Simultaneous polarimeter retrievals of microphysical aerosol and ocean color parameters from the "MAPP" algorithm with comparison to high-spectral-resolution lidar aerosol and ocean products.

We present an optimal-estimation-based retrieval framework, the microphysical aerosol properties from polarimetry (MAPP) algorithm, designed for simultaneous retrieval of aerosol microphysical properties and ocean color bio-optical parameters using multi-angular total and polarized radiances. Polarimetric measurements from the airborne NASA Research Scanning Polarimeter (RSP) were inverted by MAPP to produce atmosphere and ocean products. The RSP MAPP results are compared with co-incident lidar measurements made by the NASA High-Spectral-Resolution Lidar HSRL-1 and HSRL-2 instruments. Comparisons are made of the aerosol optical depth (AOD) at 355 and 532 nm, lidar column-averaged measurements of the aerosol lidar ratio and Ångstrøm exponent, and lidar ocean measurements of the particulate hemispherical backscatter coefficient and the diffuse attenuation coefficient. The measurements were collected during the 2012 Two-Column Aerosol Project (TCAP) campaign and the 2014 Ship-Aircraft Bio-Optical Research (SABOR) campaign. For the SABOR campaign, 73% RSP MAPP retrievals fall within ±0.04 AOD at 532 nm as measured by HSRL-1, with an R value of 0.933 and root-mean-square deviation of 0.0372. For the TCAP campaign, 53% of RSP MAPP retrievals are within 0.04 AOD as measured by HSRL-2, with an R value of 0.927 and root-mean-square deviation of 0.0673. Comparisons with HSRL-2 AOD at 355 nm during TCAP result in an R value of 0.959 and a root-mean-square deviation of 0.0694. The RSP retrievals using the MAPP optimal estimation framework represent a key milestone on the path to a combined lidar+polarimeter retrieval using both HSRL and RSP measurements.

Optical transfer diagnosis differentiating benign and malignant pigmented lesions in a simulated primary care practice.

BACKGROUND: The detection of melanoma poses a substantial challenge, particularly for primary care providers (PCPs) who may have limited training in discriminating between suspicious and benign melanocytic lesions. The noninvasive optical transfer diagnosis (OTD) method was designed to be used by PCPs in their decision-making process. OBJECTIVES: To assess the potential of the OTD method by developing, training and validating an OTD indication algorithm for automated discrimination between benign melanocytic lesions and malignant lesions, based on a set of 712 lesions. METHODS: The authors performed in vivoOTD capture and subsequent analysis of 712 pigmented lesions. Of the lesions, 415 were clinically and dermoscopically benign and 297 were dermoscopically suspicious or equivocal. After image capture, all suspicious or equivocal lesions were biopsied and examined histopathologically. RESULTS: Of the 297 suspicious or equivocal lesions, histopathological findings revealed 80 to be malignant (64 melanomas, 13 basal cell carcinomas and 3 squamous cell carcinomas). OTD misdiagnosed one of the 80 malignant lesions as benign (sensitivity, 99%). OTD specificity was 93% for the dermoscopically benign lesions, 73% for all lesions included in the study and 36% for the clinically suspicious but histopathologically benign lesions. CONCLUSIONS: High sensitivity and specificity, as provided by OTD in this preliminary study, would help PCPs reduce the number of referrals for dermatology consultation, excision or biopsy. Further studies are planned for screening patients in a primary care setting, with comparisons of OTD results with biopsy or dermoscopy results.

Optical detection and monitoring of pigmented skin lesions.

A method is presented for discriminating between malignant and benign pigmented skin lesions based on multispectral and multi-angle images. It is discussed how to retrieve maps of physiology properties and morphometric parameters from recorded images using a bio-optical model, radiative transfer calculations, and nonlinear inversion, and how to employ automated zooming to extract lesion and surrounding masks. Training and validation of a classification scheme for separation between benign and malignant tissue yielded sensitivity/specificity ranging from 97%/97% for application to a small dataset comprised of lesions not used for training and validation to 99%/93% for application to a larger dataset.

