Showing papers on "Spectroradiometer published in 1995"

Comparing ground‐level spectrally resolved solar UV measurements using various instruments: A technique resolving effects of wavelength shift and slit width

Abstract: Spectrally resolved UV measurements are important for the study of biologically relevant UV in relation to changes in atmospheric parameters. The inter-comparison of spectral instruments is essential as measurement techniques and calibrations are not standardized. The differences in slit functions cause large spectral variations when comparing the spectral readings directly. The method described, which compares spectral readings using different instruments, corrects for differences of wavelength calibrations and slit functions, and does not require knowledge of additional atmospheric parameters and UV-transfer model calculations. The wavelength alignment has an accuracy of 0.02 nm over the wavelength interval from 300–400 nm, and a reproducibility of 0.01 nm. The robustness of the methods and reproducibility of results are shown in the evaluation of a seven day intercomparison campaign with three different scanning spectroradiometers.

A spectroradiometer for the measurement of direct and scattered solar irradiance from on-board the NASA ER-2 high-altitude research aircraft

Abstract: A compact, diode-array spectrophotometer has been designed to make measurements of the downward spectral flux on a horizontal surface, the limb brightness and the apparent brightness in the nadir direction from on-board the NASA ER-2 research aircraft. The instrument was included as part of the photochemical payload used for the Stratospheric Photochemistry, Aerosol and Dynamics Expedition (SPADE) of the NASA High-Speed Research Program. The spectrophotometer is based on a 1024-element, randomly-addressable Reticon photodiode array and makes measurements covering the 300 to 775 nm spectral region at a resolution of about 1 nm. Some aspects of the design and the performance of the instrument during SPADE will be presented. The direct-beam, spectral measurements are in good agreement with modelled data (MODTRAN).

The solar UV radiation environment: measurement techniques and results

Abstract: The Australian Radiation Laboratory (ARL) has been involved for many years in the measurement of solar UV radiation (UVR) using spectroradiometers and a network of broad-band detectors at 16 sites in Australian and Antarctica. Measurement locations range from tropical to polar and as a result, there are many difficulties associated with the maintenance and calibration of the network to ensure an accurate and reliable data collection. Calibration procedures for theious detectors involve comparison with simultaneous spectral measurements obtained using a portable spectroradiometer incorporating a double monochromator, calibrated against traceable standard lamps. Detector-datalogger systems are also intercompared at the Yallambie site for a number of months before installaion at another site. As an additional check on the calibrations, computer models of solar UVR at the earth's surface for days with clear sky and known ozone values are compared with the UV radiometer measurements. The use of chemical, biological and solid state persnal UVR dosimeters is discussed and some measurement results are presented.

Bi-directional reflectance factor of 14 soil classes from Brazil

Abstract: The spectral reflectance of soils is required for effective use of remote sensing products. The absence of studies concerned with spectral reflectance of the soils from the tropical region in the 400 to 2500 nm spectral range is the main motivation of this research. The objective of this study was to present spectral reflectance data from different tropical soil types. This spectral characterization was done through measurements of the bi-directional reflectance factor of 111 selected soil samples, grouped in 14 tropical soil classes, taken from 53 sites (Sao Paulo State, Brazil). The measurements were made with a spectroradiometer operating in the 400 to 2500 nm region of the electromagnetic spectrum. Each soil sample is associated to a set of physical and chemical analyses data, with part of these published in descriptive reports of soil surveys.

Inversion of a soil bidirectional reflectance model for use with vegetation reflectance models

Abstract: The need for anisotropic soil reflectance in canopy reflectance modeling is assessed for different sampling and canopy conditions. Based on the results for grasslands, a soil model is inverted with ground-based radiometer data from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). A general solution applicable over different spectral bands, solar angles, and soil moisture levels is determined using a diverse data set. With this solution, the model can be used as a lower boundary condition in FIFE canopy modeling. Despite the previously reported independence of retrieved model parameters to data sampling conditions, solutions determined with more limited data sets vary significantly. Moreover, the semiphysically based model may not accurately predict reflectance in angular regions where data are absent in the inversion process. These findings are important for the Earth Observing System multiangle imaging spectroradiometer (MISR), which will gather data in essentially one azimuthal plane per pass like the instrument used in this study did.

