Showing papers on "Spectroradiometer published in 1996"

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

Abstract: A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-µm and 8-14-µm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-µm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-µm and 8-14-µm atmospheric windows.

Measurements of biologically effective UV doses, total ozone abundances, and cloud effects with multichannel, moderate bandwidth filter instruments

Abstract: I describe a method to derive biologically effective UV dose rates, total ozone abundances, and cloud optical depths from irradiance measurements with moderate bandwidth filter instruments that have only a few channels in the UV region. These quantities are determined when the measured irradiances are combined with radiative transfer calculations. The method was applied to a four-channel filter instrument with center wavelengths at 305, 320, 340, and 380 nm and bandwidths of 10 nm. I compared the instrument with a high-wavelength-resolution spectroradiometer during a 1-week period in San Diego, California, with variable cloudiness. The relative difference in Commission Internationale de l’Eclairage (CIE)-weighted UV dose rates for solar zenith angle’s (SZA’s) < 80° was 1.4 ± 3.2%. The relative difference for clear sky was 0.6 ± 1.5% for SZA’s < 80°. The total ozone inferred from the irradiance measurements with the filter instrument is insensitive to clouds. The instrument was compared with a Dobson and a Brewer instrument in Oslo, Norway, 60 °N, for more than 1 year. The relative difference in derived ozone abundance for the entire period, including cloudy days, was 0.3 ± 2.9%. The standard deviation was reduced to 1.9% when only clear sky and SZA’s < 60° were included. By using the total ozone and the cloud optical depth derived from the filter instrument as input to a radiative transfer model, one can compute a complete spectrum from 290 to 400 nm with 1-nm resolution. Such calculated spectra are in good agreement with spectra measured simultaneously with a high-wavelength-resolution spectroradiometer for clear as well as cloudy sky conditions and can be used to determine dose rates for any desired action spectrum. Only one UV-B channel and one UV-A channel are required to compute the spectra.

Estimation of ozone with total ozone portable spectroradiometer instruments. II. Practical operation and comparisons

Abstract: We used a microcomputer-controlled total ozone portable spectroradiometer instrument #21 (MTOPS21) to measure solar radiation at 298, 304 and 310 nm in Greenbelt, Md., during 1995. One day’s ozone measurements from a Brewer instrument (B105) were used to calibrate the 304- and 310-nm channel ratios to a theoretical model. Total ozone estimates were then determined for the entire MTOPS21 data set. Differences between individual B105 and MTOPS21 ozone estimates show a 1% drop as solar zenith angles increase and depend on atmospheric attenuation and SO2 variation at the ±2% level. Daily average values agree well (<0.5% average offset, 2% standard deviation).

A simple red:far-red sensor using gallium arsenide phosphide detectors

Abstract: 1. A low-cost portable instrument was designed to measure the red :far-red ratio (660 :730 nm) and photosynthetically active radiation (PAR, 400-700 nm). The sensor head comprises three gallium arsenide phosphide photodiodes associated with tinted-glass filters. 2. Such detection takes advantage of the spectral response curves of these diodes, which provide a narrow bandpass response when combined with an appropriate filter. No interference filtering and no amplification is required. The output is displayed on a battery-powered unit enclosing two digital voltmeters for both PAR and red :far-red ratio readings. 3. The unit is easy to produce and requires no knowledge of electronics. The instrument was tested under field conditions against a spectroradiometer. 4. This approach may provide a basis to assess other spectral vegetation indices such as NDVI.

