Uncertainty evaluation of photoluminescence quantum yield measurement in an integrating hemisphere-based instrument.

We present an integrating hemisphere-based (i.e., a variant of integrating spheres) implementation of the indirect illumination method for absolute photoluminescence quantum yield measurements, which is a recommended method in the international standard IEC 62607-3-1:2014. We rigorously formulated a mathematical model and a measurement procedure for the absolute photoluminescence quantum yield measurement in the integrating hemisphere-based system. The measurement system was calibrated using an Hg-Ar discharge lamp and spectral irradiance standard lamps for wavelength and relative spectral radiant flux scales, respectively. Furthermore, we identified and evaluated uncertainty components involved in the photoluminescence quantum yield (PLQY) measurement. To validate our measurement system, we applied it to the two de facto standard dyes: quinine bisulfate (QBS) and fluorescein (FLS). Consequently, their PLQY values were determined to be 0.563±0.024 (k=2) and 0.876±0.032 (k=2) for, respectively, QBS and FLS, which are consistent with previous reports.

Inherently high uncertainty in predicting the time evolution of epidemics.

OBJECTIVES: Amid the spread of coronavirus disease 2019 (COVID-19), with its high infectivity, we have relied on mathematical models to predict the temporal evolution of the disease. This paper aims to show that, due to active behavioral changes of individuals and the inherent nature of infectious diseases, it is complicated and challenging to predict the temporal evolution of epidemics. METHODS: A modified susceptible-exposed-infectious-hospitalized-removed (SEIHR) compartment model with a discrete feedback-controlled transmission rate was proposed to incorporate individuals' behavioral changes into the model. To figure out relative uncertainties in the infection peak time and the fraction of the infected population at the peak, a deterministic method and 2 stochastic methods were applied. RESULTS: A relatively small behavioral change of individuals with a feedback constant of 0.02 in the modified SEIHR model resulted in a peak time delay of up to 50% using the deterministic method. Incorporating stochastic methods into the modified model with a feedback constant of 0.04 suggested that the relative random uncertainty of the maximum fraction of infections and that of the peak time for a population of 1 million reached 29% and 9%, respectively. Even without feedback, the relative uncertainty of the peak time increased by up to 20% for a population of 100,000. CONCLUSIONS: It is shown that uncertainty originates from stochastic properties of infections. Without a proper selection of the evolution scenario, active behavioral changes of individuals could serve as an additional source of uncertainty.

Bright visible light emission from graphene.

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.

Measurement of rod and cone effects in mesopic visual sensitivity by varying viewing field.

To investigate the effects of rods and cones in mesopic visual sensitivity, we perform spectral sensitivity experiments by varying viewing fields and adaptation levels. We obtain mesopic spectral sensitivities for 2° and 10° centrally viewing fields and a (10°-20°) peripherally viewing field at adaptation luminance levels of 0.04 cd/m², 0.4 cd/m², and 1.8 cd/m². The spectral shapes are examined through comparison with the Commission International de l'Eclairage luminous efficiency functions. We observe a decrease of visual sensitivity with an increase of adaptation luminance and the dependence of visual sensitivity on viewing field. The observation is discussed in terms of the interaction between rods and cones.

Experimental validation of the six-port design for a highly uniform integrating sphere photometer.

We present experimental realization and validation of the six-port design of integrating sphere photometers for total luminous flux measurement, which significantly improves the uniformity of spatial response compared to the conventional single-port design. Construction, measurement procedure, and data acquisition of the realized instrument with a radius of 1 m are described. Measurement of the spatial response distribution function confirms the expected effect of improving the uniformity by averaging the signals from the six detection ports. The related spatial mismatch error is determined to be less than 1.4% for all the realistic cases of beam angles and directions of a test lamp mounted in the vicinity of the sphere center. As a result, we confirm that the realized six-port instrument allows us to eliminate the complicated spatial mismatch correction procedure by adding a relative standard uncertainty of only 1.4/3%≈0.81%, which offers a great practical benefit for testing solid-state lighting products of various beam characteristics.

Optical injection locking of a singly resonant continuous-wave optical parametric oscillator.

We report on experimental realization of optical injection locking of a singly resonant continuous-wave optical parametric oscillator (OPO) based on a thermally induced waveguide in a magnesium-oxide doped periodically poled lithium niobate. An external cavity diode laser is used to control the frequency of the resonant signal output of the OPO at 795 nm. The key to successful injection locking was the improvement of the OPO spatial modes by a special operating condition with a thermally induced waveguide. The phase coherence of injection locking is confirmed by recording a high-contrast interference fringe between the injection laser and the OPO output. High-resolution measurement of the frequency spectrum of the nonresonant idler output by the delayed self-heterodyne technique shows that the spectral linewidth of the OPO is reduced from 4.5 to 0.4 MHz by injection locking. The full locking range is assumed to be less than 1.7 MHz.

Measurement of normalized spectral responsivity of digital imaging devices by using a LED-based tunable uniform source.

We present an instrumentation solution for measurement of normalized spectral responsivity of digital imaging sensors and cameras. The instrument consists of multiple light-emitting diodes (LEDs), a single-grating monochromator, and a small-size integrating sphere. Wavelength tuning is achieved by a proper selection of LED in accordance with the monochromator setting in a range from 380 to 900 nm. High spectral purity with a bandwidth of 5 nm is realized without using double gratings and order-sorting filters. Experimental characteristics and calibration of the instrument are described with the related error and uncertainty sources. The performance is demonstrated by measuring a monochrome charge-coupled device and a trichromatic complementary metal-oxide-semiconductor device. The measurement uncertainty is evaluated to be less than 1% (k=2) except several wavelengths with low LED power.