Comparison of airborne and surface spectral bidirectional reflectance factors, spectral hemispherical reflectance and spectral vegetation indices

Abstract: Six sets of airborne advanced solid-state array spectroradiometer (ASAS) and ground modular multiband radiometer (MMR) remotely sensed bidirectional measurements acquired over one First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment site in 1987 and 1989 were compared for the following parameters: bidirectional spectral reflectance factor, spectral hemispherical reflectance, simple ratio, and normalized difference vegetation index. ASAS at-sensor radiances were atmospherically corrected and converted to surface reflectance factors. Selected MMR bands were simulated with ASAS data, and hemispherical reflectance for both ground and airborne data sets was estimated using data collected only in or close to the solar principal plane. The shapes of the reflectance factor response curves (as a function of view zenith angle) were strongly affected by solar zenith angle and compared well between instruments. Off-nadir anisotropy in reflectance factors was comparable between sensors, while actual values of reflectance factors differed 2–35% relative between instruments in the green channel and 0–38% relative in the red channel. For the data set giving the overall closest agreement, ASAS reflectance factors differed from MMR values by 5–17% relative (0.3–1.8% absolute) in the green and 5–10% relative (0.2–0.5% absolute) in the red. These differences showed no correlation with solar zenith angle. Some of the differences were arbitrarily introduced by the 5° offset in view zenith angles (except nadir) between the two instruments. Other differences were caused by the discrepancy in solar zenith angle for some of the sets, variable deviations from the solar principal plane, hotspot and associated shadowing effects not consistently recorded by both instruments, and sampled ground area variations. Estimates of hemispherical reflectance compared very well between sensors, with differences of only 3–14% relative (0.3–1.6% absolute) in the green and 11–27% relative (0.8–2.4% absolute) in the red. For the ASAS data, using atmospherically corrected reflectance factors (instead of at-sensor radiances) significantly increased the values of the spectral vegetation indices (SVIs). Off-nadir anisotropy for the SVIs derived from both sensors' data sets was less than that observed for the bidirectional reflectance factors, and NDVI off-nadir deviations were much less than those of the simple ratio (SR). Large differences in the values of SR and NDVI between sensors indicate SVIs calculated from broadband (MMR) versus narrowband (ASAS) data are not comparable.

The Image Navigation Cloud Mask for the Multiangle Imaging Spectroradiometer (MISR)

Abstract: The authors have developed a cloud mask technique that may be applied to the efficient selection of “clear enough” scenes for image navigation. While the mask can be applied generally, the motivation for its development comes from its intended use on Multiangle Imaging Spectroradiometer (MISR) imagery. The difficulties in detecting clouds in the presence of land–water boundaries when using prenavigated imagery is overcome by using a simple two-step direct threshold technique. The two steps involve the thresholding of two observables derived for each pixel. The first is a 0.86-μm reflectance. The second is a new observable, D = | NDVI |bR1−2, where NDVI = (R2 − R1)(R2 + R1)−1, R2 is the 0.86-μm reflectance, R1 is the 0.67-μm reflectance, and b is chosen so as to maximize the separation between clear and cloudy pixels. The success of the cloud mask is shown by applying it to degraded AVIRIS data. The authors make comparisons with a more popular NDVI technique to show the advantage of our method.

System and method for scene light source analysis

Abstract: System and method for rapid measurement of an exposure light source and calculation of filter selection for proper color balanced exposure of a light sensitive medium includes measurement of the light source with a spectroradiometer and calculation of color log exposure differences for successive filters taken from a database list with the filter selection being based on a minimum overall difference value derived from the summation of the absolute values of the individual color log exposure differences.

