Ambler Thompson

The 1996 North American Interagency Intercomparison of Ultraviolet Monitoring Spectroradiometers

Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. To assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks, the first North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 19–29, 1994 at Table Mountain outside Boulder, Colorado, USA. This Intercomparison was coordinated by the National Institute of Standards and Technology and the National Oceanic and Atmospheric Administration (NOAA). Participating agencies were the Environmental Protection Agency, National Science Foundation, Smithsonian Environmental Research Center, and Atmospheric Environment Service, Canada. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity, the latter with a NIST standard lamp calibrated to operate in the horizontal position. The spectral irradiance responsivity was determined once indoors and twice outdoors, and demonstrated that, while the responsivities changed upon moving the instruments, they were relatively stable when the instruments remained outdoors. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST standard lamp, and a simple convolution technique to account for the different bandwidths of the instruments, the measured solar irradiances agreed within 5 %.

Methodology for Deriving Clear-Sky Erythemal Calibration Factors for UV Broadband Radiometers of the U.S. Central UV Calibration Facility

Abstract In the United States, there are several federal agencies interested in the effects of UV radiation, which has resulted in the establishment of UV monitoring programs each with their own instrumentation and sites designed to address their specific needs. In 1993, participating agencies of the U.S. Global Change Research Program organized a UV Panel for coordinating the different agencies’ programs in order to ensure that UV data are intercalibrated, have common quality assurance and control procedures, and that the efforts among agencies are not duplicated. In order to achieve these goals, in 1994 the UV Panel recommended formation of the U.S. Central UV Calibration Facility (CUCF), which is operated by the Surface Radiation and Research Branch of the Air Resources Laboratory of National and Oceanic Atmospheric Administration. The CUCF is responsible for characterizing and calibrating UV measuring instruments from several U.S. federal agencies. Part of this effort is to calibrate UVB broadband rad...

The 1997 North American Interagency Intercomparison of Ultraviolet Spectroradiometers Including Narrowband Filter Radiometers

The fourth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 15 to 25, 1997 at Table Mountain outside of Boulder, Colorado, USA. Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NIST and NOAA, and included participants from the ASRC, EPA, NIST, NSF, SERC, USDA, and YES. The UV measuring instruments included scanning spectroradiometers, spectrographs, narrow band multi-filter radiometers, and broadband radiometers. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity. The spectral irradiance responsivity was determined two to three times outdoors to assess temporal stability. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST traceable standard lamp, and a simple convolution technique with a Gaussian slit-scattering function to account for the different bandwidths of the instruments, the measured solar irradiance from the spectroradiometers excluding the filter radiometers at 16.5 h UTC had a relative standard deviation of ±4 % for wavelengths greater than 305 nm. The relative standard deviation for the solar irradiance at 16.5 h UTC including the filter radiometer was ±4 % for filter functions above 300 nm.

Irradiance of Horizontal Quartz-Halogen Standard Lamps

Spectral irradiance calibrations often require that irradiance standard lamps be oriented differently than the normal calibration orientation used at the National Institute of Standards and Technology and at other standards laboratories. For example, in solar measurements the instruments are generally upward viewing, requiring horizontal working standards for minimization of irradiance calibration uncertainties. To develop a working standard for use in a solar ultraviolet intercomparison, NIST determined the irradiance of quartz-halogen lamps operating in the horizontal position, rather than in the customary vertical position. An experimental technique was developed which relied upon equivalent lamps with independent mounts for each orientation and a spectroradiometer with an integrating sphere whose entrance port could be rotated 90° to view either lamp position. The results presented here are limited to 1000 W quartz-halogen type lamps at ultraviolet wavelengths from 280 nm to 400 nm. Sources of uncertainty arose from the lamps, the spectroradiometer, and the lamp alignment, and increased the uncertainty in the irradiance of horizontal lamps by less than a factor of two from that of vertical NIST standard lamps. The irradiance of horizontal lamps was less than that of vertical lamps by approximately 6 % at long wavelengths (400 nm) to as much as 12 % at the shortest wavelengths (280 nm). Using the Wien radiation law, this corresponds to color temperature differences of 15.7 K and 21.3 K for lamps with clear and frosted envelopes, respectively.

Standards for Corrected Fluorescence Spectra

A set of four fluorescent standards has been produced and calibrated at the National Institute of Standards and Technology (NIST) for corrected relative spectral emission over the wavelength range from 400 to 740 nm. This new Standard Reference Material (SRM) 1931 has been produced in the form of sintered mixtures of inorganic phosphors and polytetrafluorethylene (PTFE) powder for spectral response calibration of spectrofluorimeters used in biology, medicine and studies of molecular structure. They provide a means for correcting spectrofluorimeter errors due to wavelength dependencies of optics, monochromators, and detectors, thus permitting meaningful intercomparison of results obtained in different laboratories.

