Howard W. Yoon

National Institute of Standards and Technology, Gaithersburg, MD, United States

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Publications (29)21.04 Total impact

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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.
    Applied Optics 06/2014; 53(16):3565-3573. · 1.69 Impact Factor
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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%.
    Applied Optics 07/2013; 52(21):5184-93. · 1.69 Impact Factor
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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.
    Metrologia 01/2013; 50(5). · 1.90 Impact Factor
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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.
    Applied Optics 07/2012; 51(19):4469-76. · 1.69 Impact Factor
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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.
    Proc SPIE 02/2012;
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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%.
    Metrologia 06/2010; 47(4):429. · 1.90 Impact Factor
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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.
    Proc SPIE 08/2009;
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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.
    Proc SPIE 08/2009;
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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.
    01/2009;
  • Howard W. Yoon, George P. Eppeldauer
    Experimental Methods in the Physical Sciences 01/2009; 42:133-180.
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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.
    Proc SPIE 12/2008;
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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.
    Proc SPIE 05/2008;
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    ABSTRACT: The development of a radiation thermometer calibrated for spectral radiance responsivity using cryogenic, electrical-substitution radiometry to determine the thermodynamic temperatures of the Ag- and Au-freezing temperatures is described. The absolute spectral radiance responsivity of the radiation thermometer is measured in the NIST Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility with a total uncertainty of 0.15% (k=2) and is traceable to the electrical watt, and thus the thermodynamic temperature of any blackbody can be determined by using Planck radiation law and the measured optical power. The thermodynamic temperatures of the Ag- and Au-freezing temperatures are determined to be 1234.956 K (+/-0.110 K) (k=2) and 1337.344 K(+/-0.129 K) (k=2) differing from the International Temperature Scale of 1990 (ITS-90) assignments by 26 mK and 14 mK, respectively, within the stated uncertainties. The temperatures were systematically corrected for the size- of-source effect, the nonlinearity of the preamplifier and the emissivity of the blackbody. The ultimate goal of these thermodynamic temperature measurements is to disseminate temperature scales with lower uncertainties than those of the ITS-90. These results indicate that direct disseminations of thermodynamic temperature scales are possible.
    Applied Optics 06/2007; 46(15):2870-80. · 1.69 Impact Factor
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    ABSTRACT: A new facility for measuring irradiance in the UV was commissioned recently at the National Institute of Standards and Technology (NIST). The facility uses the calculable radiation from the Synchrotron Ultraviolet Radiation Facility as the primary standard. To measure the irradiance from a source under test, an integrating sphere spectrometer-detector system measures both the source under test and the synchrotron radiation sequentially, and the irradiance from the source under test can be determined. In particular, we discuss the calibration of deuterium lamps using this facility from 200 to 400 nm. This facility improves the current NIST UV irradiance scale to a relative measurement uncertainty of 1.2% (k=2).
    Applied Optics 02/2007; 46(1):25-35. · 1.69 Impact Factor
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    ABSTRACT: Absolute stellar photometry is based on 1970s terrestrial measurements of the star Vega calibrated by using the Planck radiance from a Cu fixed-point blackbody. Significant advances in absolute radiometry have been made in the last 30 years that offer the potential to improve both terrestrial and space-based absolute stellar photometry. These advances include new high-temperature blackbody standards, absolute cryogenic radiometry, solid-state optical radiation sources, improved atmospheric transmittance modeling, and laser-based radiometric calibration. We describe the possible use of these new technologies for ground-based calibration of standard stars and their impact on stellar photometry, including present efforts to achieve highly accurate measurements from the ultraviolet to the near infrared for cosmological applications.
    Proc SPIE 01/2007;
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    George P. Eppeldauer, Howard W. Yoon
