Jisoo Hwang

Simultaneous measurement of emittance, transmittance, and reflectance of semitransparent materials at elevated temperature

A two-substrate method is developed to simultaneously determine emissivity, transmittance, and reflectance of semitransparent materials with a single measurement under the same environment at elevated temperature. The three quantities can be obtained through the emissivities of substrates and the apparent emissivities resulting from the radiance of the sample heated by substrates. The two-substrate method is compared with the conventional method by measuring sapphire samples with various thicknesses, resulting in good agreements for all the samples. The present method will be useful to measure the temperature dependence of optical properties of porous ceramic materials.

Correction of port reflection effect for integrating sphere-based reflection measurements

We investigate the effect of port reflection in reflection measurements. Due to a high reflection coating of a sphere, the spheres ports have thick edges, which cause reflection and contribute to the reflectance of the sphere. When the sphere is used in reflection measurements, the measured result may show a discrepancy with the theoretical prediction dealing with an ideal sphere with non-reflecting ports. To correct the reflection effects occurring at the port edges, we analytically calculate the contribution of the port edge reflection to the sphere reflectance and obtain a new correction factor for the absolute reflection measurement of Sharp and Little. Through a numerical calculation, we investigate the dependence of the new correction factor on the thickness of the port edges. For an experimental example, we perform a Sharp–Little measurement to determine the reflectance of a sample using the new correction factor.

Radiometric Characteristics of KOMPSAT-3 Multispectral Images Using the Spectra of Well-Known Surface Tarps

A vicarious calibration with reference to characterized surface tarps was conducted to determine the first radiometric characteristics of KOMPSAT-3. The 6S radiative transfer model was also used by inputting various initial parameters, such as the spectral response function of KOMPSAT-3, and atmospheric and geometric conditions. Moderate-Resolution Imaging Spectroradiometer atmospheric products, such as aerosol optical depth, precipitable water, and total ozone, were used as input parameters to interpret solar radiation reflection, scattering, and absorption effects. In the first field campaign, the radiometric coefficients from each of the spectral bands were estimated by calculating the predicted radiance at sensor level and the digital number (DN) of KOMPSAT-3 based on a linear least squares fit over a range of target reflectance levels. The second field campaign measurements were also used to upgrade the KOMPSAT-3 DNs to radiance coefficients. The root-mean-square error differences between simulated radiance and measured radiance during the second field campaign for “sensor-to-itself” calibration were 2.072 W/m2 sr (blue), 6.80 W/m2sr (green), 7.512 W/m2sr (NIR), and 5.712 W/m2sr (red), respectively. This highlights that radiometric calibration with tarps is a reliable method. Furthermore, the gain ratio between the first and the second one was <; 5%, indicating reasonable radiometric calibration results. Additionally, cross-validation of KOMPSAT-3 with radiometrically well-calibrated Landsat-8 was performed over bright desert. Although the difference between the vicarious calibration with surface tarps and cross-validation with Landsat-8 was significant, reasonable results were obtained under close geometrical conditions, despite inherent vicarious calibration error.

Surface Color Measurement Uncertainties

We present a surface color measurement including quantities of surface color, methods, and uncertainty evaluation. Based on a relation between spectral reflectance and surface color, we study how an uncertainty of spectral reflectance propagates to surface color. In analyzing the uncertainty propagation, we divide the uncertainty into uncorrelated components, fully correlated components, and correlated components with spectrally varying correlations. As an experimental example, we perform spectro-reflectometric measurements for ceramic color plates. With measured spectral reflectance and its uncertainty evaluation, we determine surface color and analyze uncertainties of the ceramic color plates.

A Display-based Visual Stimulator for Psychophysical and Electrophysiological Color Sensitivity Measurements

We present a display-based visual stimulator for psychophysical and electrophysiological visual sensitivity measurements. The stimulator offers various psychophysical visual stimuli and transfers the signals from external devices along with the stimulation signals to an electrophysiological recorder. As an experimental demonstration, we perform a visual sensitivity experiment in the mesopic vision range by using the display-based stimulator. The intensity of the steady-state visual evoked potential is observed to correlate with the luminance of the flickering visual stimulation. For the psychophysically determined detection thresholds, we determine the mesopic luminance, showing agreement with the perceived brightness within the uncertainty of the luminance measurement.

Updating Absolute Radiometric Characteristics for KOMPSAT-3 and KOMPSAT-3A Multispectral Imaging Sensors Using Well-Characterized Pseudo-Invariant Tarps and Microtops II

Radiometric calibration of satellite imaging sensors should be performed periodically to account for the effect of sensor degradation in the space environment on image accuracy. In this study, we performed vicarious radiometric calibrations (relying on in situ data) of multispectral imaging sensors on the Korea multi-purpose satellite-3 and -3A (KOMPSAT-3 and -3A) to adjust the existing radiometric conversion coefficients according to time delay integration (TDI) adjustments and sensor degradation over time. The Second Simulation of a Satellite Signal in the Solar Spectrum (6S) radiative transfer model was used to obtain theoretical top of atmosphere radiances for both satellites. As input parameters for the 6S model, surface reflectance values of well-characterized pseudo-invariant tarps were measured using dual ASD FieldSpec® 3 hyperspectral radiometers, and atmospheric conditions were measured using Microtops II® Sunphotometer and Ozonometer. We updated the digital number (DN) of the radiance coefficients of the satellites; these had been used to calibrate the sensors during in-orbit test periods in 2013 and 2015. The coefficients of determination, R2, values between observed DNs of the sensors, and simulated radiances for the tarps were more than 0.999. The calibration errors were approximately 5.7% based on manifested error sources. We expect that the updated coefficients will be an important reference for KOMPSAT-3 and -3A users.

Integrating sphere-based relative methods for reflection measurements

We investigate the comparison method and the substitution method of integrating sphere-based relative methods for reflection measurements. Through an analytical description, we compare the two methods with a focus on the effects of a sphere: to determine reflectance without any correction on a sphere, the substitution method requires symmetry of a sphere in addition to the requirement of the comparison method, either uniformity of a sphere or equal reflectance distribution between a sample and a reference. We experimentally compare the two methods through reflection measurements of gray-scale diffuse samples with reflectance of 10%, 40%, and 99%, which results in agreement within their measurement uncertainties. Also, we observe a reflectance change depending on sample location on a sphere in the substitution type measurements, which is attributed to spheres lack of symmetry. The experimental results are analyzed in terms of measurement uncertainty and reflection properties including bidirectional reflectance distribution function.

Measurement of relative spectral responsivity of photovoltaic detectors by using single tunable pulsed laser

We describe a novel method to measure the relative spectral responsivity of Si and Ge photovoltaic detectors from 250 nm to 1700 nm based on a tunable nano-second pulsed optical parametric oscillator (OPO).

Dispersion-relation-based method for determining gloss

A dispersion-relation-based method to determine luminous gloss and monochromatic-definition gloss is introduced. Through analysis of the spectral specular reflectance, the Sellmeier equation that determines gloss is obtained. The dispersion-relation-based method and the glossmeter-employed method are compared by determining the luminous gloss of a KRISS gloss certified reference material, resulting in agreement within a relative expanded uncertainty of 1.0% (k = 2).