Long-term comparisons of UV index values derived from a NILU-UV instrument, NWS, and OMI in the New York area.

A comparison is presented of UV index (UVI) values obtained under different cloud conditions from a Norwegian Institute for Air Research UV (NILU-UV) instrument, the ozone monitoring instrument (OMI) onboard the Aura satellite, and the National Weather Service (NWS) model for the time period of 2010-2014. The NILU-UV irradiance meter is a ground-based, multi-channel, moderate bandwidth filter instrument. UVI values derived from measurements by a NILU-UV instrument deployed in the New York area (40.74°N, -74.03°E) to monitor the erythemal UV radiation from 2010 to present is compared to UVI values derived from OMI measurements and predicted by the NWS model. OMI overestimated the UVI values by 13.06% for all cloud conditions compared with the UVI values derived from measurements by the NILU-UV instrument. The heavier the cloud cover, the higher the overestimation. The mean relative difference between the UVI derived from the NWS model and from NILU-UV measurements was 11.54%. The UVI prediction by NWS was also overestimated under cloudy conditions. Under overcast conditions the NWS predictions of UVI values differ significantly from those derived from NILU-UV measurements, yielding a correlation of only 0.8025 and a mean relative difference of 28.25%.

Comparisons of three NILU-UV instruments deployed at the same site in the New York area.

The Norwegian Institute for Air Research ultraviolet (NILU-UV) irradiance meter is a ground-based, multichannel, moderate bandwidth filter instrument that measures irradiances at ultraviolet (UV) and visible wavelengths with five channels in the UV (302, 312, 320, 340, and 380 nm) and one channel in the visible (400-700 nm) part of the spectrum. Minute-by-minute irradiances recorded in these channels are used to infer the total ozone column (TOC) amount, and a radiation modification factor (RMF) designed to have a value close to 100 under cloud-free conditions. The performance of three NILU-UV instruments deployed side-by-side in the New York area (40.74°N, -74.03°E) is assessed, and derived TOC values are compared with those derived from the ozone monitoring instrument (OMI) deployed on NASA's AURA satellite. Based on about three years of data, it was found that the three instruments yielded similar TOC values that were in close agreement with those derived from the OMI. The relative difference in TOC values derived from the three NILU-UV instruments was generally less than 2.5%. Cloud cover affects the accuracy of the inferred TOC, but reliable values can be obtained in the presence of clouds, although the accuracy deteriorates under heavy overcast conditions with RMF values smaller than 65 (low cloud transmittance).

Comparison of discrete ordinate and Monte Carlo simulations of polarized radiative transfer in two coupled slabs with different refractive indices.

A comparison is presented of two different methods for polarized radiative transfer in coupled media consisting of two adjacent slabs with different refractive indices, each slab being a stratified medium with no change in optical properties except in the direction of stratification. One of the methods is based on solving the integro-differential radiative transfer equation for the two coupled slabs using the discrete ordinate approximation. The other method is based on probabilistic and statistical concepts and simulates the propagation of polarized light using the Monte Carlo approach. The emphasis is on non-Rayleigh scattering for particles in the Mie regime. Comparisons with benchmark results available for a slab with constant refractive index show that both methods reproduce these benchmark results when the refractive index is set to be the same in the two slabs. Computed results for test cases with coupling (different refractive indices in the two slabs) show that the two methods produce essentially identical results for identical input in terms of absorption and scattering coefficients and scattering phase matrices.

Modeling optical properties of human skin using Mie theory for particles with different size distributions and refractive indices.

We used size distributions of volume equivalent spherical particles with complex refractive index to model the inherent optical properties (IOPs) in four different layers of human skin at ten different wavelengths in the visible and near-infrared spectral bands. For each layer, we first computed the size-averaged absorption coefficient, scattering coefficient, and asymmetry factor for the collection of particles in a host medium using Mie theory and compared these IOPs in each layer with those obtained from a bio-optical model (BOM). This procedure was repeated, using an optimization scheme, until satisfactory agreement was obtained between the IOPs obtained from the particle size distribution and those given by the BOM. The size distribution as well as the complex refractive index of the particles, obtained from this modeling exercise, can be used to compute the phase matrix, which is an essential input to model polarized light transport in human skin tissue.

