Showing papers on "Spectroradiometer published in 2018"

Radiometric block adjustment of hyperspectral image blocks in the Brazilian environment

Abstract: The objective of this investigation was to study and optimize a hyperspectral unmanned aerial vehicle (UAV)-based remote-sensing system for the Brazilian environment. Comprised mainly of forest and sugarcane, the study area was located in the western region of the State of Sao Paulo. A novel hyperspectral camera based on a tunable Fabry–Perot interferometer was mounted aboard a UAV due to its flexibility and capability to acquire data with a high temporal and spatial resolution. Five approaches designed to produce mosaics of hyperspectral images, which represent the hemispherical directional reflectance factor of targets in the Brazilian environment, are presented and evaluated. The method considers the irradiance variation during image acquisition and the effects of the bidirectional reflectance distribution function. The main goal was achieved by comparing the spectral responses of radiometric reference targets acquired with a spectroradiometer in the field with those produced by the five different approaches. The best results were achieved by correcting the bidirectional reflectance distribution function effects and by applying a least squares method to a radiometric block adjustment using only the image data, thereby achieving a root mean square error of 11.35%.

Assessing scale‐wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance

Abstract: Forest canopies create dynamic light environments in their understorey, where spectral composition changes among patterns of shade and sunflecks, and through the seasons with canopy phenology and sun angle. Plants use spectral composition as a cue to adjust their growth strategy for optimal resource use. Quantifying the ever-changing nature of the understorey light environment is technically challenging with respect to data collection. Thus, to capture the simultaneous variation occurring in multiple regions of the solar spectrum, we recorded spectral irradiance from forest understoreys over the wavelength range 300-800 nm using an array spectroradiometer. It is also methodologically challenging to analyze solar spectra because of their multi-scale nature and multivariate lay-out. To compare spectra, we therefore used a novel method termed thick pen transform (TPT), which is simple and visually interpretable. This enabled us to show that sunlight position in the forest understorey (i.e., shade, semi-shade, or sunfleck) was the most important factor in determining shape similarity of spectral irradiance. Likewise, the contributions of stand identity and time of year could be distinguished. Spectra from sunflecks were consistently the most similar, irrespective of differences in global irradiance. On average, the degree of cross-dependence increased with increasing scale, sometimes shifting from negative (dissimilar) to positive (similar) values. We conclude that the interplay of sunlight position, stand identity, and date cannot be ignored when quantifying and comparing spectral composition in forest understoreys. Technological advances mean that array spectroradiometers, which can record spectra contiguously over very short time intervals, are being widely adopted, not only to measure irradiance under pollution, clouds, atmospheric changes, and in biological systems, but also spectral changes at small scales in the photonics industry. We consider that TPT is an applicable method for spectral analysis in any field and can be a useful tool to analyze large datasets in general.

Experimental Results of the Balloon-Borne Spectral Camera Based on Ghost Imaging via Sparsity Constraints

Abstract: The spectral camera based on ghost imaging via sparsity constraints (GISC spectral camera) is a phase modulated compressive snapshot spectral imager. It makes use of the second-order intensity correlation of the light field to resolve the spatial and spectral information. In this paper, an optical design for GISC spectral camera which aims to obtain the desired spatial and spectral resolution is presented. A system calibration strategy based on few-mode optical fiber and monochrometer is developed. The snapshot spectral imaging experiments for the test targets and natural scenes are conducted using the prototype of GISC spectral camera loaded on the tethered balloon. The result of the spatial resolution, linearity, and spectra reconstruction error of the prototype is quantitatively evaluated. The distinguishable size at the distance of 1 km is around 0.34 m. The linearity is higher than 0.99 among the wavelength channels from 410 to 640 nm. The reconstructed spectra of eight color targets are compared with those measured by a commercial spectroradiometer. The average relative root mean squared error of the reconstructed spectra is 0.65.

