Radiometric Transfer Standards Research Articles

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.

Traceability and Quality Control in a Radiation Thermometry Laboratory

In radiation thermometry, a number of steps have been taken to improve calibration quality at temperatures below 1000 °C. These steps involve work done by national metrology institutes and standards bodies. The Fluke Infrared Calibration Laboratory in American Fork, Utah has benefitted from this progress and has established its own radiation thermometry program. The calibration range of this laboratory is -15 °C to 500 °C. This program involves calibrating radiometric transfer standards (with pyroelectric detectors) that, in turn, are used to calibrate flat-plate radiation sources, sometimes referred to as blackbodies. The transfer standards are calibrated by a sequence of blackbody cavity radiation sources that cover the entire temperature range of the laboratory. The radiometric transfer takes place between the cavity sources and the flat-plate sources. The intended use of the flat-plate sources is infrared thermometer calibration. Both the transfer standard calibrations and the flat-plate calibrations are accredited by the National Voluntary Accreditation Program (NVLAP). This paper discusses the traceability involved in this radiometric calibration program. It also discusses numerous quality control measures that have been taken to improve and assure measurement accuracy for both calibrations.

Stability of vacuum-ultraviolet radiometric transfer standards: Electron cyclotron resonance versus hollow cathode source

Established transfer standards such as Penning and hollow cathode discharge sources suffer from limited spectral range and, in particular, a limited lifetime and stability due to electrode erosion. The development of a vacuum-ultraviolet radiation source based on an electron cyclotron resonance (ECR)-created plasma might overcome these limitations. To test such a source with regard to its usefulness as radiometric transfer standard, the emission intensity of a Ne plasma was monitored over an operation period of 180 days, with regard to stability and reproducibility in the 50–75 nm wavelength range. For comparison and calibration, a hollow cathode was used as transfer standard traceable to the electron storage ring BESSY II as primary standard. It was found that the ECR source exceeded the lifetime of the hollow cathode source by far, offering a more balanced spectral emission line variety with similar stability.

Determination of the spectral radiance of deuterium lamps using the storage ring Bessy as a primary radiometric standard

The electron storage ring BESSY is a primary radiometric standard of spectral irradiance, which has been used for the calibration of deuterium lamps as radiometric transfer standards of spectral radiance in the wavelength range 115-350 nm. The radiometer used for these measurements is described in detail. For λ > 165 nm the uncertainty of the spectral radiance of the calibrated lamps is if > 3%.

Electron impact excitation and radiometric applications

The radiometric qualities of electron impact induced line radiation are discussed with emphasis on the spectral range of the vacuum ultraviolet. This is done in connection with a measurement of radiation losses from a fusion plasma, and with an investigation of electron impact induced double ionization of the Ar M shell. The excitation of He by highly charged ions is given as a third application. An appendix summarizes absolute cross sections for electron impact induced line emission, suited for a radiometric transfer standard.

Laser-Produced Plasmas as Radiometric Transfer-Standard Sources for the Vacuum-Ultraviolet and the Soft-X-ray Range

Laser-produced tungsten plasmas were investigated with respect to their suitability as radiometric transfer standard sources in the range between 7 nm and 100 nm. For plasma generation a Nd:YAG/glass laser system with λ = 1.064 μm, a pulse length of 17 ns and pulse energies between 100 mJ and 800 mJ was used in connection with a focusing system of 85 mm focal length. The spectral concentration of radiant intensity time-integrated over one plasma radiation pulse was measured with a relative uncertainty of 8% using the electron storage ring BESSY as a primary standard. Within this uncertainty the radiant emission was reproducible during several months, if one applied specific procedures that had been developed for setting the laser parameters and the focusing conditions. The maximum of emission was found at 16.5 nm. At this wavelength a 800 mJ laser pulse generates 3.5 × 1014 photons nm-1 sr-1. The results of this study let us expect that a small transportable radiation standard is practical, which is based on a low-cost, single-stage, Nd-laser system.

Establishment of a Spectral Irradiance Scale in the Visible and Near Infrared Using the Electron Storage Ring BESSY

The electron storage ring BESSY was investigated for use as a primary standard of spectral irradiance at wavelengths between 400 nm and 1000 nm. A method was devised to transfer the spectral irradiance scale realized on to tungsten halogen lamps by means of a radiometer that measured either the spectral irradiance of BESSY or that of the lamps. The relative systematic uncertainty of the spectral irradiance produced by the storage ring was determined to be 0.3%. Presently, the calibration of tungsten halogen lamps as radiometric transfer standards of spectral irradiance from synchrotron radiation yields a relative systematic uncertainty of 0.8%, which is below the uncertainty that is currently achieved using black-body radiators. Spectral irradiances of tungsten halogen lamps measured by means of synchrotron radiation and by means of black-body radiation agree within the combined uncertainties of the two methods. We conclude from the investigations that a reduction of the uncertainty of the spectral irradiance of the storage ring to approximately 0.1% can be achieved if it seems worthwhile. Also, the uncertainty in the calibration of the spectral irradiance of tungsten halogen lamps could be reduced below 0.5% if desired.