Retrieval of the physiological state of human skin from UV-Vis reflectance spectra - a feasibility study.

We test the feasibility of using an accurate radiative transfer model for the coupled air-tissue system in conjunction with a classic inversion scheme based on Bayesian optimal estimation theory for retrieval of parameters describing the physiological state of human skin. To that end, we analyse ultraviolet and visible reflectance spectra from human skin measured before, immediately after, and on each day for two weeks after photodynamic treatment with the hexyl ester of ALA and exposure to red light (632 nm). For the first time, we show that it is possible to perform a simultaneous retrieval of the melanosome concentration in both the basal and the upper layers of the epidermis.

Time-resolved polarimetry over water waves: relating glints and surface statistics.

The phenomenon of sunglint, well known in satellite remote sensing, lacks a fundamental characterization under controlled laboratory conditions. Exploiting an apparatus specifically assembled for the purpose, we examine the signal collected by a photopolarimeter, pointed at a wavy water surface with measurable statistics and illuminated by a laser source. We also analyze the wave slope distributions, retrieved with an imaging system, and correlate them with the time series of glints. More particularly, we investigate the link between the occurrence of glints and that of the slopes from which they originate. In this context, the results obtained by applying the Hilbert-Huang transform technique to the slope time series are compared with those obtained through a traditional Fourier transform. This novel study first identifies the individual atomic glints as Fresnel reflection originating from a single wave facet. It then discusses the periodic character of a sequence of glints generated by a gravity wave state, as opposed to the erratic behavior of glints typical of capillary wave states. In mixed gravity-capillary conditions, it is shown that the glint properties are governed mainly by the capillary regime.

Use of shadows to retrieve water vapor in hazy atmospheres.

Techniques aimed at retrieving water vapor from satellite data of reflected near-infrared solar radiation have progressed significantly in recent years. These techniques rely on observation of water vapor attenuation of near-infrared solar radiation reflected by the Earth's surface. Ratios of measured radiances at wavelengths inside and outside water vapor absorbing channels are used for retrieval purposes. These ratios partially remove the dependence of surface reflectance on wavelength and are used to retrieve the total column water vapor amount. Hazy atmospheric conditions, however, introduce errors into this widely used technique. A new method based on radiance differences between clear and nearby shadowed surfaces, combined with ratios between water vapor absorbing and window regions, is presented that improves water vapor retrievals under hazy atmospheric conditions. Radiative transfer simulations are used to demonstrate the advantage offered by this technique.

Optical remote sensing of waters with vertical structure.

Optical remote sensing of ocean color is a well-established technique that is used to produce maps of marine constituents on a routine basis. Retrieval algorithms used to infer pigment concentrations from measurements of ocean color are usually based on the assumption that the upper ocean column is vertically homogeneous. However, stable stratification of the water column is often encountered in coastal waters and in fjords. This stratification is decisive for the initiation, maintainance, and species composition of phytoplankton blooms. Here we present an optical remote-sensing algorithm with the ability to resolve such a vertical structure of oceanic waters. The vertical structure is assumed to consist of two homogeneous layers with different concentrations of chlorophyll a. The algorithm is designed to determine the chlorophyll-a concentrations of the two layers as well as the thickness of the upper layer. These three parameters influence the ocean color and are simultaneously retrieved through an inverse-modeling technique. This technique consists of using radiative-transfer computations for a coupled atmosphere-ocean system to simulate radiances received in various bands of the satellite sensor and to compare these simulated results with measured radiances. The sum of absolute values of differences between simulated and measured radiances is minimized by use of an optimization algorithm, and the retrieved parameters are those that yield the minimum sum of differences between measured and simulated data. The optimization algorithm that we used in our study is the simulated annealing method, which is an extension of the downhill simplex algorithm. In this study the algorithm was tested on synthetic data generated by the forward model. The results indicate that it should be possible to retrieve vertical variations in the pigment concentration. The synthetic data were generated for spectral bands that coincide with those of the Medium Resolution Imaging Spectrometer sensor, which will be a part of the instrument package of the upcoming Environmental Satellite.