Potential Utility of Spectral Angle Mapper and Spectral Information Divergence Methods for mapping lower Vindhyan Rocks and Their Accuracy Assessment with Respect to Conventional Lithological Map in Jharkhand, India

Abstract: In this study, we have evaluated the potential use of spectral mapping algorithms in deriving spectrolithological maps of metasedimentary rocks of Vindhyan group of rocks. In this regard, we have processed visible near infrared (VNIR) and shortwave infrared (SWIR) bands of Advanced Speceborne Thermal Emission and Reflection Radiometer (ASTER) data using similarity based spectral mapping algorithms such as spectral angle mapper (SAM) and spectral information divergence (SID). Laboratory spectra were collected by Fieldspec 3© spectroradiometer for main rock types of study area and the spectra were resampled to ASTER bandwidth to compare laboratory spectra with image spectra of respective rocks. Overall matching of image spectra of rocks with their ASTER resampled laboratory counterparts justified the spectral integrity of these rocks on the image. Therefore, image spectra of rocks were used as end member for deriving spectral maps using SAM and SID method. These maps were compared with the conventional field based lithological map (consequently updated using ASTER false colour image composite and band ratio images). SAM spectral map had over all accuracy of 67.41% and the SID map had overall accuracy of 69.67%. Present study has brought out the fact that spectral mapping algorithms would be useful in deriving moderate accuracy lithological maps even if the sedimentary rocks are of close mineralogy and these rocks have very close reflectance spectra within the spectral bandwidth of ASTER sensor. Spectral maps corroborate well with the discrete geochemical data.

Global spectral irradiance array spectroradiometer validation according to WMO

Abstract: Solar spectral irradiance measured by two recently developed array spectroradiometers (called UV-BTS and VIS-BTS) are compared to the results of a scanning double monochromator system which is certified as a travelling reference instrument by the Network for the detection of atmospheric composition change (NDACC) and fulfils the specifications of S-2 UV instruments of the world meteorological organization (WMO). The comparison took place between 15 and 18 May 2017 at the Institute of Meteorology and Climatology of the University of Hanover (IMuK) between 4:00 and 17:00UTC. The UV-BTS array spectroradiometer is equipped with special hardware to significantly reduce internal stray light which has been the limiting factor of many array spectroradiometers in the past. It covers a wavelength range of 200 nm–430 nm. The VIS-BTS covers a wider spectral range from 280 nm up to 1050 nm, and stray light reduction is achieved by mathematical methods. For the evaluation, wavelength integrated quantities and spectral global irradiance are compared. The deviation for UV index measured by the UV-BTS, is within ±1% for solar zenith angles (SZA) below 70° and increased to a maximum of ±3% for SZA between 70° and 85° when synchronisation between measurements was possible. The deviation of global spectral irradiance is smaller ±2.5% in the spectral range from 300 nm to 420 nm (evaluated for SZA < 70°). The VIS-BTS achieved the same deviation for blue light hazard as the UV-BTS for the UV index. The evaluations of global spectral irradiance data of the VIS-BTS show a deviation smaller than ±2% in the spectral range from 365 nm to 900 nm (evaluated for SZA < 70°). Below 365 nm, the deviation rises up to ±7% at 305 nm due to remaining stray light. The agreement within the limited time of the intercomparison is considered to be satisfactory for a number of applications and provides a good basis for further investigations.

Spectral signatures of submicron scale light-absorbing impurities in snow and ice using hyperspectral microscopy

Abstract: Light-absorbing impurities (LAI) can darken snow and ice surfaces, reduce snow/ice albedo and accelerate melt. Efforts to allocate the relative contribution of different LAI to snow/ice albedo reductions have been limited by uncertainties in the optical properties of LAI. We developed a new method to measure LAI spectral reflectance at the submicron scale by modifying a Hyperspectral Imaging Microscope Spectrometer (HIMS). We present the instrument's internal calibration, and the overall small influence of a particle's orientation on its measured reflectance spectrum. We validated this new method through the comparison with a field spectroradiometer by measuring different standard materials. Measurements with HIMS at the submicron scale and the bulk measurements of the same standard materials with the field spectroradiometer are in good agreement with an average deviation between the spectra of 3.2% for the 400–1000 nm wavelength range. The new method was used (1) to identify BC (black carbon), mineral dust including hematite and the humic substances present in an environmental sample from Plaine Morte glacier and (2) to collect the individual reflectance spectra of each of these types of impurity. The results indicate that this method is applicable to heterogeneous samples such as the LAI found in snow and ice.

