Howard W. Yoon

National Institute of Standards and Technology, GAI, Maryland, United States

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Publications (42)50.96 Total impact


  • No preview · Article · Feb 2016 · Metrologia
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    Michael R. Moldover · Weston L. Tew · Howard W. Yoon
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    ABSTRACT: The past 25 years have seen tremendous progress in thermometry across the moderate temperature range of 1 K to 1,235 K. Various primary thermometers, based on a wide range of different physics, have uncovered errors in the International Temperature Scale of 1990, and set the stage for the planned redefinition of the kelvin.
    Preview · Article · Jan 2016 · Nature Physics
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    ABSTRACT: This paper describes the establishment and application of the 0/45 reflectance factor scale in the shortwave infrared (SWIR) from 1100 to 2500 nm. Design, characterization, and the demonstration of a four-stage, extended indium-gallium-arsenide radiometer to perform reflectance measurements in the SWIR have been previously discussed. Here, we focus on the incorporation of the radiometer into the national reference reflectometer, its validation through comparison measurements, and the uncertainty budget. Next, this capability is applied to the measurement of three different diffuser materials. The 0/45 spectral reflectance factors for these materials are reported and compared to their respective 6/di spectral reflectance factors.
    No preview · Article · Apr 2015 · Applied Optics
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    ABSTRACT: An irradiance-mode absolute differential spectral response (SR) measurement system based on a light emitting diode (LED) array is described. The LEDs are coupled to an integrating sphere whose output irradiance is uniform to better than 2% over an area of 160 mm by 160 mm. SR measurements of solar cells when subject to diffuse irradiation, as provided by the integrating sphere, are compared with collimated irradiance SR measurements. Issues originating from the differences in angular response of the reference versus the test cells are also investigated. The SR curves of large-area cells with dimensions of up to 155 mm are measured and then used to calculate the cell's short circuit current (I<sub>sc</sub>), if illuminated by a defined solar spectrum. The resulting values of I<sub>sc</sub> agree well with the values obtained from secondary measurements.
    No preview · Article · Jun 2014 · Applied Optics
  • Catherine C. Cooksey · Gerald T. Fraser · Howard W. Yoon
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    ABSTRACT: The challenge of detecting small changes in the Earth's climate system over decadal and longer time scales places stringent requirements on environmental monitoring systems. Sensors must be well calibrated and maintain their calibration in challenging environments: space, oceans, mountains, tropics, polar regions, and deserts. Additionally, the assembly of regional and global time series for environmental variables, such as sea-surface temperature, vegetation coverage, and soil moisture, requires the integration of measurements from a variety of sensors of different designs, operated by different organizations, and acquired at different times. Success requires sensors that are calibrated and recalibrated against standards tied to the International System of Units (SI) to ensure that measurements are physics-based and comparable between nations, organizations, and over generations. The present talk will provide an overview of research and dissemination efforts at NIST to advance the calibration and characterization of sensors, highlighting satellite-based sensors, for application to measurements in support of climate science.
    No preview · Conference Paper · Apr 2014
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    ABSTRACT: For photocurrent measurements with low uncertainties, wide dynamic range reference current-to-voltage converters and a new converter calibration method have been developed at the National Institute of Standards and Technology (NIST). The high-feedback resistors of a reference converter were in situ calibrated on a high-resistivity, printed circuit board placed in an electrically shielded box electrically isolated from the operational amplifier using jumpers. The feedback resistors, prior to their installation, were characterized, selected and heat treated. The circuit board was cleaned with solvents, and the in situ resistors were calibrated using measurement systems for 10 kΩ to 10 GΩ standard resistors. We demonstrate that dc currents from 1 nA to 100 µA can be measured with uncertainties of 55 × 10−6 (k = 2) or lower, which are lower in uncertainties than any commercial device by factors of 10 to 30 at the same current setting. The internal (NIST) validations of the reference converter are described.
    No preview · Article · Oct 2013 · Metrologia
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    ABSTRACT: An irradiance mode, absolute differential spectral response measurement system for solar cells is presented. The system is based on combining the monochromator-based approach of determining the power mode spectral responsivity of cells with an LED-based measurement to construct a curve representing the light-overfilled absolute spectral response of the entire cell. This curve can be used to predict the short-circuit current (I<sub>sc</sub>) of the cell under the AM 1.5 standard reference spectrum. The measurement system is SI-traceable via detectors with primary calibrations linked to the NIST absolute cryogenic radiometer. An uncertainty analysis of the methodology places the relative uncertainty of the calculated I<sub>sc</sub> at better than ±0.8%.
