29
a depth where solar irradiance amounts to about 1% of surface incident radiation.
Knowledge of k d permits computation of this depth. The inverse of the diffuse
attenuation coefficient has units of meters (m), a value consequently known as the
optical depth. This value is of great use for ocean observing since about 90% of the
signal received from below the ocean surface by optical satellite sensors emerges
from waters above this threshold. Experimental downwelling k d spectra in the photosynthetically active 412 nm band in eastern Caribbean waters yield optical depths
ranging from 0.4 m in the Orinoco River delta to greater than 30 m in oligotrophic
waters to the north. More demanding applications of satellite sensors such as the
detection of photosynthetic pigments or of dissolved organic matter require instruments capable of quantifying inherent optical properties.
Early measurements of light attenuation in the ocean were performed using the
Secchi disk, a white 30 cm disk lowered into the water whose disappearance was
visually determined. The advent of photocells, incorporating optically active cadmium sulfide photoresistor sensors, rendered the Secchi disk obsolete which themselves were shortly thereafter replaced by the photodiodes that are in use today.
Submersible instruments measuring PAR are available in various configurations for
incorporation into existing instrument networks or platforms. These instruments are
known as quantum sensors because their optical range is tailored by means of
appropriate photodiodes and glass filters such that there is equal response to photon
flux across the PAR spectrum (despite the energy difference of different wavelength
photons) and zero response above or below PAR. Light attenuation is commonly
measured using paired flat diodes equipped with cosine collectors that favor direct
incident light from above. Readings from an instrument on or directly below the
surface paired to another one at depth allow computation of k d . Profiling the water
column with the lower unit allows more detailed optical characterization of layered
water masses (Fig. 2.12).
Fig. 2.12 Cosine collector quantum sensor (PAR meter)
2.2 Electronic Sensors and Instruments for Ocean Observing
a depth where solar irradiance amounts to about 1% of surface incident radiation.
Knowledge of k d permits computation of this depth. The inverse of the diffuse
attenuation coefficient has units of meters (m), a value consequently known as the
optical depth. This value is of great use for ocean observing since about 90% of the
signal received from below the ocean surface by optical satellite sensors emerges
from waters above this threshold. Experimental downwelling k d spectra in the photosynthetically active 412 nm band in eastern Caribbean waters yield optical depths
ranging from 0.4 m in the Orinoco River delta to greater than 30 m in oligotrophic
waters to the north. More demanding applications of satellite sensors such as the
detection of photosynthetic pigments or of dissolved organic matter require instruments capable of quantifying inherent optical properties.
Early measurements of light attenuation in the ocean were performed using the
Secchi disk, a white 30 cm disk lowered into the water whose disappearance was
visually determined. The advent of photocells, incorporating optically active cadmium sulfide photoresistor sensors, rendered the Secchi disk obsolete which themselves were shortly thereafter replaced by the photodiodes that are in use today.
Submersible instruments measuring PAR are available in various configurations for
incorporation into existing instrument networks or platforms. These instruments are
known as quantum sensors because their optical range is tailored by means of
appropriate photodiodes and glass filters such that there is equal response to photon
flux across the PAR spectrum (despite the energy difference of different wavelength
photons) and zero response above or below PAR. Light attenuation is commonly
measured using paired flat diodes equipped with cosine collectors that favor direct
incident light from above. Readings from an instrument on or directly below the
surface paired to another one at depth allow computation of k d . Profiling the water
column with the lower unit allows more detailed optical characterization of layered
water masses (Fig. 2.12).
Fig. 2.12 Cosine collector quantum sensor (PAR meter)
2.2 Electronic Sensors and Instruments for Ocean Observing