Spatial uniformity inspection apparatus for solar cells using a projection display.

We demonstrate a measurement apparatus to inspect spatial uniformity of quantum efficiency of solar cells using a beam projector. Deviation of irradiance from the used beam projector over the area of 1.5×0.8 m on the cell plane was flattened within ±2.6% through gray scale adjustment, which was originally about 200%. Scanning a small square image with an area of 3×3 mm over a square-shaped photovoltaic cell with an area of 15.6×15.6 cm, we could identify the locations according to efficiency level and showed that the cell had quantum efficiency deviation of more than 10%. Utilizing the advantageous feature of a projection display, we also demonstrated that this apparatus can inspect the spatial uniformity of solar modules and panels consisting of multiple solar cells.

Six-port integrating sphere photometer with uniform spatial response.

We propose an integrating sphere photometer with six detection ports for total luminous flux measurement, which significantly improves the uniformity of spatial response compared to the conventional single-port detection design. Numerical simulations based on the geometric radiative transfer equation show that a spatial response distribution function of the new design is uniform within 2% with respect to all spatial directions. The related spatial mismatch error is calculated to be less than 0.3% for all the realistic cases of angular intensity distribution of a test lamp. As a result, the new design practically eliminates the spatial mismatch error of an integrating sphere photometer, so that a high-accuracy measurement can be achieved without the complicated spatial mismatch correction procedure.

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.

Differential spectral responsivity measurement of photovoltaic detectors with a light-emitting-diode-based integrating sphere source.

We present an experimental realization of differential spectral responsivity measurement by using a light-emitting diode (LED)-based integrating sphere source. The spectral irradiance responsivity is measured by a Lambertian-like radiation field with a diameter of 40 mm at the peak wavelengths of the 35 selectable LEDs covering a range from 280 to 1550 nm. The systematic errors and uncertainties due to lock-in detection, spatial irradiance distribution, and reflection from the test detector are experimentally corrected or considered. In addition, we implemented a numerical procedure to correct the error due to the broad spectral bandwidth of the LEDs. The overall uncertainty of the DSR measurement is evaluated to be 2.2% (k = 2) for Si detectors. To demonstrate its application, we present the measurement results of two Si photovoltaic detectors at different bias irradiance levels up to 120 mW/cm(2).

Correction of self-screening effect in integrating sphere-based measurement of total luminous flux of large-area surface-emitting light sources.

We present a correction method of a systematic error that arises when total luminous flux of a large-area surface-emitting light source (SLS) is measured in an integrating sphere by substitution with a reference lamp. Putting a large-area SLS into an integrating sphere is equivalent to adding a low-reflective baffle to screen the spatial distribution of radiation inside the sphere, which severely changes the sphere responsivity. To compensate this self-screening effect, we propose to use a specially designed auxiliary lamp whose illuminating area is spatially matched to that of the SLS under test. The validity of the proposed correction method is tested by numerical simulations based on the radiative transfer equation.

Continuous-wave 532 nm pumped MgO:PPLN optical parametric oscillator with external power regulation and spatial mode filtering.

We report a continuous-wave (CW) optical parametric oscillator (OPO) based on a MgO-doped periodically poled LiNbO(3) (MgO:PPLN) crystal. The 532 nm pump generates coherent radiation that is tunable from 800 to 920 nm for the signal and from 1250 to 1580 nm for the idler, respectively. The OPO output power exhibits a slowly varying instability that we attribute to a thermal effect induced by the pump. This instability is truncated by means of a low-pass servo that includes a single-mode fiber that filters the beam into a single spatial mode. The resulting output characteristics are promising for radiometric applications in the near infrared including most fiber-optic communication bands.

Measurement uncertainty evaluation for emission color and luminance of displays.

We present a measurement uncertainty evaluation for emission color and luminance of displays. The calibration procedures and the measurement uncertainties of a CCD-based spectroradiometer and a filter-type luminance meter are discussed. As evaluation examples, a Commission Internationale de l' Eclairage illuminant A, a liquid-crystal display with a light-emitting diode backlight, and a liquid-crystal display with a cold-cathode fluorescent lamp backlight are evaluated. The uncertainties in Commission Internationale de l' Eclairage 1931 (x,y) chromaticity coordinates are determined to be in a range from 0.0020 to 0.0050 (k=2). The uncertainties in luminance are obtained from 4.8% to 8.5% by using the spectroradiometer, while the filter-type luminance meter shows the uncertainties from 1.1% to 1.6% (k=2).

Uncertainty evaluation for the spectroradiometric measurement of the averaged light-emitting diode intensity.

An uncertainty evaluation is presented for the spectroradiometric measurement of the averaged LED intensity (ALI), which is a standardized photometric quantity of LEDs introduced by the Commission Internationale de l'Eclairage. Using a spectral irradiance standard lamp as a calibration source for the spectroradiometer, 12 uncertainty components are sorted out and their propagation formulated with correlations between the components taken into account. The procedure of uncertainty evaluation is demonstrated for four LED samples of different colors; red, green, blue, and white. The relative uncertainties of the ALI of the test samples are determined to be in a range from 4.1% to 5.5% (k=2), but most of their dominant uncertainty components turn out to be systematic and correlated. In conclusion, correlations between the uncertainty components critically affect the overall uncertainty of the LED measurement using a spectroradiometer.