Spectral Attenuation Measurements Research Articles

High-resolution spectral atmospheric attenuation measurement for solar power plants

One of the main challenges in solar tower renewable technologies is measurement of solar radiation attenuation at the plants at surface level. This paper describes an improved version of the optical spectrum analysis method for measuring solar radiation attenuation in real time at solar tower plants, as presented in a previous work. The new hardware design and calibration process are explained in detail. This new system has a resolution of 0.25 nm in the VIS range and a precision of 0.5% in real time, improving over the state of the art for the measurement of spectral atmospheric attenuation. These specifications allow the assessment of the contribution of different attenuation factors at surface level, such as absorbance peaks (Na, H, H2O) or scattering and the search for correlation with different weather conditions. They also enable experimental validation of atmospheric extinction simulation methods and their parameters. This work also includes the results of a 6 month-long campaign of solar weighted atmospheric extinction measurements at Atlantica's operating central receiver solar plant PS10 at Sanlúcar la Mayor (Seville, Spain). Fluctuations in the daily averaged transmittance greater than 10% are observed, which shows the importance of monitoring this parameter during the operation of solar plants.

The Degree of Linear Polarization for Suspended Particle Fields from Diverse Natural Waters

The Degree of Linear Polarization (DoLP) for unperturbed particle fields in waters from six diverse regions around the globe was measured with the custom Multi-Angle Scattering Optical Tool (MASCOT). DoLP here is defined as the ratio of two elements of Mueller scattering matrix, i.e., -M12/M11. Field sites covered inland waters, coastal oceans and open oceans, including both ocean color Case I and II water types. The angular shape of the measured particulate DoLP was analyzed in detail for each field site and for the ensemble average. Three parameters used to quantitatively characterize DoLP shape were the symmetry with respect to 90°, peak magnitude, and peak angle of measured DoLP angular curve. Vertical profiles of particulate DoLP were analyzed with maximum recorded depth of 111 m. Converse to Rayleigh scatterers, we found measured particulate DoLPs were not symmetric with respect to 90°. On average, DoLP peaks were shifted slightly toward larger angles, with most falling between estimated values of 90° and 95°. All particulate DoLP peak magnitudes generally varied within [0.6, 0.9]. Lorenz-Mie (homogeneous sphere) light scattering theory was used to construct a new inversion for bulk particulate refractive index from a lookup table based on DoLP and spectral attenuation measurements. We compared the Mie-DoLP-based particulate refractive index retrieval with the backscattering-based model from (Twardowski et al., J. Geophys. Res., 2001, 106(C7), 14,129–14,142). Particulate refractive index retrieved with the two models were in some cases comparable. At two of the six field sites we saw good agreement between the two models, whereas at another two field sites we observed large discrepancies between the two models. Further investigation on the choice of the modeled particle shapes and compositions may improve this retrieval approach. Results are compatible with previous studies on DoLPs in natural waters and comprehensive observations are provided on the particulate DoLP angular shape, vertical profile and global distributions that are important for future vector radiative transfer simulations. This study is relevant to future PACE polarimeters and associated remote retrieval of oceanic particle composition using polarimetry.

Measuring and Monte Carlo Modelling of X-Ray and Gamma-Ray Attenuation in Personal Radiation Shielding Protective Clothing.

A collection of personal protective equipment (PPE), suitable for use in case of accident in nuclear facilities or radiological emergencies, was gathered at the National Institute for Nuclear, Chemical and Biological Protection, Czech Republic. The shielding characteristics of the various PPE materials were measured via narrow geometry spectral attenuation measurements with point radionuclide sources covering a broad range of photon energies. Photon relative penetration and attenuation for relevant energies of the spectra were the principal experimentally determined quantities for tested PPE. Monte Carlo simulations in the MCNPX™ code were carried out to determine photon attenuation for respective energies in the tested PPE, and the results were compared to those determined experimentally. Energy depositions in a unit volume of an ORNL phantom were simulated in a radioactive aerosols atmospheric environment to determine effective doses both for the whole body and in various organs in the human torso during exposure to different dispersed radioactive aerosols while wearing one of the PPE protecting against X- and gamma-ray. This work aimed to determine the effective dose and its decrease for individual PPE protecting against X- and gamma-ray.