SPECTRAL RADIANCE OF A LARGE-AREA INTEGRATING SPHERE SOURCE

The radiance and irradiance calibration of large field-of-view scanning and imaging radiometers for remote sensing and surveillance applications has resulted in the development of novel calibration techniques. One of these techniques is the employment of large-area integrating sphere sources as radiance or irradiance secondary standards. To assist the National Aeronautical and Space Administration’s space based ozone measurement program, a commercially available large-area internally illuminated integrating sphere source’s spectral radiance was characterized in the wavelength region from 230 nm to 400 nm at the National Institute of Standards and Technology. Spectral radiance determinations and spatial mappings of the source indicate that carefully designed large-area integrating sphere sources can be measured with a 1 % to 2 % expanded uncertainty (two standard deviation estimate) in the near ultraviolet with spatial nonuniformities of 0.6 % or smaller across a 20 cm diameter exit aperture. A method is proposed for the calculation of the final radiance uncertainties of the source which includes the field of view of the instrument being calibrated.

Portable High Accuracy Double Spectrometer for Ultraviolet Spectral Irradiance Measurements

A portable high accuracy double spectrometer for ultraviolet (UV) spectral irradiance measurements has been under development at the National Institute of Standards and Technology (NIST) or several years. It has been used in the comparison of NIST spectral irradiance sources: an electron storage ring, 1000 W quartz-halogen lamps, deuterium arc lamps, and a windowless argon miniarc. A UV spectral irradiance intercomparison with the Physikalische Technische Bundesanstalt of Germany has also been carried out with the instrument. This paper will discuss the modular design of this instrument, preliminary uncertainty analysis, results of standard source comparisons and results of solar ultraviolet measurements using the UV spectrometer with the NIST spectral irradiance field calibrator.

International Space Station utilization for radiometric calibration support to earth remote sensing programs

The idea of utilizing the International Space Station (ISS) to provide a platform for manned and maintained NIST/SI traceable standard sensors in the visible, infrared, and microwave spectral regions to augment the spectral calibration of other remote sensing sensors is being well received in the calibration community. Review of the current state of the art in solar monitoring as evidenced by intercomparison between different satellite sensors for total solar irradiance, solar spectral irradiance and measured spectral irradiance versus reference spectra very much proves this point. Analysis of the Microwave Sounding Unit (MSU) radiometer data to deduce global temperature trends also shows the dire need for a better calibration of these instruments flown on sequential satellites to a very high degree of accuracy. In this regard possible radiometric calibration missions on ISS as envisioned by NIST will be discussed. Also the current plan for a feasibility study for these missions on ISS that address issues...

Terrestrial ultraviolet monitoring spectroradiometer intercomparisons: results and field standard development

Two interagency solar ultraviolet spectroradiometer intercomparisons (September 1994 and June 1995) have been conducted at Table Mountain outside Boulder, Colorado. A third intercomparison is planned for June 1996 at the same location. The intercomparisons were organized by the National Institute of Standards and Technology (NIST) at a field site supported by the National Oceanic and Atmospheric Administration and included participants representing North American agencies currently monitoring solar ultraviolet irradiance. Instruments were characterized at the intercomparisons for wavelength error, bandwidth, and stray- light rejection. All instruments were calibrated for spectral irradiance responsivity by NIST personnel with specially developed quartz-halogen field irradiance standards. Thus, comparisons of synchronized solar irradiance measurements allowed true comparisons between instruments, without the added complication of differences in irradiance scales of the participants. Estimates of differences in individual network propagation of irradiance scales were made separately. In addition, there were extensive concurrent ancillary measurements in an effort to compile a standard data set. We will discuss the results of these experiments and the overall agreement between the participants.

Strategy for solar-ultraviolet instrument comparisons

Several different monitoring networks using spectroradiometers are being developed to detect changes in the solar ultraviolet irradiance at the surface of the earth due to stratospheric ozone depletion. To ensure the accurate, long-term measurements that are required from these networks, a strategy for instrument intercomparisons is necessary. This involves the characterization of the instrument parameters which affect measurements of solar ultraviolet irradiance, particularly the linearity, wavelength accuracy, irradiance responsivity, slit- scattering function, and cosine response of the instrument. We have developed laboratory techniques for determining each of these parameters and have used them to characterize a specific spectroradiometer. These techniques and results are being used to develop a strategy for a planned field intercomparison of instruments from several monitoring networks.