Application of the extended boundary condition method to particles with sharp edges: a comparison of two surface integration approaches.

We discuss, test, and compare two surface integration approaches that have been proposed for applying the extended boundary condition method (EBCM) to particles with sharp edges. One is based on approximating surface parameterization by a smooth function. By investigating the accuracy of this approach we find a quantitative condition for the radius of curvature of the approximate particle surface at the edge. The second approach is based on a special quadrature scheme for performing surface integration in the EBCM. For the simple test case of a cubic particle we find that the numerical advantages of the second method outweigh those of the first method, resulting in an overall reduction of computation time by a factor of 2. We conclude that the second method is preferable to the first when one is dealing with regularly shaped particles, for which the special quadrature scheme is reasonably simple to implement, and with particles with a relatively small number of sharp edges.

Application of the extended boundary condition method to homogeneous particles with point-group symmetries.

The numerical evaluation of surface integrals is the most time-consuming part of the extended boundary condition method (EBCM) for calculating the T matrix. An efficient implementation of the method is presented for homogeneous particles with discrete geometric symmetries and is applied to regular polyhedral prisms of finite length. For such prisms, an efficient quadrature scheme for computing the surface integrals is developed. Exploitation of these symmetries in conjunction with the new quadrature scheme leads to a reduction in CPU time by 3 orders of magnitude from that of a general EBCM implementation with no geometry-specific adaptations. The improved quadrature scheme and the exploitation of symmetries account for, respectively, 1 and 2 orders of magnitude in the total reduction of the CPU time. Test results for scattering by rectangular parallelepipeds and hexagonal plates are shown to agree well with corresponding results obtained by use of the discrete-dipole approximation. A model application for various polyhedral prisms is presented.

Validity of the diffusion approximation in bio-optical imaging.

Accurate numerical simulations based on rigorous radiative transfer theory are used to assess the validity of the diffusion approximation that is frequently used in bio-optical imaging. These simulations show that the error is large for a non-index-matched boundary between air and tissue. This weakness of the diffusion approximation underscores the need to understand how diffusion theory can be used to extract accurate values of tissue optical properties. A validity criterion for the diffusion approximation is established on the basis of the single-scattering albedo a and the asymmetry factor g for a slab with index-matched boundaries.

Image reconstructions in diffraction tomography from limited transmitted field data sets.

Diffraction tomography reconstructions of objects from limited transmitted field data sets are discussed together with theoretical analyses and results of numerical experiments. It is shown that limited data sets, representing only a small part of the complete data sets, can be used for reconstructions in diffraction tomography with satisfactory accuracy. We also find that, in diffraction tomography based on the hybrid filtered backpropagation and the first-Rytov approximation, the use of limited data sets can provide a larger range of validity than the use of complete data sets, the reason being that limited data sets pose less-severe phase-unwrapping problems.

Scattering of electromagnetic waves by spheroidal particles: a novel approach exploiting the T matrix computed in spheroidal coordinates.

A method other than the extended-boundary-condition method (EBCM) to compute the T matrix for electromagnetic scattering is presented. The separation-of-variables method (SVM) is used to solve the electromagnetic scattering problem for a spheroidal particle and to derive its T matrix in spheroidal coordinates. A transformation is developed for transforming the T matrix in spheroidal coordinates into the corresponding T matrix in spherical coordinates. The T matrix so obtained can be used for analytical calculation of the optical properties of ensembles of randomly oriented spheroids of arbitrary shape by use of an existing method to average over orientational angles. The optical properties obtained with the SVM and the EBCM are compared for different test cases. For mildly aspherical particles the two methods yield indistinguishable results. Small differences appear for highly aspherical particles. The new approach can be used to compute optical properties for arbitrary values of the aspect ratio. To test the accuracy of the expansion coefficients of the spheroidal functions for arbitrary arguments, a new testing method based on the completeness relation of the spheroidal functions is developed.

