Bettye C. Johnson

Bidirectional reflectance round-robin in support of the Earth Observing System program

Abstract Laboratory measurements of the bidirectional reflectance distribution function (BRDF) of diffuse reflectors are required to support calibration in the Earth Observing System (EOS) program of the National Aeronautics and Space Administration. To assess the ability of the instrument calibration laboratories to perform accurate BRDF measurements, a round-robin with the National Institute of Standards and Technology (NIST) as the central laboratory was initiated by the EOS Project Science Office. The round-robin parameters include sample type, wavelength, and incident and viewing angles. The results show that the participating calibration laboratories are, with a few exceptions due to experimental techniques or sample properties, generally able to measure BRDF for the round-robin parameters to within 2% of the values measured by NIST.

The NIST EOS thermal-infrared transfer radiometer

A portable thermal-infrared transfer radiometer (TXR) has been developed for use in comparisons and scale verifications of sources used to calibrate thermal-infrared (TIR) channels of the National Aeronautics and Space Administrations (NASAs) Earth Observing System (EOS) flight instruments. The TXR is designed to measure the radiance temperature of large-area black-body sources in cryogenic vacuum environments, either at the National Institute of Standards and Technology (NIST) or at the EOS instrument-calibration facilities. It can be operated in ambient conditions of room temperature and pressure, or in EOS instrument thermal/vacuum chambers at temperatures as low as 77 K. The TXR is a liquid-nitrogen-cooled filter radiometer with two channels: one centred at 5 µm based on a photovoltaic InSb detector, and the other centred at 10 µm based on a photovoltaic HgCdTe (MCT) detector. The spectral, spatial and temporal characterization of the TXR using state-of-the-art NIST ambient-infrared instrumentation is reported.

The Miami2001 Infrared Radiometer Calibration and Intercomparison. Part I: Laboratory Characterization of Blackbody Targets

Abstract The second calibration and intercomparison of infrared radiometers (Miami2001) was held at the University of Miamis Rosenstiel School of Marine and Atmospheric Science (RSMAS) during May–June 2001. The participants were from several groups involved with the validation of skin sea surface temperatures and land surface temperatures derived from the measurements of imaging radiometers on earth observation satellites. These satellite instruments include those currently on operational satellites and others that will be launched within two years following the workshop. There were two experimental campaigns carried out during the 1-week workshop: a set of measurements made by a variety of ship-based radiometers on board the Research Vessel F. G. Walton Smith in Gulf Stream waters off the eastern coast of Florida, and a set of laboratory measurements of typical external blackbodies used to calibrate these ship-based radiometers. This paper reports on the results obtained from the laboratory characteriza...

Hyperspectral image projectors for radiometric applications

We describe a Calibrated Hyperspectral Image Projector (CHIP) intended for radiometric testing of instruments ranging from complex hyperspectral or multispectral imagers to simple filter radiometers. The CHIP, based on the same digital mirror arrays used in commercial Digital Light Processing (DLP) displays, is capable of projecting any combination of as many as approximately one hundred different arbitrarily programmable basis spectra per frame into each pixel of the instrument under test (IUT). The resulting spectral and spatial content of the image entering the IUT can simulate, at typical video frame rates and integration times, realistic scenes to which the IUT will be exposed during use, and its spectral radiance can be calibrated with a spectroradiometer. Use of such generated scenes in a controlled laboratory setting would alleviate expensive field testing, allow better separation of environmental effects from instrumental effects and enable system-level performance testing and validation of space-flight instruments prior to launch. Example applications are system-level testing of complex hyperspectral imaging instruments and algorithms with realistic scenes and testing the performance of first-responder cameras under simulated adverse conditions. We have built and tested a successful prototype of the spectral engine, a primary component of the CHIP, that generates arbitrary, programmable spectra in the 1000 nm to 2500 nm spectral range. We have also built a spectral engine operating at visible wavelengths to be discussed in a separate publication. Here we present an overview of this technology and its applications and discuss experimental performance results of our prototype infrared spectral engine.

Sources of Differences in On-Orbital Total Solar Irradiance Measurements and Description of a Proposed Laboratory Intercomparison.

There is a 5 W/m2 (about 0.35 %) difference between current on-orbit Total Solar Irradiance (TSI) measurements. On 18–20 July 2005, a workshop was held at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland that focused on understanding possible reasons for this difference, through an examination of the instrument designs, calibration approaches, and appropriate measurement equations. The instruments studied in that workshop included the Active Cavity Radiometer Irradiance Monitor III (ACRIM III) on the Active Cavity Radiometer Irradiance Monitor SATellite (ACRIMSAT), the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate Experiment (SORCE), the Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO), and the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS). Presentations for each instrument included descriptions of its design, its measurement equation and uncertainty budget, and the methods used to assess on-orbit degradation. The workshop also included a session on satellite- and ground-based instrument comparisons and a session on laboratory-based comparisons and the application of new laboratory comparison techniques. The workshop has led to investigations of the effects of diffraction and of aperture area measurements on the differences between instruments. In addition, a laboratory-based instrument comparison is proposed that uses optical power measurements (with lasers that underfill the apertures of the TSI instruments), irradiance measurements (with lasers that overfill the apertures of the TSI instrument), and a cryogenic electrical substitution radiometer as a standard for comparing the instruments. A summary of the workshop and an overview of the proposed research efforts are presented here.