The MISR Calibration Program

Abstract: The Multiangle Imaging SpectroRadiometer (MISR) is currently under development for NASA's Earth Observing System. The instrument consists of nine pushbroom cameras, each with four spectral bands in the visible and near-infrared. The cameras point in different view directions to provide measurements from nadir to highly oblique view angles in the along-track plane. Multiple view-angle observations provide a unique resource for studies of clouds, aerosols, and the surface. MISR is built to challenging radiometric and geometric performance specifications. Radiometric accuracy, for example, must be within +/- 3%/ 1 sigma, and polarization insensitivity must be better than +/- 1 %. An onboard calibrator (OBC) provides monthly updates to the instrument gain coefficients. Spectralon diffuse panels are used within the OBC to provide a uniform target for the cameras to view. The absolute radiometric scale is established both preflight and in orbit through the use of detector standards. During the mission, ground data processing to accomplish radiometric calibration, geometric rectification and registration of the nine view-angle imagery, and geophysical retrievals will proceed in an automated fashion. A global dataset is produced every 9 days. This paper details the preflight characterization of the MISR instrument, the design of the OBC, and the radiance product processing.

Estimation of ozone with total ozone portable spectroradiometer instruments. I. Theoretical model and error analysis

Abstract: Inexpensive devices to measure solar UV irradiance are available to monitor atmospheric ozone, for example, total ozone portable spectroradiometers (TOPS instruments). A procedure to convert these measurements into ozone estimates is examined. For well-characterized filters with 7-nm FWHM bandpasses, the method provides ozone values (from 304- and 310-nm channels) with less than 0.4% error attributable to inversion of the theoretical model. Analysis of sensitivity to model assumptions and parameters yields estimates of ±3% bias in total ozone results with dependence on total ozone and path length. Unmodeled effects of atmospheric constituents and instrument components can result in additional ±2% errors.

Detector-Based Calibration Method for High-Accuracy Solar UV Measurements

Abstract: A novel method for calibrating the absolute responsivity of solar UV spectroradiometers has been developed and tested. The method is based on calibrated filter radiometers constructed from a detector, a precision aperture, a band-pass filter and devices for temperature stabilization. The filter radiometers utilize a trap detector with very low reflectance. The filter and the detector can therefore be characterized separately. As an example the detector-based calibration is compared at 312 nm wavelength with lamp-based calibration by measuring the irradiance of a deuterium lamp with both the filter radiometer and the lamp-calibrated spectroradiometer. The agreement between the results is at the level of 1%, well within the estimated uncertainties of both methods.

Calibration of the Improved Stratospheric and Mesospheric Sounder. Part I: Introduction, Spectral, and Radiometric Calibration

Abstract: The Improved Stratospheric and Mesospheric Sounder (ISAMS) is an infrared spectroradiometer that formed part of the science instrument payload of the Upper Atmosphere Research Satellite. An essential part of the success of ISAMS in orbit was a program of prelaunch calibration and characterization of many aspects of the instrument's performance. A brief description of ISAMS is followed by a detailed discussion of the calibration and characterization methodology, the facilities used in this program, and the results from the spectral and radiometric measurements. The results are discussed in terms of factors affecting the in-flight performance of ISAMS, particularly the spectral response of the measurement channels, the radiometric linearity, stray radiations and their dependence on the line of sight view, signal-to-noise ratios, and the sensitivity of the in-flight radiometric calibration to anticipated changes to the thermal environment within ISAMS. Some of the “lessons learned” are discussed wit...

Spectrometry of turbidity in surface water

Abstract: The objective of this paper is to investigate how to measure turbidity of water using surface spectral reflectance. Spectral data from a reservoir were collected using a field spectroradiometer, which contains 512 spectral channels ranging from 350 to 1050 nm. Five replicate scans were taken at each sampling site with a total of 23 such sites selected. The results indicated that wind-induced whitecaps on the surface of water can cause higher overall reflectance and inconsistency among the replicate spectral measurements. As result, it was almost impossible to apply reflectance from any single wavelength to accurately estimate turbidity. It was found that the difference of reflectances between 710 and 720 nm is a better indicator of turbidity.