    No preview · Article · Jul 2013 · Applied Optics
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    ABSTRACT: We present a measurement system for absolute differential spectral responsivity of solar cells based on high-powered LED arrays coupled to an optical light guide capable of large area illumination. Two different measurement techniques were developed and tested with the same measurement apparatus on a variety of solar cells. The first method is an individual LED lock-in technique that can be performed over a broad frequency range. The second method is based on synchronous multi-frequency optical excitation, called the Fourier transform (FT) technique, using the LEDs and detection with a spectrum analyzer. A scheme for providing light bias using the LEDs during either measurement scheme is discussed.
    No preview · Conference Paper · Jun 2013
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    ABSTRACT: We describe the characterization of a group of NIST spectral irradiance lamps at longer distances and larger angles than are typically issued by NIST. The spectral irradiances from the FEL lamps were measured from 50 cm to 150 cm at 8 different distances using a cosine-corrected filter radiometer to determine if the lamps adhere to the inverse square law. Using the filter radiometer, the spatial uniformities of the FEL lamps were also mapped over a 20 cm square area at 135 cm, 143 cm and 151 cm. In the NIST gonio-spectroradiometer facility, selected lamps were also mapped for the angular dependences of the spectral irradiances at a distance of 123 cm using a spectrograph which measures from 300 nm to 1100 nm for comparisons to the filter radiometer measurements. Using these measurements, an uncertainty budget for the distance and the angular uniformity correction of the FEL lamps was developed.
    No preview · Conference Paper · Oct 2012
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    ABSTRACT: Scales of spectral irradiance are disseminated by NIST using assignment of values to FEL lamp standards for defined conditions. These lamp standards can be used for absolute calibrations of irradiance radiometers, or more typically, be used in conjunction with a diffuse reflectance standard to establish a scale of spectral radiance and for subsequent absolute calibrations of radiance radiometers. The NIST FEL standards are valuable artifacts requiring special care. Many users optimize resources by in-house transfer of their primary standard to working standards. There are a number of sources of uncertainty in utilizing FEL lamps, e.g., lamp current, alignment, distance setting, instrument aperture size, drift, scattered light, and interpolation in the wavelength grid for the specified irradiance values. In this work, we validated the transfer activity by ITT of their primary, NIST-traceable FEL lamp standards. A portable irradiance bench that had kinematic mounts for an FEL lamp, on-axis baffle, and three different irradiance radiometers was built, tested, and deployed to ITT in Rochester, NY. We report the results of this comparison activity. An uncertainty budget was developed and it was found that the results agreed well within the combined uncertainties of 1.5% to 1.6% (k = 2).
    No preview · Conference Paper · Oct 2012
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    ABSTRACT: An absolute differential spectral response measurement system for solar cells is presented. The system couples an array of light emitting diodes with an optical waveguide to provide large area illumination. Two unique yet complementary measurement methods were developed and tested with the same measurement apparatus. Good agreement was observed between the two methods based on testing of a variety of solar cells. The first method is a lock-in technique that can be performed over a broad pulse frequency range. The second method is based on synchronous multifrequency optical excitation and electrical detection. An innovative scheme for providing light bias during each measurement method is discussed.
    No preview · Article · Jul 2012 · Applied Optics
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    ABSTRACT: For the past several years NIST has been developing, along with several collaborators, a Hyperspectral Image Projector (HIP). This scene projector produces high-resolution programmable spectra and projects them into dynamic two-dimensional images. The current digital micromirror device (DMD) based HIP prototype has a spatial resolution of 1024 x 768 pixels and a spectral range of 450 nm to 2400 nm, with spectral resolution from 2 nm in the visible to 5 nm in the short-wave infrared. It disperses light from a supercontinuum fiber source across two DMDs to produce the programmable spectra, which then globally-illuminate a third DMD to form the spatial images. The HIP can simulate top-of-the atmosphere spectral radiance over a 10 mm x 14 mm, f/3 image, and this can be collimated to stimulate remote sensing instruments. Also, the spectral radiance of the projected scenes can be measured with a NIST-calibrated spectroradiometer, such that the spectral radiance projected into each pixel can be accurately known. The HIP was originally developed for applications in multi-spectral and hyperspectral imager testing, calibration, and performance validation, and examples of this application will be reviewed. Conceivable applications for the HIP in photovoltaic device characterization and optical medical imaging will also be discussed.