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    ABSTRACT: Short-wave infrared (SW-IR) radiometers have been developed to extend the NIST reference responsivity scales from the silicon wavelength range to 2500 nm. In addition to spectral power responsivity measurements, where 5 mm diameter extended-InGaAs (EIGA) detectors are underfilled by the incident radiation, irradiance responsivity calibrations are needed. Irradiance measuring radiometers are used as reference detectors to calibrate field radiometers in both irradiance and radiance measurement modes. In irradiance mode, smaller detectors with high shunt resistance, such as 1 mm diameter short-wave HgCdTe and EIGA detectors are used. Mechanical, optical, thermal, and electronic design considerations of SW-IR radiometers are discussed. Noise equivalent currents (NEC) were measured to evaluate noise equivalent power (NEP) and D*.
    01/2007;
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    ABSTRACT: In this study, three different detectors, regular InGaAs, short-wave infrared extended-InGaAs (exInGaAs) with the bandgap wavelength at 2.6 µm and short-wave HgCdTe (swMCT) with the bandgap wavelength at 2.8 µm are studied. The detectors have active areas of 3 mm or 1 mm diameter with all the detectors capable of being cooled from room temperatures to -85 oC with 4-stage thermo-electric coolers. Two of the detectors have field-of-view limiting, cold shrouds attached. From room temperatures to their coldest operating temperatures, the detectors are compared for their temperature-dependent shunt resistances, absolute spectral power responsivities, and noise performances at the output of the photocurrent meter. The photodiode current measuring circuit is analyzed to determine the effect of the shunt resistance for the output offset voltage, the noise and drift amplification, the uncertainty of the current-to-voltage conversion, and the linear operation. The temperature dependences of the shunt resistances are described by Arrhenius plots, and the spectral power responsivities are determined against a pyroelectric detector standard with constant responsivity versus wavelength. We determine that the shunt resistances of regular InGaAs photodiodes can increase to 5 GΩ when cooled to -20 oC demonstrating Si-like performance. The shunt resistances of the 1 mm diameter extended InGaAs and short-wave MCT photodiodes were both measured to be about 11 MΩ at diode temperatures of -70 oC. Further increase in the shunt resistances would be possible with decreasing diode temperatures. The noise voltage at the output of the photocurrent-to-voltage converter is measured for the respective detectors to determine the noise-equivalent power.
    Proc SPIE 09/2006;
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    ABSTRACT: 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 radiometer's 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.
    Metrologia 02/2005; 42(2):89. · 1.90 Impact Factor
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    ABSTRACT: For temperatures above the freezing temperature of silver, the International Temperature Scale of 1990 (ITS-90) is defined in terms of spectral radiance ratios to one of the silver, gold or copper freezing-temperature blackbodies using the Planck radiance law. However, due to the use of spectral radiance ratios, the uncertainties in the realization of thermodynamic temperatures using ITS-90 increase as the square of the temperature ratios. Such increases in the temperature uncertainties can be reduced by using absolute radiometry with pyrometers traceable to cryogenic radiometers, and the resulting temperature uncertainties can be smaller than those measured using the ratio pyrometry as prescribed in ITS-90. We describe the development and the characterization of an absolute pyrometer (AP1) constructed at NIST and calibrated for absolute radiance responsivity. The calibrations are performed with the pyrometer as a single unit; thus separate measurements of the lens transmittance and the spectral responsivities of the filters and detector are not required. The temperature measurement capabilities of the AP1 are shown using the melt and freeze cycle of the gold-point blackbody.
    09/2003;
  • Joseph P. Rice, Keith R. Lykke, Howard W. Yoon
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    ABSTRACT: We are developing a technique for accurately measuring spectral responsivity functions of infrared cameras using tunable lasers. We present preliminary results for uniform scenes where tunable infrared lasers illuminate an integrating sphere, diffusing the light to fill the imaging system optics. A commercial camera based on a liquid nitrogen-cooled InSb focal plane array was tested in the 1.4 micrometer to 4.7 micrometer spectral range using a continuously-tunable periodically-poled lithium niobate (PPLN) optical parametric oscillator. Another commercial camera based on an uncooled microbolometer array was tested using a discrete-tunable CO2 laser in the 9 micrometer to 11 micrometer spectral range. Results from these tests show that signal-to-noise ratio, uniformity, stability, and other characteristics are favorable for use of this technique in the characterization of infrared imaging systems. We also propose a generalization of this technique, to include scenes with arbitrary, controlled spatial content such as bar patterns or even pictures, by illuminating a commercially-available digital micromirror device (DMD). Dependence on irradiance level, exposure time, and polarization can also be measured. This technique has an inherent advantage over thermal-emitter based methods in that it measures absolute spectral responsivity directly without requiring knowledge of the spectral emissivity or temperature of the source.
    Proc SPIE 01/2003;