A New Database Program for Spectral Surface UV Measurements

Abstract: A program package for the management and analysis of spectral ultraviolet radiation (UV) data on an IBM-compatible PC is described. The present version can accommodate data from UV spectroradiometers operating in Russia, the U.S. National Science Foundation Polar Programs UV Spectroradiometer Network, and the World Meteorological Organization (WMO) Ozone and UV Data Center, located in Toronto, Ontario, Canada. The package manages compact databases of measured UV spectra and supporting meteorological information and has tools for visualization, statistical analysis, calculation of biologically effective doses, and a simple built-in radiative transfer model. In this paper, the program structure and the user interface are discussed. Examples of the program's operation using spectral data from three spectroradiometers (SUVS-M, Central Aerological Observatory, Russia; Meteorological Observatory of Moscow State University, Russia; NSF Network, United States) illustrate the program's capabilities. Compa...

Out-of-band spectral correction algorithm for the Multiangle Imaging Spectroradiometer

Abstract: An out-of-band correction algorithm is evaluated for the Multi-Angle Imaging Spectro-Radiometer (MISR). The objective for MISR is to report scene radiances, ad averaged over the in-band spectral response region.

Correcting for Stray Light in Brewer Spectroradiometers

Abstract: Comparisons of measured and modelled spectral UV-B (290–25 nm) irradiance for cloudless days at four Canadian stations clearly show stray light enhancement of Brewer spectroradiometer measurements at wavelengths below about 310 nm. Since the affected wavelengths are biologically important stray light error should be removed before applying action spectra. Differences between measured and modelled irradiances can be used to correct measurements. They are sufficiently similar for the four stations that one set of spectral correction factors for stray light can be calculated and applied to all four stations. Brewer measurements between 310 and 325 nm, which are unaffected by stray light, are important for calibrating models such as the one used in this paper so that models can be used to calculate irradiances in the UV-A band (325–400 nm) where measurements are not made.

Improving color measurements of displays

Abstract: It is generally believed that the most accurate means of measuring the CIE tristimulus values X, Y, Z or chromaticity coordinates x,y from a display is by using a spectroradiometer. Nevertheless, tristimulus colorimeters employing three or four colored filters find wide use because of their simplicity and lower cost. These devices cannot be calibrated to give accurate results in all situations because the spectral responsivities of their filtered detectors are not exactly the CIE color-matching functions. However, for a display that produces a linear superposition of three primary colored lights of fixed spectra, a tristimulus colorimeter can be correctly calibrated to measure all colors on that display. Signals from all of the filtered detectors are used to compute each of the X, Y, z values. The calibration matrix is computed by data fitting to a reference colorimeter. An improvement to the previously published method is reported, an a numerical example is shown. This technique is more tractable with today's digital instrumentation than it was when it was discovered, yet it remains underused. The American Society for Testing and Materials, through its Committee on Color and Appearance, is revising its standard on display measurements using tristimulus colorimeters to encourage the adoption of the technique.

Estimation of soil colour through spectral reflectance characteristics

Abstract: Surface samples (Ap horizon) of twenty one soils widely differing in their physical appearance of colour were collected from different parts of India. Air-dried soils, passed through 2 mm sieve, were packed in black round trays for reflectance measurements. Soil colour of air-dry soils was designated using Munsell colour charts. Spectral reflectance patterns were obtained by a portable spectroradiometer using sunlight as the illuminant. For each sample, the XYZ tristimulus values; x,y chromaticity coordinates were obtained from reflectance patterns using the CIE standard methods. Munsell. notations were also converted to tristimulus and chromaticity coordinates using published tables of Wyszecki and Stiles (1982). The RGB colour coordinates obtained from both the systems were compared. The RGB values from spectral data were well correlated with reflectance in TM bands, while those from Munsell notation were correlated to a lesser extent. The tristimulus values were used to designate each soil with specific set of values to locate them in Universal Colour Space.

