Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.
We discuss a new versatile setup for goniometric measurements of spectral radiances with two modes of operation: (1) it can operate as a 2-D goniometer for measurements in a horizontal plane of the singly scattered radiance from particles in suspension and (2) it can be used as a 3-D goniometer for measuring spectral radiances over an entire hemisphere. In our setup, various kinds of light sources and detectors can easily be inserted. Among the detectors, a spectral imager is designed and used. Proper hardware and software is chosen so as to make our setup fully automated and easy to operate. We present results from two different investigations to demonstrate the utilization of our setup. The first investigation is concerned with measurements of the volume scattering function (VSF) over a large forward and backward angular range. Our experimental results for the VSF show good agreement with theoretical simulations. We also use our setup to obtain a series of 1-D angular spectral images of the skin on the dorsal side of a human hand in vivo by employing various illumination angles. Our setup provides a robust, highly automated, and flexible framework for carrying out goniometric measurements in a variety of applications.
Successful retrieval of surface properties from space is hampered by the presence of atmospheric aerosol particles that contribute significantly to the measured signal. Our ability to obtain reliable information about surface properties depends to a large extent on how well we can account for the influence of aerosols. The problem is complicated by the fact that aerosols often consist of a multicomponent mixture of particles with different chemical compositions and different affinities to water. For example, to predict how the optical properties of such particles change with increasing humidity, we must make assumptions about how the particles grow, change their refractive indices, and mix as a function of humidity. We discuss possible strategies for reliable atmospheric correction over dark as well as bright surfaces. Also, we discuss the role of realistic simulations of the radiative transfer process in the coupled atmosphere-ocean system in the solution of the inverse problem required to retrieve surface properties is also discussed.
Recent work has shown the need for accurate treatment of radiative transfer in ocean color retrieval. The plane-parallel coupled atmosphere-ocean discrete ordinate model CAO-DISORT has been used to investigate the validity of current approximative inverse methods and to study new techniques for improved ocean color retrieval. In this paper we show that CAO-DISORT is fully differentiable with respect to its input optical properties, so that we can define analytic Jacobians with respect to any profile element in the atmosphere and ocean. A single call to the linearized model will produce radiances and Jacobians at arbitrary optical depth and viewing geometry in either medium. The model also has a pseudo-spherical treatment for solar beam attenuation in a curved atmosphere. The linearized model can be used directly in iterative least-squares retrievals requiring forward model simulations of backscatter measurements and their parameter derivatives; there is no need for approximations involving an atmospheric correction. We demonstrate the model's new capability by performing closed-loop least squares fitting to simultaneously retrieve the aerosol optical thickness and marine chlorophyll concentration from a set of 6 synthetic measurements at SeaWifs wavelengths.
A new method for simultaneous retrieval of aerosol properties and marine constituents in turbid waters is described. This method is an extension to turbid waters of an approach developed previously for simultaneous retrieval of aerosol properties and chlorophyll concentrations in clear waters. This extension is accomplished by employing near-infrared (NIR) channels not available on the SeaWiFS and MERIS instruments to help retrieve aerosol parameters over turbid waters. Optimal estimation theory is used to retrieve in-water parameters from multi- and hyperspectral information. Both forward and inverse modeling strategies will be discussed, as well as the uniqueness of the solutions, the information content available in multi- and hyperspectral data, and the error analysis approach. Our results indicate that it is important to use forward models that accurately treat the radiative transfer in the coupled (combined) atmosphere-ocean system, and to carefully select the most suitable bio-optical models for the in-water inherent optical properties (IOPs). Synthetic data, as well as multi- and hyperspectral images of data obtained over clear as well as turbid waters, are used to test the validity of the new retrieval approach.
Retrieval of surface properties of highly reflecting targets such as snow and ice is a challenging problem due to the influence of aerosols
which varies considerably in space and time. Also, accounting for the bidirectional properties of a bright surface such as snow is very important for reliable retrievals. The main purpose of the work described in this paper is to explore the opportunities and possibilities offered by multi- and hyperspectral data such as those available provided by MODIS, GLI, the Advanced Land Imager (ALI), and Hyperion to retrieve reliable aerosol and surface properties. Over snow and ice surfaces these include aerosol optical depth and single scattering albedo, the mean size of snow grains and ice "particles" (inclusions), and the spectral and broadband snow/ice albedo. In particular the following question will be addressed: To what extent can multi- and hyperspectral data help improve our knowledge of snow and ice parameters that are important for understanding global climate change?