A multidirectional spectral measurement method and instrument to investigate non-image-forming effects of light

Abstract: Light directionality, spectrum and relevant radiometric or photometric quantity are believed to be essential factors influencing the magnitude of non-image-forming effects. In this paper, a measurement method and an instrument (the multidirectional spectroradiometer - MuS) is proposed, which considers different light incidents and spectra simultaneously, therefore enabling the measurement of light characteristics relevant for non-image-forming effects. The MuS consists of four spectroradiometers measuring in different directions. Four spectrometers using optical fibers were configured to measure the spectral irradiance within a wavelength range from 200-1100 nm with ∼5.7 nm pixel resolution. Application of the MuS facilitates a better understanding of the non-image-forming light characteristics of space. The MuS was tested with stable electric lighting as well as dynamic daylight conditions and proved reliable to perform continuous spectral measurements for different directions simultaneously. Results of the test measurements in dynamic daylight conditions show differences in the spectral distribution and measured irradiance of up to 30 times in different directions.

Hyperspectral radiometer for automated measurement of global and diffuse sky irradiance

Abstract: An automated hyperspectral radiometer for the measurement of global and diffuse sky irradiance, SkySpec, has been designed for providing the SMEAR-Estonia research station with spectrally-resolved solar radiation data The spectroradiometer has been carefully studied in the optical radiometry laboratory of Tartu Observatory, Estonia Recorded signals are corrected for spectral stray light as well as for changes in dark signal and spectroradiometer spectral responsivity due to temperature effects Comparisons with measurements of shortwave radiation fluxes made at the Baseline Surface Radiation Network (BSRN) station at Toravere, Estonia, and with fluxes simulated using the atmospheric radiative transfer model 6S and Aerosol Robotic Network (AERONET) data showed that the spectroradiometer is a reliable instrument that provides accurate estimates of integrated fluxes and of their spectral distribution The recorded spectra can be used to estimate the amount of atmospheric constituents such as aerosol and column water vapor, which are needed for the atmospheric correction of spectral satellite images

Design of a Novel Spectral Albedometer for Validating the MODerate Resolution Imaging Spectroradiometer Spectral Albedo Product

Abstract: Land surface shortwave broadband albedo is a key parameter in general circulation models and surface energy budget models. Multispectral satellite data are typically used to generate broadband albedo products in a three-step process: atmospheric correction, for converting the top-of-atmosphere observations to surface directional reflectance; angular modeling, for converting the surface directional reflectance to spectral albedo of each individual band; and finally, narrowband-to-broadband conversion, for transforming the spectral albedos to broadband albedos. Spectroradiometers can be used for validating surface directional reflectance products and pyranometers or broadband albedometers, for validating broadband albedo products, but spectral albedo products are rarely validated using ground measurements. In this study, we designed a new type of albedometer that can measure spectral albedos. It consists of multiple interference filters and a silicon detector, for measuring irradiance from 400–1100 nm. The linearity of the sensors is 99%, and the designed albedometer exhibits consistency up to 0.993, with a widely-used commercial instrument. A field experiment for measuring spectral albedo of grassland using this new albedometer was conducted in Yudaokou, China and the measurements are used for validating the MODerate Resolution Imaging Spectroradiometer (MODIS) spectral albedos. The results show that the biases of the MODIS spectral albedos of the first four bands are −0.0094, 0.0065, 0.0159, and −0.0001, respectively. This new instrument provides an effective technique for validating spectral albedos of any satellite sensor in this spectral range, which is critical for improving satellite broadband albedo products.