Calibration of the broadband radiometers of the Finnish solar ultraviolet monitoring network

Abstract: The present solar ultraviolet (UV) monitoring network of the Finnish Meteorological Institute consists of seven sites equipped with erythemally-weighted broadband radiometers. Two of the sites are also equipped with a Brewer spectroradiometer. The Finnish Centre for Radiation and Nuclear Safety is responsible for testing and calibration of the broadband radiometers. An Optronic 742 spectroradiometer is used as a reference instrument, and the broadband radiometers are calibrated against the Optronic 742 in solar radiation. The Optronic 742 is calibrated against 1000 W FEL lamps traceable to the NIST. The errors caused by nonideal radiometric characteristics of the Optronic 742 are numerically corrected. The uncertainty (2 σ) of solar UV measurements with the Optronic 742 is estimated to be ±8 % at 310 nm and the uncertainty of the spectroradiometric solar calibration of the Solar Light Model 501 radiometers is estimated to be 11 %.

A Multichannel Cloud Pyranometer System for Airborne Measurement of Solar Spectral Reflectance by Clouds

Abstract: The design and performance of a spectral radiometer system are described for airborne measurements of solar flux reflectance by clouds. The system consists of a pair of identical multichannel pyranometers: one installed on the top and the other on the bottom of an aircraft fuselage to measure the downward and upward solar irradiances, respectively. This measurement scheme has an advantage in that reflectances derived from ratios between the upward and downward irradiances can avoid the need for absolute radiometric calibrations. The multichannel cloud pyranometer (MCP) system measures near-monochromatic solar irradiances at nine discrete wavelengths between 420 and 1650 nm by using interference filters with very narrow bandwidths. Included among these wavelengths are 760 and 938 nm in the oxygen and water vapor absorption bands, respectively. Solar radiation passing through the filters is instantly detected by a silicon photodiode for wavelength λ 1 µm. Go...

Spectroradiometer with wedge interference filters (SWIF): measurements of the spectral optical depths at Mauna Loa Observatory

Abstract: A spectroradiometer with wedge interference filters (SWIF) (the filters were produced by Carl Zeiss, Jena, Germany) and a CCD matrix (which was of Russian production) that functions as the sensor has been designed and built for use in ground-based optical sensing of the atmosphere and the Earth’s surface in the spectral range of 0.35–1.15 μm. Absolute calibration of this instrument was performed through a series of observations of direct solar radiation at Mauna Loa Observatory (MLO) in Hawaii in May and June 1993. Spectral optical depth (SOD) measurements that were made during these field experiments provided detailed spectral information about both aerosol extinction (scattering plus absorption) and molecular absorption in the atmosphere above the site at MLO. The aerosol-SOD measurements were compared with narrow-band radiometer measurements at wavelengths of 380, 500, and 778 nm The SWIF and narrow-band radiometer measurements are in agreement to within the experimental error. At a wavelength of 500 nm, the aerosol SOD was found to be approximately 0.045. A description of the SWIF instrument, its absolute calibration, and the determination of atmospheric SOD’s at MLO are presented.

Estimation of aerosol columnar size distribution and optical thickness from the angular distribution of radiance exiting the atmosphere: simulations.

Abstract: We report the results of simulations in which an algorithm developed for estimation of aerosol optical properties from the angular distribution of radiance exiting the top of the atmosphere over the oceans [Appl. Opt. 33, 4042 (1994)] is combined with a technique for carrying out radiative transfer computations by synthesis of the radiance produced by individual components of the aerosol-size distribution [Appl. Opt. 33, 7088 (1994)], to estimate the aerosol-size distribution by retrieval of the total aerosol optical thickness and the mixing ratios for a set of candidate component aerosol-size distributions. The simulations suggest that in situations in which the true size–refractive-index distribution can actually be synthesized from a combination of the candidate components, excellent retrievals of the aerosol optical thickness and the component mixing ratios are possible. An exception is the presence of strongly absorbing aerosols. The angular distribution of radiance in a single spectral band does not appear to contain sufficient information to separate weakly from strongly absorbing aerosols. However, when two spectral bands are used in the algorithm, retrievals in the case of strongly absorbing aerosols are improved. When pseudodata were simulated with an aerosol-size distribution that differed in functional form from the candidate components, excellent retrievals were still obtained as long as the refractive indices of the actual aerosol model and the candidate components were similar. This underscores the importance of component candidates having realistic indices of refraction in the various size ranges for application of the method. The examples presented all focus on the multiangle imaging spectroradiometer; however, the results should be as valid for data obtained by the use of high-altitude airborne sensors.

