for practical field applications where the object is to determine the penetration of visible light or photosynthetically active radiation (PAR). Diffuse attenuation, incorporating both components, provides a useful measure of light penetration
in the ocean allowing the derivation of a diffuse attenuation coefficient denoted as
k d (Kirk 1994).
Photosynthesis is restricted to depths above a compensation depth for solar irradiance below which respiration exceeds photosynthesis, a point which is reached at
2 Electronic Sensors and Instruments for Coastal Ocean Observing
2.2.7 Light in the Sea
Light in the ocean is attenuated through absorption by water molecules and dissolved constituents and through scattering by individual molecules and particulate
matter. Absorption is least in the violet/blue region of the spectrum (380–450 nm)
and increases at lower and higher wavelengths such that absorption is strong in the
ultraviolet (UV) and infrared (IR) bands. Absorption at individual wavelengths is
characterized by exponential decay such that
I I
e
al
/ 0 =
−
(2.1)
where I 0 is the incident radiation, I is the radiation measured after passage of light
through a sample over a distance l, e is the base of the natural logarithm (2.718), and
a is the absorption coefficient. Across a homogeneous medium, a perfect hyperbola
is observed and the absorption coefficient is derived through logarithmic transformation such that
ln /
I I
al
0
(
)= −
(2.2)
and
2 303
0
.
log /
/
I I
l
a
(
) = −
(2.3)
Light absorption by pure water (Pope and Fry 1997) interferes with the optical
detection of dissolved constituents in seawater so that the corresponding absorption
of water must be subtracted from the seawater absorption coefficient to obtain the
absorption coefficient of these constituents. Since the irradiance ratio is unitless,
the resulting coefficient is given as inverse distance. If l is expressed in meters, then
the absorption coefficient is expressed as m
−1
. Normally these measurements are
performed upon filtered samples to avoid scattering by particulate matter.
Light attenuation in the ocean medium (c) is given by the sum of absorption (a)
and scattering (b). These inherent optical properties are mostly measured in the
laboratory using spectrophotometers and scatterometers or by means of submersible research instruments available to measure these parameters. Nevertheless, measurement of apparent optical properties, dependent on prevailing irradiance, is more
suited for practical field applications where the object is to determine the penetration of visible light or photosynthetically active radiation (PAR). Diffuse attenuation, incorporating both components, provides a useful measure of light penetration
in the ocean allowing the derivation of a diffuse attenuation coefficient denoted as
k d (Kirk 1994).
Photosynthesis is restricted to depths above a compensation depth for solar irradiance below which respiration exceeds photosynthesis, a point which is reached at
2 Electronic Sensors and Instruments for Coastal Ocean Observing