Uncertainty analysis of total ozone derived from direct solar irradiance spectra in the presence of unknown spectral deviations

Abstract: . We demonstrate the use of a Monte Carlo model to estimate the uncertainties in total ozone column (TOC) derived from ground-based direct solar spectral irradiance measurements. The model estimates the effects of possible systematic spectral deviations in the solar irradiance spectra on the uncertainties in retrieved TOC. The model is tested with spectral data measured with three different spectroradiometers at an intercomparison campaign of the research project “Traceability for atmospheric total column ozone” at Izana, Tenerife on 17 September 2016. The TOC values derived at local noon have expanded uncertainties of 1.3 % (3.6 DU) for a high-end scanning spectroradiometer, 1.5 % (4.4 DU) for a high-end array spectroradiometer, and 4.7 % (13.3 DU) for a roughly adopted instrument based on commercially available components and an array spectroradiometer when correlations are taken into account. When neglecting the effects of systematic spectral deviations, the uncertainties reduce by a factor of 3. The TOC results of all devices have good agreement with each other, within the uncertainties, and with the reference values of the order of 282 DU during the analysed day, measured with Brewer spectrophotometer #183.

Advantages of Measuring the Q Stokes Parameter in Addition to the Total Radiance I in the Detection of Absorbing Aerosols

Abstract: A simple but novel study was conducted to investigate whether an imager-type spectroradiometer instrument like MODIS, currently flying on board the Aqua and Terra satellites, or MERIS, which flew on board Envisat, could detect absorbing aerosols if they could measure the $Q$ Stokes parameter in addition to the total radiance $I$, that is if they could also measure the linear polarization of the light. Accurate radiative transfer calculations were used to train a fast neural network forward model, which together with a simple statistical optimal estimation scheme was used to retrieve three aerosol parameters: aerosol optical depth at 869 nm, optical depth fraction of fine mode (absorbing) aerosols at 869 nm, and aerosol vertical location. The aerosols were assumed to be bimodal, each with a lognormal size distribution, located either between 0 and 2 km or between 2 and 4 km in the Earth's atmosphere. From simulated data with 3\% random Gaussian measurement noise added for each Stokes parameter, it was found that by itself the total radiance $I$ at the nine MODIS VIS channels was generally insufficient to accurately retrieve all three aerosol parameters ($\sim$ 15\% to 37\% successful), but that together with the $Q$ Stokes component it was possible to retrieve values of aerosol optical depth at 869 nm to $\pm$ 0.03, single-scattering albedo at 869 nm to $\pm$ 0.04, and vertical location in $\sim$ 65\% of the cases. This proof-of-concept retrieval algorithm uses neural networks to overcome the computational burdens of using vector radiative transfer to accurately simulate top-of-atmosphere (TOA) total and polarized radiances, enabling optimal estimation techniques to exploit information from multiple channels. Therefore such an algorithm could, in concept, be readily implemented for operational retrieval of aerosol and ocean products from moderate or hyperspectral spectroradiometers.

LED-based UV source for monitoring spectroradiometer properties

Abstract: A compact and stable UV monitoring source based on state-of-the-art commercially available ultraviolet light emitting diodes (UV-LEDs) has been developed. It is designed to trace the radiometric stability—both responsivity and wavelength scale—of array spectroradiometers measuring direct solar irradiance in the wavelength range between 300 nm and 400 nm. The spectral irradiance stability of the UV-LED-based light source observed in the laboratory after seasoning (burning-in) the individual LEDs was better than 0.3% over a 12 h period of continuous operation. The integral irradiance measurements of the source over a period of several months, where the UV-LED source was not operated continuously between the measurements, showed stability within 0.3%. In-field measurements of the source with an array spectroradiometer indicated the stability of the source to be within the standard uncertainty of the spectroradiometer calibration, which was within 1% to 2%.

Efficient, Real-Time Global Spectral and Broadband Irradiance Acquisition

Abstract: Two global solar spectral irradiance meters (SolarSIM-Gs) were deployed and tested at the National Renewable Energy Laboratory in Golden, USA from 18 September to 6 October 2017. The instruments were installed in the global horizontal orientation at the Solar Radiation Research Laboratory and their measurements were compared against colocated secondary reference spectroradiometers and secondary standard pyranometers. The SolarSIM-Gs’ spectral global horizontal irradiance (GHI) accuracy was shown to be $\lt 5$% on average per wavelength in the 290-1650 nm range for over 1,900 analyzed spectra. The broadband GHI measurement accuracy for two instruments was shown to have mean bias and standard deviation of 0.97, 1.23 W/m2 and 5.27, 5.65 W/m2, respectively, for over 10,000 analyzed data points.