Moderate resolution imaging spectroradiometer (MODIS) engineering model development

Abstract: The moderate resolution imaging spectroradiometer is a space-based imaging spectroradiometer designed to observe small changes in Earth system processes over long periods of time. The first of several MODIS instruments is scheduled to fly on the Earth Observation System (EOS)-AM spacecraft in 1998. The engineering model for the MODIS is well into build and will be in system test later this year. This paper provides an overview of the MODIS instrument and highlights many of the technical achievements during the engineering model development. Results of subsystem testing of critical assemblies is presented indicating a high probability of success in critical performance areas at the system level.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Monitoring the evolution of desertification processes from 1973 to 1987 in Damagaram (Niger) with Landsat multispectral scanner and thematic mapper

Abstract: Desertification of the Damagaram region (Sahelian zone of Niger) is represented by several processes: decrease or modification of the vegetation, deflation of sandy soils by the wind and exhumation of lateritic crusts, modification of the pattern of dunes, increase of erosion by water. A field survey was performed by a pluridisciplinary team in order to observe and define the expressions of desertification. The spectral signatures of various soils, rocks and vegetation were measured using a high resolution field spectroradiometer. The measured spectral properties of well defined objects were used to validate the calibration of the satellite data and to define the endmembers in models of spectral unmixing. Landsat multi spectral scanner (MSS) scenes of March 11, 1973 and January 10, 1976, and a thematic mapper (TM) scene of January 4, 1987, were made compatible, and several techniques of image processing were tested and compared: ratios (leaf area index, normalized difference vegetation index, redness, clay index), principal components analysis and spectral unmixing. The interpretation of the resulting images allowed a semi-quantitative monitoring of the evolution of the significant components of the landscape. Local variations of soils and vegetation are particularly well enhanced, whereas the global situation did not change drastically from 1973 to 1987.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Improved color image calibration

Abstract: The technique for calibrating color imagery which has been employed by the Tank-Automotive Research Development and Engineering Center (TARDEC) includes measurement of red, green, and blue color panels using a colorimeter during the approximate time that the calibration image is captured This method has the advantage that the luminance and chromaticity coordinates of the color panels are recorded in real time However, the disadvantage is the amount of time it takes to measure each individual panel Outside of a laboratory, the environment cannot be controlled, so the light level and correlated color temperature from the source may shift during the calibration period A new technique using a spectroradiometer has been developed whereby the spectral reflectance of the color panels are measured beforehand and only the light level and spectral content from the source is monitored during the calibration period This drastically reduces the time required for calibration, thus rendering insignificant any temporal changes in the light level or correlated color temperature of the panels The actual luminance and chromaticity of the color panels can be calculated subsequently© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Retrieval of Atmospheric Turbidity Coefficient and Water Column Density from Solar Irradiance Data

Abstract: Ground-based measurement of solar irradiance has been made using a spectroradiometer at wavelengthsin apx-water absorption band. An algorithm is formulated to retrieve simultaneously the atmospheic water column density and Angstrom's turbidity coefficient. The transmission models used to account for the contribution from diverse atmospheic absorbing and scattering elements are presented.

A Fourier-transform spectrometer for spectral-irradiance comparisons

Abstract: A Fourier-transform spectrometer (FTS) for the wavelength region between 1100 nm (cut-off wavelength of the Si photodiode detector) and 190 nm (absorption edge of air) has been developed for use in spectral-irradiance comparisons. The main advantages of the FTS over traditional spectroradiometers based on a monochromator are: (i) high light throughput; (ii) circular entrance aperture; (iii) high wavelength accuracy; and (iv) multiplex advantage if the detector is the dominant source of noise. The main problems encountered in achieving highly accurate irradiance comparisons are discussed. The uncertainties due to the size of the source are studied. It was found that sources with a diameter of up to 20 mm can be compared with an uncertainty of 2,5% between 1100 nm and 250 nm, and 3% below 250 nm. In the case of a homogeneous radiance distribution the uncertainty is below 1,5 %.