Optical remote sensing of marine constituents in coastal waters: a feasibility study.

Optical remote sensing of ocean color is a well-established technique for inferring ocean properties. However, most retrieval algorithms are based on the assumption that the radiance received by satellite instruments is affected only by the phytoplankton pigment concentration and correlated substances. This assumption works well for open ocean water but becomes questionable for coastal waters. To reduce uncertainties associated with this assumption, we developed a new algorithm for the retrieval of marine constituents in a coastal environment. We assumed that ocean color can be adequately described by a three-component model made up of chlorophyll a, suspended matter, and yellow substance. The simultaneous retrieval of these three marine constituents and of the atmospheric aerosol content was accomplished through an inverse-modeling scheme in which the difference between simulated radiances exiting the atmosphere and radiances measured with a satellite sensor was minimized. Simulated radiances were generated with a comprehensive radiative transfer model that is applicable to the coupled atmosphere-ocean system. The method of simulated annealing was used to minimize the difference between measured and simulated radiances. To evaluate the retrieval algorithm, we used simulated (instead of measured) satellite-received radiances that were generated for specified concentrations of aerosols and marine constituents, and we tested the ability of the algorithm to retrieve assumed concentrations. Our results require experimental validation but show that the retrieval of marine constituents in coastal waters is possible.

Radiative transfer in nonuniformly refracting layered media: atmosphere-ocean system.

We have applied the discrete-ordinate method to solve the radiative-transfer problem pertaining to a system consisting of two strata with different indices of refraction. The refraction and reflection at the interface are taken into account. The relevant changes (as compared with the standard problem with a constant index of refraction throughout the medium) in formulation and solution of the radiative-transfer equation, including the proper application of interface and boundary conditions, are described. Appropriate quadrature points (streams) and weights are chosen for the interface-continuity relations. Examples of radiative transfer in the coupled atmosphere-ocean system are provided. To take into account the region of total reflection in the ocean, additional angular quadrature points are required, compared with those used in the atmosphere and in the refractive region of the ocean that communicates directly with the atmosphere. To verify the model we have tested for energy conservation. We also discuss the effect of the number of streams assigned to the refractive region and the total reflecting region on the convergence. Our results show that the change in the index of refraction between the two strata significantly affects the radiation field. The radiative-transfer model we present is designed for application to the atmosphere-ocean system, but it can be applied to other systems that need to consider the change in the index of refraction between two strata.

Comparison of numerical models for computing underwater light fields.

Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

Derivation of total ozone abundance and cloud effects from spectral irradiance measurements.

We describe a method to infer total ozone abundance and effective cloud transmission from global (diffuse plus direct) spectral irradiance measurements taken at the Earth's surface. The derivation of total ozone abundance relies on the comparison of measured irradiance ratios at two wavelengths in the UV part of the spectrum with a synthetic chart of this ratio computed for a variety of ozone abundances. One of these wavelengths should be appreciably absorbed by ozone (e.g., 305 nm) compared with the other one (e.g., 340 nm). This synthetic ratio (and therefore also the inferred total ozone abundance) is insensitive to the value of the surface albedo used in the model computations. Comparison with independent in situ and remote (from ground and space) determinations of total ozone abundance shows that measurements of global irradiances provide a reliable means of inferring the total column ozone amount for clear as well as cloudy sky conditions. Computer simulations are used to demonstrate that the ozone abunance inferred from global irradiance measurements is quite insensitive to cloud effects, whereas the use of the scattered irradiance only or the zenith sky intensity (measured routinely in the Dobson network on overcast days) requires substantial corrections for cloud effects. Effective cloud transmission is estimated from the data by comparing the measured irradiance at a wavelength where ozone absorption is minimal (e.g., 350 nm) to the clear-sky value. Irradiances generated by a plane-parallel radiation model as a function of cloud optical thickness are used to estimate an equivalent stratified cloud optical depth. These estimates of cloud transmission and optical depth are sensitive to ground reflection, implying that the accurate determination of cloud attenuation requires precise knowledge of the surface albedo.