Langley Calibration Analysis of Solar Spectroradiometric Measurements: Spectral Aerosol Optical Thickness Retrievals

Abstract: Aerosol optical thickness (tau(aer))) is a fundamental parameter for analyzing aerosol loading and associated radiative effects. Tau(aer) can constrain many inversion algorithms using passive/active sensor measurements to retrieve other aerosol properties and/or the abundance of trace gases. In the next wave of spectroradiometric observations from geostationary platforms, we envision that a strategically distributed network of robust, well-calibrated ground-based spectroradiometers will comprehensively complement spaceborne measurements in spectral and temporal domains. Spectral tau(aer) can be accurately obtained from direct-Sun measurements based on the Langley calibration method, which allows for the analysis of distinct spectral features of the calibration results. In this study, we present a spectral tau(aer) retrieval algorithm for an in-house developed, field deployable spectroradiometer instrument covering wavelengths from ultraviolet to near infrared (UV-Vis-NIR). The spectral total optical thickness obtained from the Langley calibration method is partitioned into molecular and particulate components by utilizing a least-squares method. The resulting high temporal-resolution tau(aer) and Angstrom Exponent can be used effectively for cloud screening. The new algorithm was applied to months-long measurements acquired from the rooftop at NASA Goddard Space Flight Center's Building 33. The retrieved tau(aer) demonstrated excellent agreement with those from well-calibrated Aerosol Robotic Network (AERONET) sunphotometers at all overlapping wavelengths (correlation coefficients higher than 0.98). In addition, empirical stray light corrections considerably improved tau(aer) retrievals at short wavelengths in the UV. The continuous spectrum of tau(aer) from UV-Vis-NIR spectroradiometers is expected to provide more informative constraints for retrieval of additional aerosol properties such as refractive indices, size, and bulk vertical distribution.

Using a MODIS Index to Quantify MODIS-AVHRRs Spectral Differences in the Visible Band

Abstract: Spectral band differences are a critical issue for progressing into an integrated earth observation framework and in particular, in sensor intercalibration. The differences are currently normalized using spectral band adjustment factor (SBAF) that is generated from hyperspectral data. In this context, the current study proposes a method for calculating moderate-resolution imaging spectroradiometer (MODIS)-advanced very high resolution radiometers (AVHRRs) SBAF in the visible band, using the MODIS surface reflectance data. The method involves a uniform ratio index calculated using the MODIS 552-nm and 645-nm bands, and a sensor-specific quadratic equation, producing SBAF data at 500-m spatial resolution. The calculated SBAFs are in good agreement at site scale with literature reported data (relative error 1.0) over densely vegetated areas and extremely low over deserts and barren lands (0.96–0.98), indicative of large MODIS-AVHRRs differences. Deserts show temporally stable SBAF values, while still suffer from intra-annual BRDF effects and short-term cloud contamination. By means of daily MODIS data, the proposed method can produce ongoing SBAF data at a spatial scale that is comparable to AVHRRs. It increases the sampling of MODIS-AVHRRs image pairs for intercalibration, and offers insight into spectral band conversion, finally contributing to an integrated earth observation at moderate spatial resolutions.

New algorithms for estimating chlorophyll-a in the Spanish waters of the Western Mediterranean Sea from multiplatform imagery

Abstract: The utility of global and regional algorithms for retrieving surface chlorophyll-a values from satellite images of MODIS (Moderate-Resolution Imaging Spectroradiometer) onboard Aqua and Terra Satel...