Spectrally Tunable Sources for Advanced Radiometric Applications

A common radiometric platform for the development of application-specific metrics to quantify the performance of sensors and systems is described. Using this platform, sensor and system performance may be quantified in terms of the accuracy of measurements of standardized sets of source distributions. The prototype platform consists of spectrally programmable light sources that can generate complex spectral distributions in the ultraviolet, visible and short-wave infrared regions for radiometric, photometric and colorimetric applications. In essence, the programmable spectral source is a radiometric platform for advanced instrument characterization and calibration that can also serve as a basis for algorithm testing and instrument comparison.

Radiometric Measurement Comparison on the Integrating Sphere Source used to Calibrate the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Landsat 7 Enhanced Thematic Mapper Plus (ETM

As part of a continuing effort to validate the radiometric scales assigned to integrating sphere sources used in the calibration of Earth Observing System (EOS) instruments, a radiometric measurement comparison was held in May 1998 at Raytheon/Santa Barbara Remote Sensing (SBRS). This comparison was conducted in support of the calibration of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) instruments. The radiometric scale assigned to the Spherical Integrating Source (SIS100) by SBRS was validated through a comparison with radiometric measurements made by a number of stable, well-characterized transfer radiometers from the National Institute of Standards and Technology (NIST), the National Aeronautics and Space Administration’s Goddard Space Flight Center (NASA’s GSFC), and the University of Arizona Optical Sciences Center (UA). The measured radiances from the radiometers differed by ±3 % in the visible to near infrared when compared to the SBRS calibration of the sphere, and the overall agreement was within the combined uncertainties of the individual measurements. In general, the transfer radiometers gave higher values than the SBRS calibration in the near infrared and lower values in the blue. The measurements of the radiometers differed by ±4 % from 800 nm to 1800 nm compared to the SBRS calibration of the sphere, and the overall agreement was within the combined uncertainties of the individual measurements for wavelengths less than 2200 nm. The results of the radiometric measurement comparison presented here supplement the results of previous measurement comparisons on the integrating sphere sources used to calibrate the Multi-angle Imaging SpectroRadiometer (MISR) at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, CA and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) at NEC Corporation, Yokohama, Japan.

Radiometric and Engineering Performance of the SeaWiFS Quality Monitor (SQM): A Portable Light Source for Field Radiometers

Abstract A portable and stable source of radiant flux, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM), was developed as a field instrument for use in experiments away from the calibration laboratory such as those encountered during oceanographic cruises. The SQM monitors the radiometric stability of radiance and irradiance sensors during these field experiments; a companion paper gives results acquired during the third Atlantic Meridional Transect cruise. In conjunction with laboratory calibration sources, the SQM can be used to transfer the calibration to the field experiment. Two independent lamp assemblies generate three flux levels, and the lamps are operated at constant current using active control. The exit aperture of the SQM is large and homogeneous in radiance. The SQM was designed to approximate a Lambertian radiator. An internal heater provides operational stability and decreased warmup intervals, which minimizes lamp hours. Temperature-controlled silicon photodiodes ...

Development of a Portable Integrating Sphere Source for the Earth Observing System's Calibration Validation Programme

We have designed and built a portable integrating sphere source for use at the National Aeronautics and Space Administration (NASA) satellite sensor calibration facilities in support of the Earth Observing System (EOS) calibration programme. The source is designed to complement existing detector-based validation methodologies over the spectral range 400-2500 nm, affording extra flexibility in implementing the programme. Details of the design and operation of the portable source will be described and discussed.

Radiometric Measurement Comparison Using the Ocean Color Temperature Scanner (OCTS) Visible and Near Infrared Integrating Sphere.

As a part of the pre-flight calibration and validation activities for the Ocean Color and Temperature Scanner (OCTS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color satellite instruments, a radiometric measurement comparison was held in February 1995 at the NEC Corporation in Yokohama, Japan. Researchers from the National Institute of Standards and Technology (NIST), the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC), the University of Arizona Optical Sciences Center (UA), and the National Research Laboratory of Metrology (NRLM) in Tsukuba, Japan used their portable radiometers to measure the spectral radiance of the OCTS visible and near-infrared integrating sphere at four radiance levels. These four levels corresponded to the configuration of the OCTS integrating sphere when the calibration coefficients for five of the eight spectral channels, or bands, of the OCTS instrument were determined. The measurements of the four radiometers differed by −2.7 % to 3.9 % when compared to the NEC calibration of the sphere and the overall agreement was within the combined measurement uncertainties. A comparison of the measurements from the participating radiometers also resulted in agreement within the combined measurement uncertainties. These results are encouraging and demonstrate the utility of comparisons using laboratory calibration integrating sphere sources. Other comparisons will focus on instruments that are scheduled for spacecraft in the NASA study of climate change, the Earth Observing System (EOS).