Improvement of diffuse solar irradiance measurements with noncosine instruments

Abstract: As a result of sky radiance anisotropy, significant errors spoil diffuse solar irradiance data obtained by means of noncosine-true instruments, which happens in spite of knowledge of the relative cosine response. To reduce these errors we recommend a method that involves an additional measurement of diffuse irradiance from a horizon band. The efficiency of the so-called double-measurement method is found to be optimum for a 25° horizon band elevation. We confirm the feasibility of the method with relevance tests that use diffuse solar spectral measurements carried out on cloudless skies by means of an UV spectroradiometer.

Validation of the ASTER thermal infrared surface radiance data product

Abstract: The advanced spaceborne thermal emission and reflection radiometer (ASTER) is a 14 channel high spatial resolution instrument selected for flight on the EOS AM-1 platform. This instrument has a 60 km pointable cross-track swath and five thermal infrared channels between 8 and 12 micrometers with 90 m spatial resolution. Correction for the effect of atmospheric attenuation and emission will be made using a radiative transfer model and atmospheric parameters either from the EOS AM-1 platform instruments MODIS (moderate- resolution imaging spectroradiometer) and MISR (multi-angle imaging spectroradiometer) or temperature and moisture profiles from global numerical assimilation models. The correction accuracy depends strongly on the accuracy of the atmospheric information used. To provide an objective assessment of the validity of the atmospheric correction in situ measurements of water surfaces under a variety of atmospheric conditions will be used to estimate the surface leaving radiance at the scale of an ASTER pixel. The procedure will use an array of continuously recording temperature buoys to establish the bulk water temperature, broadband radiometers to determine the near surface water temperature gradient and radiosonde and sunphotometer measurements and a radiative transfer model to deduce the sky irradiance. These measurements and the spectral emissivity of the water will be combined with the relative system spectral response to provide an estimate of thermal infrared surface leaving radiance for each ASTER thermal channel. An example of this approach using a multichannel thermal aircraft scanner as a stand in for ASTER is described. It is expected this approach will provide estimates of surface radiance accurate, in temperature terms, to better than 1 K.

Simulation of Light Environment with Fluorescent lamps in a Growing Room

Abstract: Spectral distributions of 18 new fluorescent lamps were measured using spectroradiometer. The wavelength specific data of lamps were transformed into cumulative distributions that then formulated using regression technique. The 18 high order nonlinear equations derived can be of great use for the further study. For example, the spectral bin data in either photometric, radiometric or quantum unit of any waveband among 300 to 1100 nm can be derived. Light conversion factors from photometric to quantum to radiometric unit for all lamps were derived theoretically and validated experimentally. Amounts of red (600-700 nm) vs. far-red light (700-800 nm) were also given. The computerization of spectral data of 18 lamps enables the analysis of the performance of combine-lamps. With a given luminaries' combination and arrangement, the spectral distribution, the spatial illuminance level, PPFD level and uniformity over a specified growing area can be calculated. A software was developed for this purpose.

In-flight radiometric calibration plans for the Earth Observing System-Multi-angle Imaging SpectroRadiometer

Abstract: The Multi-angle Imaging SpectroRadiometer (MISR) will fly on the EOS-AM1 spacecraft, and provide global data sets with nine discrete view directions per scene. The instrument's radiometric scale is achieved by use of detector standards. On-orbit, photodiodes measure reflected light from solar-illuminated deployable Spectralon panels. The cameras simultaneously view the panels, providing the needed calibration data inputs. Other calibration methodologies include vicarious calibration and histogram equalization. Coefficients, as derived from the various methods, are weighted to produce a single determination of the gain and offset parameters. This process is repeated at monthly intervals to insure the calibration is maintained. Routine product generation makes use of these calibration coefficients, and also corrects for instrument-dependent errors in the radiance determination. These latter processing steps include corrections for camera out-of-band response, focal-plane scattering, and detector-to-detector nonuniformity of response. This paper reviews the MISR standard radiometric product and the in-flight radiometric calibration and characterization plans.