Direct Spectral Irradiance Measurements from Rotating Shadowband EKO Grating Spectroradiometer

Abstract: In this paper, we explore a rotating shadow band (RSB) configuration for the EKO MS-711 spectroradiometer to measure the global, diffuse and direct components of spectral irradiance. An RSB configuration allows lowering the costs associated with the instrumentation and maintenance required to measure the components of spectral irradiance. Since only one spectroradiometer is used for the measurement of the spectral irradiance components, discrepancies associated to sensor calibration can be minimized. This work presents a study to validate the EKO RSB spectroradiometer accuracy in measuring the direct normal irradiance (DNI). A comparison is made between the measurements performed with the RSB spectroradiometer and a collimated spectroradiometer in Mauna Loa Observatory. The results of the comparison show an agreement within 2% to 5% between the DNI estimated by the RSB and collimation configurations for solar zenith angles smaller than 70 °. Larger deviations approximated to 10% are found for larger angles of incidence.

Radiometric Evaluation of SNPP VIIRS Band M11 via Sub-Kilometer Intercomparison with Aqua MODIS Band 7 over Snowy Scenes

Abstract: A refined intersensor comparison study is carried out to evaluate the radiometric stability of the 2257 nm channel (M11) of the first Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. This study is initiated as part of the examination into the performance of key shortwave infrared (SWIR) bands for SNPP VIIRS ocean color data processing and applications, with Band M11 playing key role over turbid and inland waters. The evaluation utilizes simultaneous nadir overpasses (SNOs) to compare SNPP VIIRS Band M11 against Band 7 of the MODerate-resolution Imaging Spectroradiometer (MODIS) in the Aqua satellite over concurrently observed scenes. The standard result of the radiance comparison is a seemingly uncontrolled and inconsistent time series unsuitable for further analyses, in great contrast to other matching band-pairs whose radiometric comparisons are typically stable around 1.0 within 1% variation. The mismatching relative spectral response (RSR) between the two respective bands, with SNPP VIIRS M11 at 2225 to 2275 nm and Aqua MODIS B7 at 2125 to 2175 nm, is demonstrated to be the cause of the large variation because of the different dependence of the spectral responses of the two bands over identical scenes. A consistent radiometric comparison time series, however, can be extracted from SNO events that occur over snowy surfaces. A customized selection and analysis procedure successfully identifies the snowy scenes within the SNO events and builds a stable comparison time series. Particularly instrumental for the success of the comparison is the use of the half-kilometer spatial resolution data of Aqua MODIS B7 that significantly enhances the statistics. The final refined time series of Aqua MODIS B7 radiance over the SNPP VIIRS M11 radiance is stable at around 0.39 within 2.5% showing no evidence of drift. The radiometric ratio near 0.39 suggests the strong presence of medium-grained snow of a mixed-snow condition in those SNO scenes leading to successful comparison. The multi-year stability indicates the correctness of the on-orbit RSB calibration of SNPP VIIRS M11 whose result does not suffer from long-term drift.

Uncertainty evaluation and propagation for spectral measurements

Abstract: The measurement of the spectral power distribution (SPD) of a radiation source by array spectroradiometers is a technique that is widely used. In many applications, quantities that are derived from the SPD by a weighted integral over a wavelength interval are of interest. These integral quantities ought to be accompanied by a reliable uncertainty statement, for example, to assess conformity with prescribed limits or in order to judge the consistency of results obtained at different laboratories. We have developed a generally applicable Monte Carlo procedure for evaluating the uncertainty of spectral measurements. The procedure naturally accounts for correlations in the SPD which turn out to be crucial. Means are provided to handle and transfer these large-scale correlation matrices easily. The proposed approach is illustrated by the determination of the SPD of colored LEDs from array spectroradiometer measurements, together with the derived CIE 1931 color coordinates. MATLABTM software implementing the proposed analysis procedure is made available.

Determination of rubber-tree clones leaf diseases spectral using Unmanned Aerial Vehicle compact sensor