    No preview · Article · Feb 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: In 2004, the National Institute of Standards and Technology established the ultraviolet spectral irradiance scale from 200 nm to 400 nm using the calculable irradiance of the Synchrotron Ultraviolet Radiation Facility (SURF). Since the establishment of the scale, spectral irradiance calibrations of many customer lamps have been performed in direct comparison with synchrotron radiation. However, to ensure long-term stability of the scale, three check-standard deuterium lamps were calibrated using SURF III at the same time as customer lamps were being calibrated. Here, we present the results of the long-term monitoring of the scale using these check-standard lamps to ensure customer lamps are calibrated within the expanded calibration uncertainty (k = 2) of 1.2%.
    No preview · Article · Jun 2010 · Metrologia
  • Howard W. Yoon · George P. Eppeldauer
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    ABSTRACT: The chapter discusses the instrument design and characterization of spot-radiation thermometers, capable of transferring temperature scales with the lowest uncertainties. Radiation thermometers can be calibrated using the International Temperature Scale of 1990 (ITS-90) or using absolute radiometry for thermodynamic temperature determinations, although the latter is still rare. The chapter discusses radiation thermometers that operate in the visible to the near-infrared wavelength regions. The choice of the detector, optimization of the detector for stability and linearity, and characterization of radiation thermometers for size-of-source effect (SSE) are discussed. The primary consideration in the design of a radiation thermometer is the desired range of operational temperatures. The chapter presents a figure that shows the Planck radiances for blackbodies ranging in temperatures from the freezing point of indium, or 430 K, to 3,000 K. Radiation thermometers typically has either a reflective optic or a refractive lens to collect the radiation and to define the angular field of view (AOV) and the target size. Photodiodes are photovoltaic detectors that usually have advantageous electronic and radiometric characteristics. The chapter also presents the simplest version of a widely used photocurrent meter. Radiation thermometers that operate in the visible or the near-infrared wavelength region are typically constructed using spectral filters to restrict the incident radiation on the diode to a selected wavelength region. The spectral filters can be constructed using interference layers deposited on glass substrates or using broadband absorbing glasses. The chapter also discusses radiation thermometry below the silver point, calibration of radiation thermometers using ITS-90, thermodynamic temperature measurements using radiometric techniques, and so on.
    No preview · Article · Dec 2009 · Experimental Methods in the Physical Sciences
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    ABSTRACT: The spectroradiometric characterization of the NIST indoor pulsed solar simulator is described. The solar simulator has a flash duration of 36.4 ms and is designed for solar panels having a maximum size of 2.0 m by 1.6 m. As per industry standards, the performance of the solar simulator is evaluated on the basis of three criteria: spatial uniformity, temporal stability, and spectral irradiance. Results from evaluating the NIST solar simulator on all three criteria is reported, but a greater focus is given to the spectral characterization. Reported spectral irradiance measurements were made using a high-speed, diode-array spectroradiometer that was calibrated using NIST standards. An uncertainty analysis of the spectral irradiance measurements is developed, and the extent that the calibrated spectroradiometer can be used to improve solar module measurements is explored.
    Full-text · Article · Aug 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The spectral irradiance responsivity calibrations of InSb radiometers measured on the tunable-laser based Infrared Spectral Irradiance and Radiance Responsivity Calibration with Uniform Sources (IR-SIRCUS) facility are discussed. This work describes the following changes undertaken to reduce the uncertainties of the calibrations: improve the spatial uniformity, reduce the laser-induced speckle from the gold-coated integrating spheres between 1 mum and 5 mum, improve the stability of the optical parametric oscillator (OPO) tunable laser, reduce the noise from the signal-to-monitor ratio, increase the repeatability of measurements, and reduce the stray light and fringe problems of the radiometer under test. Measurements of the spatial uniformity with the use of polytetrafluoroethylene (PTFE) and gold-coated integrating spheres at different wavelengths have been performed. Different approaches for generating a uniform source, removing the speckle, stabilizing the laser, and improving the signal-to-monitor ratio are also described. The spatial non-uniformity after using these approaches has been shown to be reduced to < 1 %. The uncertainty budget of spectral irradiance responsivity calibrations is discussed, and is found to be mainly due to the measurement repeatability uncertainty component of 1 %. Calibrated radiometers are tested against a source-based scale from the calculated spectral irradiances obtained using a precision aperture and a blackbody (BB) with a known temperature.
