Incandescent lamp-based facility for measuring the long-term radiometric stability of spectrographs.

The long-term temporal stability of a spectrograph is one of the most important characteristics affecting the spectrograph's radiometric performance. For many applications, from monitoring ocean color and lunar irradiance to laboratory irradiance measurement standards, the stability of a spectrograph is a primary factor in the overall measurement uncertainty and therefore is the major criterion for the suitability of the spectrograph as an optical-scale transfer standard. Here we report a facility built for testing the long-term radiometric stability of commercial, fiber-coupled spectrographs. The facility uses tungsten quartz-halogen irradiance standard lamps, type "FEL," of the National Institute of Standards and Technology (NIST) as light sources. To ensure the highest stability of these lamps during spectrograph tests, parameters such as lamp current, lamp voltage, and signals from an independent filter radiometer were continuously recorded to monitor any possible instability caused by such effects as lamp aging. Using this facility, we report the stability study of four spectrographs with spectral coverage from the UV to short-wave infrared over an interval of two months during which the lamp irradiance was stable to better than 0.02%. The tested spectrographs show good stability in general, ranging from 0.02% to 0.1% in the visible over a span of 11 days. For a longer two-month test, the variation in spectrograph responses increases by less than 0.1% with no discernable long-term drifts. In addition, we measured the response variation of two of the test spectrographs before and after they were sent to remote field locations and subjected to adverse environmental conditions. In this case, a larger response variation of up to 1.0% dependence on the wavelength was observed. We discuss the performance of the facility and the implications for using these spectrographs for several of NIST's remote sensing projects as radiometric transfer standards based on these stability measurements.

Impact of spectral resolution of in situ ocean color radiometric data in satellite matchups analyses.

The spectral resolution requirements for in situ remote sensing reflectanceRRS measurements aiming at supporting satellite ocean color validation and System Vicarious Calibration (SVC) were investigated. The study, conducted using sample hyperspectral RRS from different water types, focused on the visible spectral bands of the ocean land color imager (OLCI) and of the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite sensors. Allowing for a ±0.5% maximum difference between in situ and satellite derived RRS solely due to the spectral band characteristics of the in situ radiometer, a spectral resolution of 1 nm for SVC of PACE is needed in oligotrophic waters. Requirements decrease to 3 nm for SVC of OLCI. In the case of validation activities, which exhibit less stringent uncertainty requirements with respect to SVC, a maximum difference of ±1% between in situ and satellite derived data indicates the need for a spectral resolution of 3 nm for both OLCI and PACE in oligotrophic waters. Conversely, spectral resolutions of 6 nm for PACE and 9 nm for OLCI appear to satisfy validation activities in optically complex waters.

A method to extrapolate the diffuse upwelling radiance attenuation coefficient to the surface as applied to the Marine Optical Buoy (MOBY).

The upwelling radiance attenuation coefficient (KLu) in the upper 10 m of the water column can be significantly influenced by inelastic scattering processes, and thus will vary even with homogeneous water properties. The Marine Optical BuoY (MOBY), the primary vicarious calibration site for many ocean color sensors, makes measurements of the upwelling radiance (Lu) at 1 m, 5 m, and 9 m and uses these values to determine KLu and propagate the upwelling radiance directed toward the zenith, Lu, at 1 m to and through the surface. Inelastic scattering causes the KLu derived from the arm measurements to be an underestimate of the true KLu from 1 m to the surface at wavelengths greater than 575 nm, thus the derived water leaving radiance is underestimated at wavelengths longer than 575 nm. A method to correct this KLu, based on a model of the upwelling radiance including Raman scattering and chlorophyll fluorescence has been developed which corrects this bias. The model has been experimentally validated, and this technique can be applied to the MOBY data set to provide new, more accurate products at these wavelengths. When applied to a 4 month MOBY deployment, the corrected water leaving radiance, Lw, can increase by 5 % (600 nm), 10 % (650 nm) and 50 % (700 nm). This method will be used to provide additional more accurate products in the MOBY data set.

Immersion Coefficient for the Marine Optical Buoy (MOBY) Radiance Collectors.

The immersion coefficient accounts for the difference in responsivity for a radiometer placed in the air versus water or another medium. In this study, the immersion coefficients for the radiance collectors on the Marine Optical Buoy (MOBY) were modeled and measured. The experiment showed that the immersion coefficient for the MOBY radiance collectors agreed with a simple model using only the index of refraction for water and fused silica. With the results of this experiment, we estimate that the uncertainty in the current value of the immersion coefficient used in the MOBY project is 0.05 % (k = 1).

Goniometric and hemispherical reflectance and transmittance measurements of fused silica diffusers.