Particle dynamics in the rising plume at Piccard Hydrothermal Field, Mid‐Cayman Rise

AbstractProcesses active in rising hydrothermal plumes, such as precipitation, particle aggregation, and biological growth, affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments. However, observations to date of particle size distribution within these systems are both limited and conflicting. In a novel buoyant hydrothermal plume study at the recently discovered high‐temperature (398°C) Piccard Hydrothermal Field, Mid‐Cayman Rise, we report optical measurements of particle size distributions (PSDs). We describe the plume PSD in terms of a simple, power‐law model commonly used in studies of upper and coastal ocean particle dynamics. Observed PSD slopes, derived from spectral beam attenuation and laser diffraction measurements, are among the highest found to date anywhere in the ocean and ranged from 2.9 to 8.5. Beam attenuation at 650 nm ranged from near zero to a rarely observed maximum of 192 m−1 at 3.5 m above the vent. We did not find large (>100 μm) particles that would settle rapidly to the sediments. Instead, beam attenuation was well‐correlated to total iron, suggesting the first‐order importance of particle dilution, rather than precipitation or dissolution, in the rising plume at Piccard. Our observations at Piccard caution against the assumption of rapid deposition of hydrothermal, particulate metal fluxes, and illustrate the need for more particle size and composition measurements across a broader range of sites, globally.

Characterization of the sensitization degree in the AISI 304 stainless steel using spectral analysis and conventional ultrasonic techniques

Specimens of AISI 304 stainless steels exposed to sensitization and carbide solution heat treatment employing ultrasonic testing have been evaluated. The study involved measurements of the longitudinal wave velocity, attenuation and spectral analysis. Despite the large size difference between the ultrasonic wave length and the precipitated carbides, the results showed a clear attenuating effect in the sensitized specimens. This effect suggests a relation between carbide precipitation in the material and the attenuation coefficient of the ultrasonic wave. The attenuation increase is mainly attributed to the continuous distribution and possible coalescence of the carbide along the grain boundaries. Power spectra exhibit an increase of the amplitude at specific frequencies in specimens with the longest sensitization times. Ultrasonic velocity measurements did not provide significant information in order to predict any sensitization grade of the evaluated materials. The combined assessment by spectral analysis and attenuation measurements is discussed.

Properties of phosphorus-related defects induced by γ-rays and pulsed X-ray irradiation in germanosilicate optical fibers

Time dependent radiation-induced attenuation changes of germanosilicate (∼13 wt%) optical fibers have been measured under steady-state γ-ray and pulsed X-ray irradiations. Particularities of phosphorus (P)-codoped cladding fibers for these environments are presented. For each irradiation type, spectral attenuation measurements show that permanent radiation-induced losses at 1.55 μm are due to the infrared absorption band of P 1 centers. We assume that particular kinetics of recovery and an increase of attenuation measured for P-codoped fibers, after pulsed X-ray irradiation, are induced by a charge retrapping phenomenon from phosphorous oxygen hole (POH) centers to P 1 ones. With this mechanism, we explain the P-codoped fiber characteristics after both types of irradiation. Our study shows that the effect of P-related color centers is increased when the P-codoped cladding (∼0.3 wt%) contains also germanium (∼0.3 wt%).

Underwater light characterisation for correction of remotely sensed images

Objective measurement of habitat change using remote sensing requires processing of the images to enhance the bottom reflectance signal. This process typically uses correction techniques to remove the influence of the water column on bottom reflectance, and to enable the accurate correction of the imagery for varying bathymetry. Such correction measures depend on reliable estimates of water column light attenuation. An investigation into the spatial variation in attenuation in a typical tropical region was undertaken. Measurements of gross spatial variations in downwelling attenuation around San Andres and Old Providence islands in the western Caribbean were made using a PAR sensor. Measurements of specific attenuation were also made for blue, green and red light using filters fitted to the sensor. High spectral resolution attenuation measurements were also made using a spectroradiometer. Results showed a four-fold variation in light attenuation in shallow littoral regions alone. Spectral attenuation measurements suggested that this variation was largely the result of scattering by particulate matter rather than varying concentrations of dissolved yellow substances. These findings suggest that the results of studies where single measurements of 'average' attenuation have been used to depth-correct remotely sensed imagery should be interpreted with a high degree of caution. The paper goes on to show that simple models can be empirically obtained where attenuation can be spatially predicted with confidence, based on the variables of water depth, distance to and size of mangrove beds, and distance to and size of towns. The models obtained showed high statistical significance, with 89% and 78% of the spatial variation in attenuation explained for San Andres and Old Providence, respectively. It is postulated that the use of such approaches for the estimation of attenuation will lead to more accurate depth correction and hence improved interpretation of remotely sensed imagery for littoral regions.