Abstract: Currently, one of the remote sensing platform that adequatly been used is Unmanned Aerial Vehicle (UAV) which suitable in monitoring and mapping for agriculture sector at large area payload by compact sensor. Thus, this study is deploy the UAV compact sensor to identify the characteristics of rubber tree clone leaf diseases based on two groups of spectral wavelength which is visible (RGB: 0.4 µm – 0.7 µm) and near infrared (NIR: 0.7µm – 2.0 µm), respectively. Spectral obtained using UAV platform is then to be validated with ground observation handheld spectroradiometer. Eight types of rubber tree clones leaf at three different conditions (healty, unhealty and severe) were randomly selected within the 9.4 hectare Experimental Rubber Plot, Rubber Research Institute of Malaysia (RRIM), Kota Tinggi, Johor whereby consist RRIM 2000 series, RRIM 3000 series and PB series, respectively. As a result, this study has found that the spectral trend based on UAV compact sensor from eight different types of rubber tree clones leaf shows the similarity to the general basic vegetation spectral in transition from blue to NIR spectral. However, referring to spectral obtained from handheld spectroradiometer there is no drastically changes of spectral in visible region but drastically increase in NIR region. Thus, this study has conclude that the spectral signature characteristics for healthy, unhealthy and severe for leaf diseases from every single rubber tree clones can be identified obviously in NIR region using UAV compact sensor.

Ultraviolet spectral irradiance measurements: an intercomparison of spectroradiometers in laboratory combined with a workplace field test

Abstract: Ultraviolet (UV) irradiance measurements are usually associated with large inaccuracies and uncertainties complicating the comparability of corresponding measurement equipment and its data. For supervisors or safety experts, though, it is mandatory to measure reliable UV irradiances with regard to occupational safety regulations. The following work aims at an intercomparison of five different types of spectroradiometers regarding their wavelength alignment and irradiance accuracy in the UV spectral region that provides the reader with information on how to determine and compare the measurement accuracies of their own devices. In a first step, measurements of two UV irradiance standards, a deuterium and a halogen lamp, were carried out. The percentage deviations of the measured total UV irradiances from their calibrated ones are smaller than ±10% for all spectroradiometers. The quality of wavelength accuracy as well as of the spectral bandwidth, both investigated by means of a low pressure mercury argon lamp are consistent with those stated by the manufacturers. In a second step, UV radiation from a metal active gas welding arc was examined at three distances in combination with a variation of welding current and arc length to check the behaviour of UV irradiance accuracy in field. The overall averaged standard deviation of these field measurements for all CCD array detectors is given by ±8%. For high welding currents this accuracy decreases to ±21%.

The calibration of integrating sphere with spectroradiometer with traceability to goniophotometer

Abstract: This article has task to inform a reader calibration procedure of an integrating sphere with spectroradiometer as detector with traceability to goniophotometer and double monochromator. In the first part there is a description of each step of calibration including preparations for necessary measurement and equipment. In the second part there is a description of practical calibration in light laboratory at Brno University of Technology with results and recommendations. Last part concludes results of calibration along with uncertainty budget evaluation.

Uncertainty Propagation of Spectral Matching Ratios Measured Using a Calibrated Spectroradiometer

Abstract: The international standard IEC62670-3 (International Electrotechnical Committee) “Photovoltaic Concentrators (CPV) Performance Testing—Part 3—Performance Measurements and Power Rating” sets the guidelines for power measurements of a CPV device, both in indoor and outdoor conditions. When measuring in outdoor conditions, the acquired data have to be filtered a posteriori, in order to select only those points measured with ambient conditions close to the Concentrator Standard Operating Conditions (CSOC). The most stringent requirement to be met is related to the three Spectral Matching Ratios (SMR), which have all to be within the limit of 1.00 ± 0.03. SMR are usually determined by the ratio of the currents of component cells to monitor the outdoor spectral ratio conditions during the CPV device power measurements. Experience demonstrates that obtaining real world data meeting these strict conditions is very difficult in practice. However, increasing the acceptable range would make the entire filtering process less appropriate from a physical point of view. Given the importance of correctly measuring the SMR, an estimation of their associated measurement uncertainties is needed to allow a proper assessment of the validity of the 3% limit. In this study a Monte Carlo simulation has been used, to allow the estimation of the propagation of uncertainties in expressions having the and integral form. The method consists of applying both random and wavelength correlated errors to the measured spectra and to the measured spectral responses of the three CPV cell junctions, according to the measurement uncertainties of the European Solar Test Installation (ESTI). The experimental data used in this study have been acquired during clear sky conditions in May 2016, at ESTI’s facilities in Ispra, northern Italy (45°49′ N 8°37′ E).