Patricia Yvonne Barnes

Realization of the National Institute of Standards and Technology Detector-Based Spectral Irradiance Scale

A detector-based spectral irradiance scale has been realized at the National Institute of Standards and Technology (NIST). Unlike the previous NIST spectral irradiance scales, the new scale is generated with filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST high-accuracy cryogenic radiometer instead of with the gold freezing-point blackbody. The calibrated filter radiometers are then used to establish the radiance temperature of a high-temperature blackbody (HTBB) operating near 3,000 K The spectral irradiance of the HTBB is then determined with knowledge of the geometric factors and is used to assign the spectral irradiances of a group of 1,000-W free-electron laser lamps. The detector-based spectral irradiance scale results in the reduction of the uncertainties from the previous source-based spectral irradiance scale by at least a factor of 2 in the ultraviolet and visible wavelength regions. The new detector-based spectral irradiance scale also leads to a reduction in the uncertainties in the shortwave infrared wavelength region by at least a factor of 2-10, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized.

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.

Measurements and predictions of light scattering by clear coatings

Comparisons are made between calculated and measured angle-resolved light-scattering distributions from clear dielectric isotropic epoxy coatings over a range of rms roughness conditions, resulting in strongly specular scattering to diffuse scattering characteristics. Calculated distributions are derived from topography measurements performed with interferometric microscopes. Two methods of calculation are used. One determines the intensity of scattered light waves with a phase integral in the Kirchhoff approximation. The other is based on the reflection of light rays by locally flat surfaces. The angle-resolved scattering distributions for the coatings are measured with the spectral trifunction automated reference reflectometer (STARR) developed by the National Institute of Standards and Technology. Comparisons between measured and calculated results are shown for three surfaces with rms roughness values of approximately 3, 150, and 800 nm for an angle of incidence of 20 degrees .

The realization of the NIST detector-based spectral irradiance scale

The realization of a detector-based spectral irradiance scale at the National Institute of Standards and Technology is described. The new scale is established using filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST High-Accuracy Cryogenic Radiometer unlike the previous NIST spectral irradiance scales based upon the gold freezing-point blackbody. The radiance temperatures of a high-temperature blackbody (HTBB) operating near 3000 K are found using calibrated filter radiometers. The spectral irradiances of a group of 1000 W FEL lamps are assigned using the spectral irradiance of the HTBB determined using the knowledge of the geometric factors and the detector-based radiance temperatures. The detector-based spectral irradiance scale leads to a reduction in the uncertainties from the previous, source-based, spectral irradiance scales by at least a factor of two in the ultraviolet and visible wavelength regions, and also leads to a reduction in the uncertainties in the short-wave infrared wavelength region by at least a factor of two to ten, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized, and beginning in the calendar year 2001, all spectral irradiance sources are issued using the detector-based scale.

Near infrared 45 degrees/0 degrees reflectance factor of pressed polytetrafluoroethylene (PTFE) powder

Pressed polytetrafluoroethylene (PTFE) powder is commonly used as a reflectance standard for bidirectional and hemispherical geometries. The wavelength dependence of the reflectance factor of PTFE is presented for the near-infrared spectral region (800 nm to 1600 nm) for the 45°/0° geometry, as well as in the visible spectral region (380 nm to 800 nm) for comparison with previously published results.

Comparison of ultraviolet bidirectional reflectance distribution function (BRDF) measurements of diffusers used in the calibration of the Total Ozone Mapping Spectrometer (TOMS)

The measurement and long-term monitoring of global total ozone by ultraviolet albedo measuring satellite instruments require accurate and precise determination of the Bi-directional Reflectance Distribution Function (BRDF) of laboratory-based diffusers used in the pre-launch calibration of those instruments. To assess the ability of laboratories to provide accurate UltraViolet (UV) diffuse BRDF measurements, a BRDF measurement comparison was initiated by the NASA Total Ozone Mapping Spectrometer (TOMS) Project. From December 1998 to September 1999, NASAs Goddard Space Flight Center (GSFC), TPD TNO (formerly the TNO Institute of Applied Physics), and the National Institute of Standards and Technology (NIST) made BRDF measurements on four Spectralon diffusers used in the pre-launch calibration of three TOMS instruments. The diffusers were measured at the six TOMS wavelengths and at the incident and scatter angles used in the TOMS pre-launch calibration. The participation of GSFC, TPD TNO, and NIST in the comparison establishes a link between the diffuser calibrations of the on-orbit TOMS instruments, the Ozone Monitoring Instrument (OMI), and a national standards laboratory. The results of the comparison show that all of the BRDF measurements on the four diffusers agreed within +0.85 % to -1.10 % of the average BRDF and were well within the combined measurement uncertainties of the participating laboratories.

Diffuse reflectance of sintered and pressed polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is widely used in applications requiring a material with a diffuse reflectance factor close to unity. The bidirectional reflectance distribution function (BRDF) of both pressed and sintered samples was measured as a function of wavelength, polarization, and incident and viewing angles. The results for both samples show that the BRDF is constant at wavelengths from 400 nm to 1000 nm and depends on the polarization of the incident beam of radiant flux, that the BRDF varies with viewing angle, and that Helmholtz reciprocity holds for angles from 10 degrees to 60 degrees.

UV bidirectional reflectance distribution function measurements for diffusers

Bidirectional reflectance distribution function (BRDF) is an important quantity to describe the scattering condition from a diffuse surface. The current presentation describes the instrumentation, instrument characterization and diffuser calibration in the ultraviolet (UV) spectral region.

Reflectance standards at ultraviolet wavelengths

Polytetrafluoroethylene (PTFE) is widely used in remote sensing applications requiring a diffuse reflectance standard for detector calibration. The bi-directional and directional-hemispherical reflectance properties of both pressed and sintered PTFE were measured at ultraviolet wavelengths to provide information for their use as standards in this spectral range. The reflectance decreases with decreasing wavelength for both geometries, and the ratio between the reflectances for these geometries remains constant for wavelengths from 300 nm to 400 nm.

Intercomparisons of reflectance measurements

A comparison of spectral diffuse reflectance between different national standards laboratories is being planned under the direction of the Comite Consultatif de Photometrie et Radiometrie (CCPR). A similar comparison of bidirectional reflectance distribution factor among laboratories in the United States in support of optical remote sensing measurements is nearing completion. Since this comparison provides valuable lessons for the one organized by the CCPR, pertinent results and their implications are presented.