Calibration and characterization of an airborne pushbroom spectroradiometer

Abstract: Improved methods for calibrating and characterizing the CCD array based off-nadir tiltable advanced solid-state array spectroradiometer (ASAS) were developed and applied. Sensor characteristics such as radiometric sensitivity, polarization sensitivity, signal-to-noise-ratio, temperature sensitivity, spectral bandpass, spectral distortion, spatial distortion, and spatial resolution were measured. Radiometric sensitivity, array temperature sensitivity, and signal-to-noise were measured using a barium sulfate coated integrating hemisphere whose output calibration is traceable to NIST. Polarization sensitivity was measured for 48 of 62 spectral bands across all 512 spatial pixels. Spectral bandpass and spectral distortion were measured using a 0.5 meter doublepass monochromator. Spatial resolution (given as the modulation-transfer-function -- MTF) and distortion were measured using a combination of monochromatic collimated light to directly measure the point-spread-function (PSF) and the edge spread function (ESF) derived from actual image data. The MTF obtained using the two techniques are compared. Potential improvements to the test setups and methods are described.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Technical progress report: Completion of spectral rotating shadowband radiometers and analysis of atmospheric radiation measurement spectral shortwave data

Abstract: Our goal in the Atmospheric Radiation Measurement (ARM) Program is the improvement of radiation models used in general circulation models (GCMs), especially in the shortwave, 1) by providing improved shortwave radiometric measurements for the testing of models and 2) by developing methods for retrieving climatologically sensitive parameters that serve as input to shortwave and longwave models. At the Atmospheric Sciences Research Center (ASRC) in Albany, New York, we are acquiring downwelling direct and diffuse spectral irradiance, at six wavelengths, plus downwelling broadband longwave, and upwelling and downwelling broadband shortwave irradiances that we combine with National Weather Service surface and upper air data from the Albany airport as a test data set for ARM modelers. We have also developed algorithms to improve shortwave measurements made at the Southern Great Plains (SGP) ARM site by standard thermopile instruments and by the multifilter rotating shadowband radiometer (MFRSR) based on these Albany data sets. However, the major objective of our program has been the development of two spectral versions of the rotating shadowband radiometer. The MFRSR contains six filtered detectors, and one unfiltered silicon detector that serves as a surrogate total shortwave sensor. The MFRSR was completed in the first phase of this program and is deployed at the SGP cloud and radiation testbed (CART) site. It has been selected as the core instrument of the ARM solar and infrared observing system (SIROS) A small part of our ARM funding is used to acquire a radiation instrument suite that is being deployed at each of high-quality radiation data set on the roof of the ASRC twenty extended facilities within the SGP CART site. The using the MFRSR and the same standard thermopile rotating shadowband spectroradiometer (RSS) contains a pyranometers and pyrheliometers that are used at the SGP 256-channel diode array that spans the wavelengths CART site. With the instruments accessible we are able to 350-1050 nm with resolution varying between 0.6 nm and provide frequent cleaning, alignment, and calibration to 8 nm. The prototype of the RSS has acquired data on the roof of the ASRC at the State University of New York in Albany, but a field-worthy unit is not yet complete. A calibration facility for these instruments is being built at the ASRC to characterize cosine response, to determine absolute spectral calibration, and to characterize spectral response. The facility is operational, and upgrades are continuing. The MFRSR, which was developed during the first years of the ARM Program, has become a workhorse at the CART sites in Oklahoma and Kansas, and it is widely deployed in other climate programs. We have spent most of our effort this year developing techniques to retrieve column aerosol, water vapor, and ozone from direct beam spectral measurements of the MFRSR. Additionally, we have had success in calculating shortwave surface albedo and aerosol optical depth from the ratio of direct to diffuse spectral irradiance. Using the surface albedo and the global irradiance, we have calculated cloud optical depths. From cloud optical depth and liquid water measured with the microwave radiometer, we have calculated effective liquid cloud particle radii. In each case, we have attempted to validate our approach using independent measurements or retrievals of the parameters under investigation. With the exception of the ozone intercomparison, the corroborative measurements have been made at the SGP CART site. This report highlights these results. ASRC Data Acquisition

