Robert D. Saunders

Spectral irradiance standard for the ultraviolet: the deuterium lamp

A set of deuterium lamps is calibrated as spectral irradiance standards in the 200-350-nm spectral region utilizing both a high accuracy tungsten spectral irradiance standard and a newly developed argon mini-arc spectral radiance standard. The method which enables a transfer from a spectral radiance to a spectral irradiance standard is described. The following characteristics of the deuterium lamp irradiance standard are determined: sensitivity to alignment; dependence on input power and solid angle; reproducibility; and stability. The absolute spectral radiance is also measured in the 167-330-nm region. Based upon these measurements, values of the spectral irradiance below 200 nm are obtained through extrapolation.

Results of a CCPR Intercomparison of Spectral Irradiance Measurements by National Laboratories

An intercomparison of spectral irradiance measurements by 12 national laboratories has been carried out between 1987 and 1990. The intercomparison was conducted under the auspices of the Comité Consultatif de Photometrie et Radiometrie (CCPR) of the Comité International des Poids et Mesures, and the National Institute of Standards and Technology (NIST) served as the pilot laboratory. The spectral range of the intercomparison was 250 to 2400 nm and the transfer standards used were commercial tungsten-halogen lamps of two types. The world-wide consistency of the results (one standard deviation) was on the order of 1% in the visible spectral region and 2 to 4% in the ultraviolet and infrared portions of the spectrum. The intercomparison revealed no statistically significant differences between spectral-irradiance scales based on blackbody physics and absolute detector radiometry.

Spectroradiometric Determination of the Freezing Temperature of Gold

A direct spectroradiometric determination of the temperature of freezing gold was performed by measuring the spectral radiances of a gold blackbody relative to those of a laser-irradiated integrating sphere which was calibrated with absolute silicon detectors and an electrically calibrated radiometer. The measurements were performed at three laser wavelengths near 600 nm, and the temperature of the blackbody was calculated by substituting the measured spectral radiances into Planck’s radiation formula. The result obtained, TAu=(1337.33± 0.34) K, is 0.25 K below the gold-point assignment in the International Practical Temperature Scale of 1968 (IPTS-68) and has been adopted in September 1990 as the new gold-point value in the International Temperature Scale of 1990 (ITS-90). The effect of this change in the gold-point assignment on pyrometric, radiometric, and photometric measurement services provided by the National Institute of Standards and Technology is assessed.

Radiative Calibration of Heat-Flux Sensors at NIST: Facilities and Techniques

We present an overview of the National Institute of Standards and Technology high temperature blackbodies, both in operation and in development, suitable for heat-flux sensor calibration. Typical results of calibrations using the transfer technique in the 25 mm Variable-Temperature Blackbody are presented to demonstrate the long-term repeatability of the calibration technique. A comparative study of the absolute and transfer calibrations of a Gardon gage in a spherical blackbody with a cooled enclosure surrounding the gage housing was conducted. Results of this study demonstrated the influence of convection associated with absolute calibration of sensors in a cooled enclosure. Plans for further development of the transfer technique to higher heat-flux levels and the associated technical issues are discussed.

Methods to reduce the size-of-source effect in radiometers

In radiometry, photometry and radiation thermometry, accurate measurements of the radiance, luminance or the radiance temperatures of sources requires a knowledge of the contribution from the surroundings to the measured signal from the target area. The dependence of the radiometer or the radiation thermometer on the area surrounding the target area is described as the size-of-source effect (SSE), and minimizing the radiometers sensitivity to SSE is critical in the lowest-uncertainty optical measurements. We describe the dominant effects that influence the SSE, and show that the SSE can be reduced to <5 ? 10?5 as measured using a 50?mm diameter radiance source with a 2?mm diameter, central obscuration. The SSE is found to be dependent on the internal scatter and the optical design of the radiometer. For testing the contributions to SSE, a radiometer is constructed with a 50.8?mm diameter lens in f/12 geometry with a 1?mm diameter target size. If the internal radiometer scatter is reduced, then the SSE is found to be primarily dependent on the scatter from the objective lens such as surface finish, internal lens scatter and the particulate contamination of the lens. Various combinations of objective lenses are measured for SSE, and the relative merits of increasing optical performance at the expense of additional optical elements are also discussed.

Automated radiometric linearity tester

U.S. National Bureau of Standards, Washington, D.C. 20234. Received 22 June 1984. For many years the basic radiometric scales maintained by NBS (spectral radiance, spectral irradiance, pyrometric temperature) have relied on a beam addition device called a beam conjoiner to estabhsh the level or intensity dimension of the scales. This device consists of two or more sources, such as tungsten strip lamps, whose fluxes are combined by beam splitters. By means of shutters for each source the contribution from each source to the combined beam is measured separately as is the combined beam with all shutters open. Repetitions of such sets of measurements for a number of source levels permit, in principle, evaluation of the response function (linearity) of any measuring radiometer. To improve the speed and precision of this technique we have recently built and begun using an automated beam conjoiner. Since the construction of this device offers a relatively simple alternative to comparable automated multiple-aperture linearity testers, we feel that its description may be of interest to other laboratories which perform detector linearity evaluations. The schematic diagram of the automated beam conjoiner is shown in Fig. 1. In our application the source at position S is either a tungsten strip lamp or a quartz-halogen irradiance lamp. The flux from this source is collimated by spherical mirror M2 and split into two branches by the first beam splitter BS1. The fluxes in the two branches are independently attenuated by one of four neutral-density filters mounted in each of the stepping-motor driven filter wheels W1 and W2. The two fluxes are then recombined by the final beam splitter BS2, further attenuated by one of four filters in filter wheel W3, and brought to a focus at P. Perfect superposition of the two source images at P is intentionally avoided to preclude coherence effects.

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.

A Method of Realizing Spectral Irradiance Based on an Absolute Cryogenic Radiometer

A technique is presented for realizing spectral irradiance using a large-area, high temperature, uniform, black-body source and filter-radiometers that are calibrated using a High Accuracy Cryogenic Radiometer. The method will be studied by calibrating irradiance lamps with this new technique and comparing the results with those obtained by the method currently employed at the National Institute of Standards and Technology (NIST). Progress to date and preliminary results are presented. The ultimate goal of the programme is to reduce the measurement uncertainties in the spectral irradiance scales that are made available to industry by calibrating deuterium and tungsten-halogen irradiance lamps.

High-heat-flux sensor calibration using black-body radiation

This paper deals with the radiative calibration aspects of high-heat-flux sensors using black-body radiation. In the last two years, several heat-flux sensors were calibrated up to 50 kW/m2 using a 25 mm diameter aperture variable-temperature black body and a reference room-temperature electrical-substitution radiometer. Tests on a typical Schmidt-Boelter heat-flux sensor showed long-term repeatability of calibration is within 0.6%. Plans for extending the present calibration capability to 100 kW/m2 are discussed.

The 1990 NIST Scales of Thermal Radiometry

Following an absolute NIST measurement of the freezing temperature of gold and the adoption of the International Temperature Scale of 1990 (ITS-90), NIST has adopted new measurement scales for the calibration services based on thermal radiometry. In this paper, the new scales are defined and compared to the ITS-90, and the effects of the scale changes on NIST measurement services in optical pyrometry, radiometry, and photometry are assessed quantitatively. The changes in reported calibration values are within quoted uncertainties, and have resulted in small improvements in accuracy and better consistency with other radiometric scales.