Spectral light absorption and attenuation measurements from a towed undulating vehicle

Measurements from a nine-wavelength light absorption and attenuation instrument mounted on a towed, undulating vehicle (SeaSoar) capable of rapidly profiling the water column – complete up-down cycles in 1.5–12 min depending on the maximum sampling depth – are used to characterize strong spatial variations in the distributions of upper-ocean bio-optical properties. Water sampled from adjacent to the conductivity and temperature sensors located in the nose of the vehicle is pumped through the 25-cm dual optical flow tubes of a Western Environmental Technology Laboratories (WET Labs) ac-9 instrument mounted on top of SeaSoar. A three-stage algorithm for post-processing the optical data to insure high-quality measurements concurrent with conductivity–temperature–depth data is described. After synchronizing the optical, navigational and conductivity–temperature–depth data streams, the method relies on finding the optimal time delay between when a water parcel is sampled first by the temperature and conductivity sensors and then by the absorption and attenuation optics. After applying the calculated time-dependent lag, a correction for the dependence of light absorption on temperature and salinity and a scattering correction to absorption are made. The final processed optical data from SeaSoar compare well with the same parameters sampled by a slowly lowered vertical profiling package deployed from a nearby stationary vessel. A 2-h, 30-km long cross-shelf section consisting of 184 vertical profiles separated by 150–200 m reveals strong horizontal variations on short spatial scales (1 km or less) of the vertical distributions of bio-optical properties.

Consequences of radiation effects on pure-silica-core optical fibers used for Raman-scattering-based temperature measurements

Two types of pure-silica-core fibers (one low-OH, Al-jacketed, one medium-OH, polyimide jacketed) suitable for use as sensing fibers for Raman-scattering-based temperature measurements in nuclear environments have been subjected to gamma and fission reactor irradiation tests. Spectral attenuation measurements were performed between 500 and 1500 nm with samples kept at room temperature, 80 and 300/spl deg/C. The Al-jacketed fiber was developed for use under ionizing radiation and showed lower loss compared with the polyimide-jacketed fiber at room temperature under gamma irradiation. Both fiber types showed similar spectra at room temperature with the main part of the loss originating from a band tail extending from the ultraviolet. Thermal bleaching of the radiation-induced defects was found to be effective in both fiber types. At 80/spl deg/C the loss in both fibers was compounded of a band at 625 nm together with the band tail from the ultraviolet, which now had a strength several times lower compared with room-temperature irradiations. At 300/spl deg/C, both fibers exhibited similar low-loss spectra, except for the band at 625 nm which reached levels of approximately /spl sim/2000 dB/km at an accumulated dose of 2.8/spl times/10/sup 4/ Gy(SiO/sub 2/). In light of the experimental spectral findings, selection of suitable Raman-distributed temperature sensors for nuclear plants can be made.

Spectral Attenuation Measurements of Optical Fibers: Design of an Instrument based on a Pulsed-light Source

Spectral Attenuation Measurements of Optical Fibers: Design of an Instrument based on a Pulsed-light Source

対流圏工一ロゾルの放射効果の航空機観測

The size distribution and the complex refractive index of tropospheric aerosols have been retrieved from the simultaneous observations of solar radiation and aerosols described in Part I of this study (Asano and Shiobara, 1989). Three sets of observations over the Tsukuba area have been analyzed, for which the aerosol optical thicknesses were available from ground-based spectral attenuation measurements of direct solar radiation.The bimodal size distribution for tropospheric aerosols was approximated by a composite powerlaw distribution function. The parameters of the distribution function were determined by fitting the function to the columnar size distributions inverted from photometrically measured spectral optical thicknesses. Estimation was made of wavelength-mean effective values of the complex refractive index m*=mr-mii which optimally simulate the observed solar fluxes and heating rates for the total solar spectral region. In the simulated calculations, the observed profiles of aerosols and water vapor were considered. The single scattering properties were computed from Mie theory for the determined size distribution and for a reasonably assumed real part of the index of refraction of mr=1.52. The optimal value of mi=0.03×0.02 was estimated for the imaginary part of the index of refraction of the tropospheric aerosols.

Absorption characteristics of CO2 molecules dissolved in ZrF4-based fluoride glass

Absorption characteristics of CO2 molecules dissolved in ZrF4-based glass are measured using a Fourier transform spectral attenuation measurement system. Absorptions due to vibrational modes appear at 2353, 3606, and 3716 cm−1, respectively. The influence of the CO2 impurity on transmission loss is clarified. The interaction between dissolved CO2 molecules and the glass matrix is discussed on the basis of the measured spectra.