Radiometric comparison of a laser-produced plasma and a BRV-source plasma at normal incidence

A comparison of radiometric properties, e.g., time structure, source size, reproducibility, and time-integrated relative spectral radiance of a laser-produced plasma and a BRV-source plasma, is described. At normal incidence the laser-produced plasma proved superior to the BRV-source plasma in every aspect concerning its use as a radiometric transfer standard. Reproducibility in spectral radiance within 5% from shot to shot has been achieved with the laser-produced plasma.

The use of synchrotron radiation for detector calibrations

The National Bureau of Standards (NBS) VUV radiometric detector program calibrates photodiode detectors as radiometric transfer standards over the wavelength range between 50 and 2500 Å (5–250 eV photon energy). Our calibration uncertainty is 10% or better over this full range. The source of radiation used for calibration in the spectral region below 600 Å is synchrotron radiation from the NBS Synchrotron Ultraviolet Radiation Facility (SURF) while a duoplasmatron source is used above 600 Å. A noble gas double ionization chamber is used as the absolute detector between 400–1000 Å while a single ionization chamber is used below 400 Å and a thermopile referred to the double ionization chamber is used above 1000 Å. At wavelengths above 1200 Å magnesium flouride windowed photodiodes with cesium or rubidium telluride photocathodes serve as transfer standards. For wavelengths below 1200 Å specially prepared Al 2O 3 photodiodes are used. Their photocathodes are made by evaporating about 1800 Å of aluminum onto a quartz disc. Then a 150 Å thick layer of Al 2O 3 is formed through anodization to serve as the photoemitting surface. The properties of these diodes prove to be very sensitive to contamination especially at wavelengths below 200 Å. It is particularly important that hydrocarbon oils not be cracked on their surface since this tends to form a carbonaneous photoemitting surface. We are also investigating tungsten photocathodes as a possible transfer standard for the shorter wavelengths. In collaboration with groups at LASL and NRL studies have been made of the photoefficiency of several “practical” photocathode materials at wavelengths between 50–584 Å. Results will be shown for natural aluminum, gold, and carbon. The studies include evaluation of the stability of photoefficiency with time and the sensitivity of the photoefficiency to contamination with use in plasma machine diagnostics. In collaboration with DoE and the University of Hawaii a study was made of some high photoefficiency materials for radiometric diagnostic work. Results for CuI and CsI will be reported as will a study of some “disposable” radiometric detectors.

Calibration of Radiometric Transfer Standards in the UV and VUV by Electron Synchrotron Radiation Using a Normal Incidence Radiometer

In order to utilize the electron synchrotron radiation for the calibration of radiometric transfer standards a radiometer has been developed which is in use at the Deutsches Elektronen-Synchrotron (DESY, Hamburg). The radiometer is designed for calibrating transfer standards of the spectral radiance and irradiance for wavelengths between 80 nm and 350 nm. By the method used it is avoided that after calibration with synchrotron radiation the transfer standards have to be converted to an absolute scale by comparing them with a conventional standard in the uv. This is especially vital for those vuv standards which do not emit any measurable radiation in the uv. In this paper the quality of the underlying principle of measurement is stressed by determining the spectral radiance of commercial deuterium lamps (165 nm to 340 nm). The uncertainty of the measured spectral radiance is ±2%. This uncertainty is confirmed by comparative measurements (290 nm to 340 nm) based on the radiation of tungsten tube black-body at a temperature of about 2500 K.

Measurements of the operating-time variation of the spectral radiance of deuterium lamps

For the calibration of radiometric transfer standards by means of electron synchrotron radiation an apparatus has been developed which is in service at DESY, Hamburg. The equipment is used to calibrate commercial deuterium lamps (165 nm to 340 nm). The uncertainty of the “absolute” spectral radiance is ± 4%, whereas that of the relative spectral radiance is ± 2%.

Absolute Radiometric Calibration of Detectors Between 200–600 Å

Radiometric transfer standards consisting of windowless diodes with cathodes made of anodized aluminum oxide on aluminum are now available from the National Bureau of Standards with calibrations in the 200-600-A wavelength range. This extends the previously existing range of calibration for these diodes (600-1200 A). For wavelengths shorter than 600 A, synchrotron radiation at NBS-SURF is used as the source of radiant energy. A noble gas double ionization chamber is used to calibrate a secondary standard diode that is then intercompared with the transfer standards. Monitors take into account variations in the intensity of synchrotron radiation and in beam position. Methods of accounting for the effects of second-order radiation in the incident flux and secondary ionization in the double ionization chamber are discussed. Calibration uncertainties are about 10%.

Measurement of transition probabilities of C I multiplets in the visible and vacuum-ultraviolet spectral region utilizing arc-plasma emission and synchrotron-calibrated radiometric transfer standards

Measurement of transition probabilities of C I multiplets in the visible and vacuum-ultraviolet spectral region utilizing arc-plasma emission and synchrotron-calibrated radiometric transfer standards