410 research outputs found
Asymptotic light field in the presence of a bubble-layer
We report that the submerged microbubbles are an efficient source of diffuse
radiance and may contribute to a rapid transition to the diffuse asymptotic
regime. In this asymptotic regime an average cosine is easily predictable and
measurable.Comment: 4 pages, 3 Postscript figures, opex2.sty (enclosed), also available
from the Optical Society of America
htpp://epubs.osa.org/oearchive/pdf/11948.pd
The influence of bottom morphology on reflectance: Theory and two-dimensional geometry model
The reflectance of the bottom is of importance when interpreting optical data in shallow water. Closure studies of radiative transfer, interpretation of laser line scanner data, lidar, and remote sensing in shallow waters require understanding of the bottom reflectance. In the Coastal Benthic Optical Properties experiment (CoBOP), extensive measurements of the material reflectance (reflectance very close to the bottom) were made. Far field reflectance will be needed in carrying out closure of the radiative transfer model and observed radiometric and inherent optical properties. The far field reflectance is the bottom reflectance that includes the effect of bottom morphology (such as sand ripples) as well as the material reflectance. We present here a first-order analytical model to derive the relationship between the material and far field reflectances. We show that the effective reflectance of the bottom is proportional to the average cosine of the bottom slope. Using a simple two-dimensional geometry without scattering and absorption, we show that errors in ignoring the bottom morphology can lead to overestimations of the far field reflectance on the order of 30%
The effect of bottom substrate on inherent optical properties: Evidence of biogeochemical processes
Measurements of inherent optical properties (IOP) were conducted over bottoms with different substrates by use of a sampling package mounted on and operated by a SCUBA diver. It was found that in areas of low ambient currents the distribution of IOP varies with bottom type in (1) its value relative to a nearby bottom of different type, (2) its vertical gradient, and (3) its variability. This implies that radiative transfer modeling in shallow environments may need to include, besides the bottom characteristics, the bottom effect on in-water IOP. In tidally flushed shallow banks, vertical and horizontal gradients over scales of O(1, 10 m), respectively, are as large as temporal gradients over scales of minutes and cannot be separated in our measurements. However, bottom-substrate-related processes over the banks result in gradients over large horizontal spatial scales and tidal timescales. The distribution of IOP is consistent with several biogeochemical processes that may be active at a given bottom substrate and suggest that optical measurements may provide a useful tool to infer and quantify bulk rates of biogeochemical processes
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A theoretical derivation of the dependence of the remotely sensed reflectance of the ocean on the inherent optical properties
An expression for the ratio of the upwelling nadir radiance L(π, z) and the downwelling scalar irradiance Eod(Z) is derived from the following equation of radiative transfer. This expression is given by RSR(z)=[L(π, z)]/Eod(Z) = [fb(z)bb(z)]/2π[k(π, z) + c(z) – fL(z)bf(z)], where bb(z) is the backscattering coefficient, k(π, z) is the vertical attenuation coefficient of the nadir radiance, c(z) is the beam attenuation coefficient, and fb(z) and fL(z) are shape parameters that depend on the shape of the volume scattering function and the radiance distribution. Successive approximations are subsequently applied to the above exact equation. These are fb(z) = [2πβ(π – θm, z)]/[bb(z)], where β(π – θm, z) is the volume scattering function at 180° minus the zenith angle of the maximum radiance and k(π, z) = am = c[1 – 0.52 b/c – 0.44 (b/c)²], where m is a parameter that is numerically equal to the inverse of the average cosine of the asymptotic light field for a medium with the same inherent optical properties, a is the absorption coefficient, and b/c is the single scattering albedo. Together with fL(z) = 1.05 and application of Gershun’s equation, it is shown that for nearly all oceanic cases RSR(z) ≡ L(π, z)/Eod(z) = [β(π – θm, z)]/{a(z)[1 + m(z)]}
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Remotely sensed reflectance and its dependence on vertical structure: a theoretical derivation
An exact expression for the remotely sensed reflectance (RSR, upwelling radiance divided by downwelling scalar irradiance) just beneath the surface of the ocean is derived from the equation of radiative transfer. It is shown that the RSR at a given depth in the ocean depends only on the inherent optical properties, the attenuation coefficient for upwelling radiance, and two shape factors that depend on the radiance distribution and volume scattering function. The shape factors are shown to be close to unity. An exact expression for the RSR just beneath the surface as a function of the vertical structure of inherent and apparent optical properties is derived. This expression is solved for an N-layered system, which presents the possibility of inverting remotely sensed reflectance data to obtain the vertical structure of chlorophyll in the ocean.This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ao/home.cfm. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law
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Optical and hydrographic observations of the Cromwell Current between 92 ÌŠ00'west and the Galapagos Islands
Microscale Quantification of the Absorption by Dissolved and Particulate Material in Coastal Waters with an ac-9
Measuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m−1 in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated.
Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale vertical profiles of spectral total absorption, at(λ), and spectral absorption of dissolved materials, ag(λ), were then measured concurrently using multiple meters, corrected for drift, temperature, salinity, and scattering errors and subsequently compared. Particulate absorption, ap(λ), was obtained from at(λ) − ag(λ). CTD microstructure was simultaneously recorded. Vertical profiles of ag(λ), at(λ), and ap(λ) were replicated with different meters within ±0.005 m−1, and spectral relationships compared well with laboratory measurements and hydrographic structure
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