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

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

The Cromwell Current on the east side of the Galapagos Islands

Observations made during October and December 1971 on the Yaloc 71 Cruise of Oregon State University indicate the presence of the Cromwell Current on the east side of the Galapagos Islands. Light scattering, particle size distribution, nutrients, and standard hydrographic parameters were measured in water samples collected at 154 stations. The distribution of water properties shows that the extension of the current to the east side of the islands derives primarily from water flowing around the north side of the islands. This flow consists of two branches that tend to merge into one at about 84°W. There was no clear indication of a branch around the south side of the islands within the area of the observations

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

Influence of surface waves on measured and modeled irradiance profiles

Classical radiative transfer programs are based on the plane-parallel assumption. We show that the Gershun equation is valid if the irradiance is averaged over a sufficiently large area. We show that the equation is invalid for horizontal areas of the order of tens of meters in which horizontal gradients of irradiance in the presence of waves are much larger than vertical gradients. We calculate the distribution of irradiance beneath modeled two-dimensional surface waves. We show that many of the features typically observed in irradiance profiles can be explained by use of such models. We derive a method for determination of the diffuse attenuation coefficient that is based on the upward integration of the irradiance field beneath waves, starting at a depth at which the irradiance profile is affected only weakly by waves.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

Theoretical Dependence of the Near-Asymptotic Apparent Optical Properties on the Inherent Optical Properties of Sea Water

The apparent optical properties of sea water are a set of measurables that describe the geometry of the submarine light field. These properties are related to the inherent optical properties—the volume attenuation coefficient and the volume scattering function—through the process of radiative transfer. A numerical approximation to the equation of radiative transfer was programmed for solution on a high-speed digital computer. The technique is based on resolving the hydrosol into a system of thin slabs, finding approximate transmission and reflection operators for each slab, and then systematically applying these operators to determine the geometry of the light field in the interior of the system of slabs. Thus each set of calculations produces the radiance solid as a function of depth. The apparent optical properties were found by numerical integration of the radiance. A simple three-parameter model was used to simplify study of the dependence of the apparent optical properties on the directional characteristics of the volume scattering function.This paper was published in the Journal of the Optical Society of America 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/josa/Issue.cfm. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Keywords: Oceanography, Image formation, Scattering, Luminance, Water, Spread function, Modulation transferKeywords: Oceanography, Image formation, Scattering, Luminance, Water, Spread function, Modulation transfe

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.This paper was published in Optics Express 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/oe/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.Keywords: Ocean optics, Multiple Scatterin