Spectral attenuation measurement for fluoride glass single-mode fibres by Fourier transform techniques

A new spectral attenuation measurement system using a Fourier transform spectrometer is developed. The Fourier transform spectral attenuation measurement (FT-SAM) system makes the signal acquisition time for a wide wave-length range of 1.0–4.2μ much shorter than measurement with a conventional dispersive spectral attenuation measurement system. The spectral attenuation for a fluoride glass single-mode fibre, with a minimum loss of 3±1 dB/km at 2.5μm, is measured with this system.

Determining the effective cutoff wavelength of single-mode fibers: An interlaboratory comparison

The National Bureau of Standards (NBS), in cooperation with the Electronic Industries Association, conducted an interlaboratory measurement comparison among six fiber manufacturers to determine the effective cutoff wavelength of single-mode fibers. Measurement techniques based on transmitted power were used to determine cutoff wavelength on four fibers designed for single-mode operation at 1300 nm. NBS also contributed results using a spectral near-field technique. One standard deviation measurement spreads for the various techniques range from 6 to 12 nm. With the appropriate data analysis, single bend attenuation and power step methods give the same results. Both techniques are easily implemented as extensions to the usual spectral attenuation measurement.

Comparative spectral attenuation measurements on optical fibres: final report by members of COST 208

Comparative spectral attenuation measurements on a graded-index fibre have been perfomed by nine participants, covering the range 700–1800 nm. Details of measurement equipment and techniques are being reported. Since the attenuation of a graded-index fibre depends on the energy distribution launched into the fibre, only careful control of this parameter leads to comparable results. A method was developed to check the energy distribution in the fibre over a broad spectral range. By this method agreement of attenuation values to within ± 5% from 800 to 1500 nm has been achieved. This is about half of the deviation found in other comparison measurements which are usually restricted to one or two wavelengths.

Spectral characterization and attenuation measurements in ultrasound

Various means of characterizing ultrasonic attenuation in tissue are reviewed. A simple method for estimating frequency-dependent attenuation via measurement of the zero crossing density of the signal is presented and validated. Both the effects of the frequency dependence of scatter and stochastic variability of the measurement are considered and discussed. Results of measurements made in phantoms, animals and humans are presented and compared to the theoretical model. The technique is shown to be technically feasible.

Spectral characterization and attenuation measurements in ultrasound.

Various means of characterizing ultrasonic attenuation in tissue are reviewed. A simple method for estimating frequency-dependent attenuation via measurement of the zero crossing density of the signal is presented and validated. Both the effects of the frequency dependence of scatter and stochastic variability of the measurement are considered and discussed. Results of measurements made in phantoms, animals and humans are presented and compared to the theoretical model. The technique is shown to be technically feasible.

Wavelength dependence of the numerical aperture of optical fibers

Wavelength resolved measurements of the numerical apertures and the profiles of graded index fibers have been performed by the far- and near-field methods, respectively. Quantitative results for the spectral dispersion of the numerical aperture of fibers are in very good agreement with theoretical results, which are obtained by using a 3-term Sellmeier series. Together with spectral attenuation measurements these results allow the approximate determination of fiber dopants.

Broadband spectral attenuation measurements on optical fibres: an interlaboratory comparison by members of COST 208

Comparative loss measurements on optical fibres are being performed by members of COST 208, and in this paper some preliminary results are reported. Full spectral attenuation measurements have been made, covering the range from 700 to 1800 nm, on graded-index and single-mode fibres. In the multimode fibre the measured loss depends on the energy distribution launched into the fibre. By ensuring similar energy distributions the deviations between loss spectra have been maintained to within about ± 5%. In single-mode fibre the launched power is very small so the transmitted intensity was closer to the noise limit of the detector. Differences between loss spectra amounted to about ± 8% in the low-loss ranges.

Aerosol Size Distributions Obtained by Inversions of Spectral Optical Depth Measurements

Columnar aerosol size distributions have been inferred by numerically inverting particulate optical depth measurements as a function of wavelength. An inversion formula which explicitly includes the magnitude of the measurement variances is derived and applied to optical depth measurements obtained in Tucson with a solar radiometer. It is found that the individual size distributions of the aerosol particles (assumed spherical), at least for radii ≳ 0.1 μm, fall into one of three distinctly different categories. Approximately 50% of all distributions examined thus far can best be represented as a composite of a Junge distribution plus a distribution of relatively monodispersed larger particles centered at a radius of about 0.5 μm. Scarcely 20% of the distributions yielded Junge size distributions, while 30% yielded relatively monodispersed distributions of the log-normal or gamma distribution types. A representative selection of each of these types will be presented and discussed. The sensitivity of spectral attenuation measurements to the radii limits and refractive index assumed in the numerical inversion will also be addressed.