Estimating hemispherical reflectance and selected biophysical parameters for boreal forest canopies using spectral bidirectional reflectance data acquired by ASAS

Abstract: Advanced Solidstate Array Spectroradiometer (ASAS) hyperspectral, multiangle data were obtained over BOREAS sites in Saskatchewan and Manitoba, Canada during four field campaigns in 1994. Flown aboard the NASA C-130B at an altitude of approximately 5000 m above ground level, ASAS acquired off-nadir data from 70 degrees forward to 55 degrees aft along-track, in 62 contiguous spectral bands ranging from 400-1025 nm. These measurements were collected to develop linkages between optical remote sensing data and biophysical parameters at the canopy level, and to provide an intermediate level in the process of scaling local ground conditions to satellite observations. ASAS at-sensor radiances over various canopies were atmospherically corrected using the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) and at-surface reflectance factors were derived. Using the multiangle spectral reflectance factors, spectral hemispherical reflectance (PAR, red, and near-IR) was estimated, and spectral vegetation indices, including hemispherical measures, were calculated. The values of the SVIs varied widely depending on the particular angular inputs to the calculation.

An Application of a Spectroradiometer to Measure the Optical Radiant Conditions for Plant Growth

Abstract: In maintaining environments for plant growth effectively and properly, it is necessary to perceive and to evaluate optical radiant conditions not only in indoor areas but also under natural radiation. This is true especially under natural radiation, where optical radiant conditions change frequently as time passes. So it is desirable to be able to measure optical radiant conditions quickly, accurately and easily. In this paper, we report on an application of a spectroradiometer to measure the optical radiant conditions for plant growth. In this system, a fiber optic probe was used to improve the freedom and workability for measurements. A spectral dispersion system and a linear image sensor, which is inexpensive and easy to obtain because of recent developments in solid state technology, was utilized instead of a mechanical wavelength scanning system. The spectral dispersion system and a linear image sensor together are called multi-channel spectroradiometer. High speed measurement can be insured by its use under natural radiation. In designing this measurement system, two points were improved to reduce errors. One cosine-corrected the angular responsibility of the spectroradiometer for the measurement of spectral irradiance under natural radiation. The fiber probe of the spectro-radiometer is cosine-corrected by a shielding tube with a diffuser head. The other point is to provide an automatic wavelength scale calibration system using a 7 line spectrum. The combination and error reduction methods of these instruments contributed to the application of a high-speed and high-accuracy multi-channel spectroradiometer to measure the optical radiant conditions for plant growth.

Calibration of MODTRAN3 with PGAMS observational data for atmospheric correction applications

Abstract: The portable ground-based atmospheric monitoring system (PGAMS) is a spectroradiometer system that provides a set of in situ solar and hemispherical sky irradiance, path radiance, and surface reflectance measurements. The observations provide input parameters for the calibration of atmospheric algorithms applied to multispectral and hyperspectral images in the visible and near infrared spectrum. Presented in this paper are the results of comparing hyperspectral surface radiances calculated using MODTRAN3 with PGAMS field measurements for a blue tarp and grass surface targets. Good agreement was obtained by constraining MODTRAN3 to only a rural atmospheric model with a calibrated visibility and surface reflectance from PGAMS observations. This was accomplished even though the sky conditions were unsteady as indicated by a varying aerosol extinction. Average absolute differences of 11.3 and 7.4 percent over the wavelength range from 400 to 1000 nm were obtained for the grass and blue tarp surfaces respectively. However, transformation to at-sensor radiances require additional constraints on the single-scattering albedo and scattering phase function so that they exhibit the specific real-time aerosol properties rather than a seasonal average model.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.