A number of remote sensing instruments with multi-spectral imaging capabilities (SeaWiFS, MODIS, GLI on ADEOS-II, and others) have recently been launched on earth-orbiting satellites or will soon be launched into space. Many of these sensors offer unique opportunities for studies of sea ice and ocean properties at high latitudes. There are a number of challenges associated with the inversion of data received from satellite such instruments in order to retrieve meaningful information. Here we discuss some of these challenges with emphasis on the derivation of sea ice and marine parameters from satellite data.
For optical satellite remote sensing of the marine arctic environment
it is essential to establish appropriate knowledge of its optical
properties. To that end we used a multi stream radiative transfer (RT)
code to study the penetration of spectral irradiances in a coupled
atmosphere-ice-ocean system. The code account for the change in the
refractive index at the atmosphere-ice interface. To validate and tune this coupled RT model we ran the code with input from in situ measurements of optical properties and compared the computed results with in situ spectral irradiance measurements. The field work was done in Kongsfjorden, Svalbard, and the measurements were performed for first year ice. One goal of this study was to establish a model for the optical properties of the marine arctic environment that can be used in the interpretation of satellite remote sensing data.
Successful retrieval of surface properties from space is hampered by the presence of atmospheric aerosol particles that contribute significantly to the measured signal. Our ability to obtain reliable information about surface properties depends to a large extent on how well we can account for the influence of aerosols. The problem is complicated by the fact that these aerosols often consist of a multi-component mixture of particles with different chemical compositions and different affinities to water. For example, in order to predict how the optical properties of such particles change with increasing humidity, we need to make assumptions about how the particles grow, change their refractive indices, and mix as a function of humidity. The purpose of this paper is to discuss possible strategies for reliable atmospheric correction over dark as well as bright surfaces. The role of realistic simulations of the radiative transfer process in the coupled atmosphere-surface system in order to solve the inverse problem required to retrieve surface properties will also be discussed.
At Stevens Institute of Technology, Hoboken, NJ we have operated a site with NILU-UV instruments for nearly two years. For most of this time only one instrument has been in operation, but we also have
data for extended periods of time when up to three instruments have been working in parallel. The site is in close proximity to New York City and it is equipped with basic radiation sensors in addition to the NILU-UV sensors. In a companion paper we present results from intercomparisons between filter-based instruments, such as the NILU-UV, and the Dobson and Brewer instruments. Here we describe our experience operating filter-based radiation instruments. In particular, we discuss data quality issues and describe how one can detect and correct for drift in filter-based instruments. We also investigate the effect of elevated detector temperatures due to over-heating of the instrument by solar radiation on very warm days. Our experience with the newer versions of the filter instruments is that most of them have only minor problems with filter drift over time, and that this drift (if any) is easily detectable and can be corrected for. A potential problem is that varying detector temperature can degrade the instrument performance. Since filter UV instruments are normally set to operate with detector temperatures much higher than ambient temperatures this is a minor issue for most locations, and one that can easily be prevented.
KEYWORDS: Ray tracing, Diffraction, Image quality, Point spread functions, Radio propagation, Electromagnetism, Lenses, Imaging systems, Monochromatic aberrations, Computing systems
Images formed by optical systems comprised of many elements are difficult to assess in a quantitative manner. Because the overall size of the system usually is very large compared to the wavelength, a full rigorous treatment based on numerical solutions of Maxwell+s equations is out of the question even with the fast computers of today. Although ray tracing is extremely important in optical design, ray methods alone can not be used for a quantitative assessment of image quality because these methods break down in the region of the image. Here we give a review of recent developments aimed at combining ray tracing and diffraction calculations to obtain an efficient yet accurate assessment of image quality. This hybrid procedure consists of two steps. Ray tracing is used in step 1 to propagate the field from a point source in object space to a reference plane behind the last optical surface in the system, and the first Rayleigh-Sommerfeld diffraction formula is used in step 2 to propagate the field throughout the focal region. Also, we consider cases of imaging by low f- number diffractive lenses where ray tracing must be replaced by more rigorous methods in step 1, and where asymptotic techniques can be used in step 2 to reduce the computation time significantly.