Spectral characteristics of the Earth Observing System (EOS) Moderate-Resolution Imaging Spectroradiometer (MODIS)

Abstract: The Moderate Resultion Imaging Spectroradiometer (MODIS) is a key instrument scheduled for flight on the AM (1030 hours equator crossing time) and the PM (1330 equator crossing time) platforms of the Earth Observing System (EOS). It has a considerable multispectral capability for observations of land, ocean, and atmospheric features. The 36 bands sampling the visible, near IR, and thermal IR portions of the spectrum along with 250 to 1000 meter resolution will offer very powerful observing advantages over heritage instruments such as the Advanced Very High Resolution Radiometer on the NOAA environmental satellites. The engineering model of the MODIS is under construction and so far the performance being observed is generally meeting specifications. Preliminary analyses of components of the engineering model indicate that it will meet most of the spectral specifications with some variances relative to the specifications being required and approved. Overall MODIS fabrication is on schedule for its first flight in 1998.

Some problems in realizing an infrared spectral-irradiance scale from 1500 nm to 2400 nm at the NRC

Abstract: The spectral-irradiance scale in the infrared at the National Research Council of Canada is being realized using electrical-substitution absolute radiometers in conjunction with interference filters to calibrate FEL tungstenhalogen lamps as primary source standards. This paper discusses some preliminary measurements associated with the second phase of this work, the realization of a scale in the range 1500 nm to 2400 nm. Some characteristics of the spectroradiometer that will be used to measure the transmittance of the interference filters under conditions as close as possible to the final configuration are presented. Various problems encountered in configuring the liquid-nitrogen-cooled InSb detector used are discussed. The larger effect of atmospheric water absorption bands in this wavelength range presents a greater challenge in determining the choice of passband wavelengths for the filters. Measurements to determine the extent of these absorption bands are presented.

Development and Operation of a Material Identification and Discrimination Imaging Spectroradiometer

Abstract: Many imaging applications require quantitative determination of a scene's spectral radiance. This paper describes a new system capable of real-time spectroradiometric imagery. Operating at a full-spectrum update rate of 30Hz, this imager is capable of collecting a 30 point spectrum from each of three imaging heads: the first operates from 400 nx m to 950 nm, with a 2% bandwidth; the second operates from 1.5 micrometers to 5.5 micrometers with a 1.5% bandwidth; the third operates from 5 micrometers to 12 micrometers , also at a 1.5% bandwidth. Standard image format is 256 X 256, with 512 X 512 possible in the VIS/NIR head. Spectra of up to 256 points are available at proportionately lower frame rates. In order to make such a tremendous amount of data more manageable, internal processing electronics perform four important operations on the spectral imagery data in real-time. First, all data in the spatial/spectral cube of data is spectro-radiometrically calibrated as it is collected. Second, to allow the imager to simulate sensors with arbitrary spectral response, any set of three spectral response functions may be loaded into the imager including delta functions to allow single wavelength viewing; the instrument then evaluates the integral of the product of the scene spectral radiances and the response function. Third, more powerful exploitation of the gathered spectral radiances can be effected by application of various spectral-matched filtering algorithms to identify pixels whose relative spectral radiance distribution matches a sought- after spectral radiance distribution, allowing materials-based identification and discrimination. Fourth, the instrument allows determination of spectral reflectance, surface temperature, and spectral emissivity, also in real-time. The spectral imaging technique used in the instrument allows tailoring of the frame rate and/or the spectral bandwidth to suit the scene radiance levels, i.e., frame rate can be reduced, or bandwidth increased to improve SNR when viewing low radiance scenes.

Status of the Multi-angle Imaging SpectroRadiometer instrument for EOS-AM1 and its application to remote sensing of aerosols

Abstract: The Multi-angle Imaging SpectroRadiometer (MISR) instrument is currently under development at JPL for the AM1 spacecraft in the Earth Observing System (EOS) series. MISR consists of nine pushbroom cameras, and will provide global coverage in four visible/near-infrared spectral bands. This measurement strategy provides systematic multi-angle imagery of the Earth for studies of aerosols, surface radiation, and clouds. An on-board calibrator consisting of deployable solar diffusers and a set of stable photodiodes provides a high-accuracy detector-based calibration. In this paper the authors report on the progress of the instrument fabrication and testing and focus on the application of MISR's unique observational strategy to studies of tropospheric aerosols.