    Full-text · Article · Aug 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: In the International Temperature Scale of 1990 (ITS-90), temperatures above the freezing temperature of silver are determined with radiation thermometers calibrated using spectral radiance ratios to one of the Ag-, Au-or Cu-freezing temperature blackbodies and the Planck radiance law. However, due to the use of spectral radiance rats, the temperature uncertainties of the ITS -90 increase as the square of the temperature ratios. Recent acoustic-gas thermometry measurements have also shown that the underlying thermodynamic temperatures used in the radiance ratios in determining the Ag-and Au-fixed point temperatures could be in error. Since the establishment of ITS-90, much progress has been made in the development of radiation thermometers and blackbody sources. Cryogenic electrical -substitution radiometry is widely used in detector and radiometer calibrations, and stable, high-temperature metal-carbon eutectic blackbodies are under development. Radiation thermometers can be calibrated for absolute radiance responsivity, and blackbody temperatures determined from measurement of optical power without the use of any fixed points thus making possible direct dissemination of thermodynamic temperatures. We show that these temperatures can be measured with lower final uncertainties than the ITS -90 derived temperatures. We have shown that these "Absolute Pyrometers" can be used to determine the thermodynamic temperatures of the ITS-90 fixed points as well as also being used in bilateral comparisons of temperature scales. Many leading national measurement institutes are already utilizing detector-based temperatures in establishing spectroradiometric source scales. We believe, that due to these developments, the international temperature scale should be revised so that a thermodynamic temperature scale can be directly disseminated.
    Full-text · Article · Jan 2009
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    ABSTRACT: Measurements of bi-directional reflectance factor for diffuse reflectance from 1100 nm to 2500 nm using extended-range indium gallium arsenide (exInGaAs) detectors in the NIST Spectral Tri-function Automated Reference Reflectometer (STARR) facility are described. The determination of bi-directional reflectance factor with low uncertainties requires the exInGaAs radiometer to be characterized for low-noise performance, linearity and spatial uniformity. The instrument characterizations will be used to establish a total uncertainty budget for the reflectance factor. To independently check the bi-directional reflectance factors, measurements also were made in a separate facility in which the reflectance factor is determined using calibrated spectral irradiance and radiance standards. The total combined uncertainties for the diffuse reflectances range from < 1 % at 1100 nm to 2.5 % at 2500 nm. At NIST, these measurement capabilities will evolve into a calibration service for diffuse spectral reflectance in this wavelength region.
    Full-text · Article · Dec 2008 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Short-wave infrared detectors and regular-glass optics are used to construct a calibrator for infrared collimators. The advantages of using short-wave infrared detectors with thermo-electric cooling instead of cryogenically-cooled infrared detectors are shown. Diffraction-limited imaging is obtained using off-the-shelf achromats for rejection of stray radiation and for collection of the thermal radiation. The design of a prototype calibrator is shown and the noise-equivalent irradiances (NEI) are determined using a separately calibrated, off-axis infrared collimator. The measured NEI of 7 fW/cm2 demonstrates at least several orders of magnitude better performance than existing infrared calibrators.
    Full-text · Article · May 2008 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The Technical Specification IEC 62492-1 TS: Industrial process control devices - Radiation thermometers - Part 1: Technical data for radiation thermometers will define the technical data, i.e. metrological data, to be given in data sheets and operating instructions for radiation thermometers with one wavelength range and one measurement field. It has been developed within the working group IEC SC 65B WG5 "Temperature Sensors". The content and structure of the IEC 62492-1 TS is briefly explained and an outlook on further work on radiation thermometry planned within the IEC SC 65B WG5 is given.
    Full-text · Article · Jan 2008 · Proceedings of SPIE - The International Society for Optical Engineering