Howard W. Yoon

Ultraviolet radiation is becoming increasingly popular for applications such as germicidal disinfection and photo-polymer additive manufacturing. However, calibrations of UV detectors have been limited by relative expanded uncertainties typically larger than 2% and a lack of National Measurement Institutes that measure wavelengths from 200 nm to 250 nm because of the difficulty working in that range. The National Institute of Standards and Technology (NIST) has established a new facility that achieves diode-to-diode spectral power responsivity transfers with uncertainties of 0.1% or better across the 200 nm to 400 nm wavelength range and a relative expanded uncertainty ( k =2) of less than 1%. The Ultraviolet Spectral Comparator Facility (UVSCF) provides spectral power and spectral irradiance responsivity calibrations of detectors and filter radiometers from 200 nm to 1800 nm. Its low UV transfer uncertainties derive from a combination of common-mode rejection, nitrogen purging the ozone from the system, a careful selection of mirrors, and strategic usage of broadband optical radiation sources, including a laser-driven light source and a deuterium lamp. The measurement stability over long time periods enables spatial mapping of detectors’ responsivities using calibrated x-y motion stages, which allows the facility to directly calibrate the spectral irradiance responsivity. Spectral irradiance is a critical measurement in many UV applications, and legacy calibration methods for spectral irradiance require multiple facilities and measurements. Our detector-based spectral irradiance responsivity method determines the spectral irradiance responsivity of silicon carbide detectors with 1% relative expanded uncertainty ( k =2). Those results agreed to within 0.70% with spectrally integrated irradiance measurements using a calibrated deuterium lamp with an independent scale traceability. This novel comparison of UVC scales validates the performance and uncertainties of the UVSCF and the efficacy of scanning spectral irradiance calibrations down to 200 nm.

A technique for characterizing and correcting the linearity of radiometric instruments is known by the names the ‘flux-addition method’ and the ‘combinatorial technique’. In this paper, we develop a rigorous uncertainty quantification method for use with this technique and illustrate its use with both synthetic data and experimental data from a ‘beam conjoiner’ instrument. We present a probabilistic model that relates the instrument readout to a set of unknown fluxes via a set of polynomial coefficients. Maximum likelihood estimates (MLEs) of the unknown fluxes and polynomial coefficients are recommended, while a non-parametric bootstrap algorithm enables uncertainty quantification including standard errors and confidence intervals. The synthetic data represent plausible outputs of a radiometric instrument and enable testing and validation of the method. The MLEs for these data are found to be approximately unbiased, and confidence intervals derived from the bootstrap replicates are found to be consistent with their target coverage of 95%. For the polynomial coefficients, the observed coverages range from 91% to 99%. The experimental data set illustrates how a complete calibration with uncertainties can be achieved using the method plus one well-known flux level. The uncertainty contribution attributable to estimation of the instrument’s non-linear response is less than 0.025% over most of its range.

Novel atomic and molecular systems for radiative thermometry |

Improvements in a lamp-monochromator-based facility at the National Institute of Standards and Technology (NIST), the Visible near-infrared Spectral Comparator Facility (VisSCF) which is used to calibrate optical detectors for spectral radiant power responsivity from 300 nm to 1100 nm, are described. These changes include extending the VisSCF operational range down to 300 nm from 350 nm, thereby fully covering the ultraviolet-A (UVA) spectral region and partially covering the UVB range. These improvements have lowered the magnitudes of most of the components in the uncertainty budget and have led to combined 0.005% transfer (k = 1) uncertainties in the spectral power responsivity calibrations over most of the spectral range. Redevelopment of the uncertainty budget results in total expanded uncertainties of spectral responsivities of less than 0.1% (k = 2) over the spectral range from 380 nm to 980 nm, with the greatest uncertainty term coming from the calibrations of the transfer standards.

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.

The Ocean Color component of the global Aerosol Robotic Network (AERONET-OC) utilizes CE-318 sun photometers modified for above-water radiometry from fixed structures such as oil rigs, lighthouses, and service platforms. Primarily, AERONET-OC measurements allow determination of the water-leaving radiance required for the validation of ocean color satellite data products. One instrument from the AERONET–OC network, identified as AERONET #080, was studied in this work. A laser-illuminated integrating sphere of known radiance enabled determination of the linearity with flux and absolute radiance responsivity at multiple wavelengths within seven of the AERONET #080 filter bands. We compared the results to calibrations from the AERONET facility at the Goddard Space Flight Center of the National Aeronautics and Space Administration and from the Joint Research Centre of the European Commission. These results agree within the estimated mean comparison uncertainty of 1.88 % ( k = 2 ). We also assessed these results using calibrated lamp-illuminated integrating spheres and observed a spectral dependence to the comparison results that is unexplained.

We report new developments in instrumentation and techniques for both acoustic (speed of sound) and radiometric primary thermometry methods. These include both new cylindrical resonators for extending acoustic gas thermometry to higher temperatures and absolute radiation thermometers incorporating InGaAs detectors to extend primary radiometry to lower temperatures than can be achieved using Si-detector based instruments. These new approaches have been established in order to determine the difference between thermodynamic temperature, T , and the International Temperature Scale of 1990 (the ITS-90), T ₉₀ , over the temperature range from 430 K to 1358 K as part of the three-year EMPIR project ‘Implementing the new kelvin 2' (InK2). This paper describes the facilities and measurement methodologies for measuring T — T ₉₀ at each of the different institutes, along with an assessment of the target uncertainties. The work is ongoing, but we anticipate that the results of these measurements will ultimately be pooled to provide consensus values of T — T ₉₀ with associated estimated uncertainties. These consensus values will initially feed into the technical annex of the mise en pratique for the definition of the kelvin (MeP-K-19) and, if required, will be used to help to provide a foundation for any future temperature scale.

A new design for thermal-infrared radiation thermometer and sensors is described. Critical optical elements, such as the field stop, Lyot stop, collimating lens, and detector, are placed inside a thermally stabilized assembly that is controlled using thermo-electric coolers and thermistors. The assembled radiation thermometer is calibrated using both variable-temperature fluid-bath and heat-pipe blackbodies from −45 °C to 75 °C and the use of a modified-Planck function and these blackbodies. The size-of-source effect both with and without the Lyot stop has been measured. This new design, during operations without the need for cryogenic cooling, demonstrates sub millikelvin temperature measurement resolution with few millikelvin, week-long stable operations while measuring room-temperature objects.