Optical remote sensing of ocean color is a well-established technique. But most algorithms developed hitherto have been based on the assumption that only the phytoplankton affect the optical properties of the ocean. Such algorithms are often based on assumptions that become questionable in coastal areas. The assumption of a near-infrared dark pixel in the satellite image, will no longer be valid, and the band-ratio technique used for computing the algae concentration will also become inaccurate. To overcome these limitations we have developed an inverse-modeling algorithm for retrieval of marine constituents. Here the determination of ocean color is based on a three-component optical model consisting of chlorophyll-a, suspended matter, and yellow substance. We also use one parameter to describe the thickness of the aerosol layer. A simulated-annealing optimization scheme is employed to minimize the difference between measured satellite data and corresponding simulated data obtained using a coupled atmosphere-ocean radiative transfer code. The same optimization method has also been applied to the problem of retrieving the algae concentration in waters with vertical structure. In this case the marine parameters of interest are the algae concentration in two different layers as well as the thickness of the first layer.
In this article we consider laboratory measurements of scattering by algae. We investigated the optical properties of a culture of the chlorophyte Chlamydomonas vectensis over almost two division cycles. The cell culture was synchronized which means that all cells were at the same stage in the growth cycle and thus divided at approximately the same time. By measuring the scattering properties of the synchronous culture during the full growth cycle, we get information about the influence of size changes of the individual cells as well as changes in the cell concentration. We measured of the scattering phase function in the interval from 1.7 degrees to 7.7 degrees at a wavelength of 633 nm using a CCD camera from Astrocam. In addition we measured the scattering phase function at 90 degrees, including the degree of polarization. We checked the experimental setup by doing a similar experiment in which we used monosized spheres from 'Dyno Particles AS,' so that the results can be compared with Mie calculations. The absorption and attenuation coefficients were measured by an AC9 instrument from WetLabs. The cell size was measured with a Coulter Counter, and the concentration was determined using a microscope. As expected, the optical properties were strongly dependent on the concentration of cells. Furthermore the shape of the scattering phase function and the degree of polarization were seen to depend not only on the concentration but also on the time of day.
The hybrid filtered backpropagation algorithm of diffraction tomography is known to give good reconstructions of 2D objects with a low contrast relative to the background medium. Here we show how the method can be extended (1) to reconstruct objects with rather large contrasts, and (2) to reconstruct 3D objects.
By using a new method of solving the radiative transfer equation, we calculate the diffuse (i.e. scattered) radiance due to a Gaussian beam incident on a slab of finite thickness filled with scattering particles. The radiance is calculated for several observation angles and at any point inside or at the boundaries of the slab, both for isotropically and weakly anisotropically scattering media.
In this paper we discuss diffraction effects on laser computed tomography and time-resolved imaging in a random medium. Our basic point is that light diffracted by an object embedded in a random medium contains useful and important information about its structure and therefore should be included in the imaging process. To support this point of view, we compute diffracted fields from opaque and semi-transparent cylindrical objects of various sizes. Based on these results we conclude that to improve imaging in random media, diffraction effects should be taken into account, even for cases in which the size of the observed object is much larger than the optical wavelength.
Optical diffraction tomography (ODT) is a novel imaging technique in which a transparent object is illuminated from many different directions. The object's complex refractive index distribution is reconstructed digitally from the scattered field data by means of a reconstruction algorithm. Preliminary experimental results are presented for the imaging of an object whose refractive index and shape are known a priori. The results show that both the refractive index and the shape may be reconstructed accurately by the proposed ODT technique.
Threshold criteria for the determination of width, thickness and shape of dielectric line structures with wedge shaped edges, negligible attenuation and small refractive index n (n = 1.02, tan 5 = 0.0004) are examined in the electromagnetic nearfield (distance ::: 0.L). It is demonstrated that height and position of extreme values in the polarization dependent amplitude and phase distributions are particularly suited as thresholds.
Two dterent methodsfor computing the ftracon OTF in complex optical systemsediscussed. The theoretical beis1 numerical aspects and computer imp''ementations e also dscussed. 1.
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