Fused silica diffusers, made by forming scattering centers inside fused silica glass, can exhibit desirable optical properties, such as reflectance or transmittance independent of viewing angle, spectrally flat response into the ultraviolet wavelength range, and good spatial uniformity. The diffusers are of interest for terrestrial and space borne remote sensing instruments, which use light diffusers in reflective and transmissive applications. In this work, we report exploratory measurements of two samples of fused silica diffusers. We will present goniometric bidirectional scattering distribution function (BSDF) measurements under normal illumination provided by the National Institute of Standards and Technology (NIST)'s Goniometric Optical Scatter Instrument (GOSI), by NIST's Infrared reference integrating sphere (IRIS) and by the National Aeronautics and Space Administration (NASA)'s Diffuser Calibration Laboratory. We also present hemispherical diffuse transmittance and reflectance measurements provided by NIST's Double integrating sphere Optical Scattering Instrument (DOSI). The data from the DOSI is analyzed by Prahl's inverse adding-doubling algorithm to obtain the absorption and reduced scattering coefficient of the samples. Implications of fused silica diffusers for remote sensing applications are discussed.

Optical Passive Sensor Calibration for Satellite Remote Sensing and the Legacy of NOAA and NIST Cooperation.

This paper traces the cooperative efforts of scientists at the National Oceanic and Atmospheric Administration (NOAA) and the National Institute of Standards and Technology (NIST) to improve the calibration of operational satellite sensors for remote sensing of the Earth's land, atmosphere and oceans. It gives a chronological perspective of the NOAA satellite program and the interactions between the two agencies' scientists to address pre-launch calibration and issues of sensor performance on orbit. The drive to improve accuracy of measurements has had a new impetus in recent years because of the need for improved weather prediction and climate monitoring. The highlights of this cooperation and strategies to achieve SI-traceability and improve accuracy for optical satellite sensor data are summarized.

Results of aperture area comparisons for exo-atmospheric total solar irradiance measurements.

Exo-atmospheric solar irradiance measurements made by the solar irradiance community since 1978 have incorporated limiting apertures with diameters measured by a number of metrology laboratories using a variety of techniques. Knowledge of the aperture area is a critical component in the conversion of radiant flux measurements to solar irradiance. A National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) sponsored international comparison of aperture area measurements of limiting apertures provided by solar irradiance researchers was performed, the effort being executed by the National Institute of Standards and Technology (NIST) in coordination with the EOS Project Science Office. Apertures that had institutional heritage with historical solar irradiance measurements were measured using the absolute aperture measurement facility at NIST. The measurement technique employed noncontact video microscopy using high-accuracy translation stages. We have quantified the differences between the participating institutions' aperture area measurements and find no evidence to support the hypothesis that preflight aperture area measurements were the root cause of discrepancies in long-term total solar irradiance satellite measurements. Another result is the assessment of uncertainties assigned to methods used by participants. We find that uncertainties assigned to a participant's values may be underestimated.

The Use of Filtered Radiometers for Radiance Measurement.

A methodology for using a calibrated filter radiometer to measure and monitor the spectral radiance of calibration sources is described. An example is presented using the NIST calibration sphere source that is used to support the NASA Earth Observing remote-sensing program.

Simple spectral stray light correction method for array spectroradiometers.

A simple, practical method has been developed to correct a spectroradiometer's response for measurement errors arising from the instrument's spectral stray light. By characterizing the instrument's response to a set of monochromatic laser sources that cover the instrument's spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instrument's response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instrument's software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.

Radiometric validation of NASA's Ames Research Center's Sensor Calibration Laboratory.

The National Aeronautics and Space Administration's (NASA's) Ames Research Center's Airborne Sensor Facility (ASF) is responsible for the calibration of several airborne Earth-viewing sensor systems in support of NASA Earth Observing System (EOS) investigations. The primary artifact used to calibrate these sensors in the reflective solar region from 400 to 2500 nm is a lamp-illuminated integrating sphere source. In September 1999, a measurement comparison was made at the Ames ASF Sensor Calibration Facility to validate the radiometric scale, establish the uncertainties assigned to the radiance of this source, and examine its day-to-day repeatability. The comparison was one of a series of validation activities overseen by the EOS Calibration Program to ensure the radiometric calibration accuracy of sensors used in long-term, global, remote-sensing studies. Results of the comparison, including an evaluation of the Ames Sensor Calibration Laboratory (SCL) measurement procedures and assigned radiometric uncertainties, provide a validation of their radiometric scale at the time of the comparison. Additionally, the maintenance of the radiance scale was evaluated by use of independent, long-term, multiyear radiance validation measurements of the Ames sphere source. This series of measurements provided an independent assessment of the radiance values assigned to integrating sphere sources by the Ames SCF. Together, the measurements validate the SCF radiometric scale and assigned uncertainties over the time period from September 1999 through July 2003.

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 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).

Intercomparison of the ITS-90 Radiance Temperature Scales of the National Physical Laboratory (U.K.) and the National Institute of Standards and Technology.

An intercomparison of radiance temperature scales has been performed by the National Physical Laboratory (NPL) and the National Institute of Standards and Technology (NIST) using a standard transfer pyrometer operating at a wavelength of approximately 1000 nm. It was found that the radiance temperature scales established by the two laboratories were in agreement to 0.1% or better of the temperature over the range 1000 °C to 2500 °C.