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.

Experimental method for measuring color appearance shifts in high-dynamic-range luminance conditions.

We present an experimental method to determine color appearance shifts under high-dynamic-range conditions. A couple of light booths with variable luminance provide high-dynamic-range luminance conditions, and a perceptual color shift between the two booths is determined using color appearance matching. For red, green, yellow, and blue groups of four surface color samples, color shifts were measured for nine subjects under a dual illumination at background luminance levels of $100\,\,{{\rm cd/m}^2}$100cd/m2 and $4700\,\,{{\rm cd/m}^2}$4700cd/m2. We observed significant perceptual hue shifts toward blue with magnitudes of 2.5 to 3.9 and 5.0 to 6.9 CIELAB units, for the red and green samples, respectively, and decreases in chroma for most samples when changed from low to high luminances.

Dual-photodiode radiometer design for simultaneous measurement of irradiance and centroid wavelength of light sources with finite spectral bandwidth.

We present the design of a radiometer that can simultaneously measure both centroid wavelength and irradiance of a light source without recording its spectrum, when the light source has a finite spectral bandwidth. It consists of two photodiodes separated with a beam splitter in its basic construction, which can be referred to as a dual-photodiode radiometer. This radiometer is calibrated by measuring the spectral responsivities of two photodiodes against the spectral irradiance at the input aperture. The concept of the simultaneous measurement is valid under the condition that the spectral responsivities are linear against wavelengths within the spectral bandwidth of the source to be tested. The feasibility and expected accuracy are analyzed by numerical simulations and experimentally tested for the realization of a UVA irradiance meter, which shows an agreement within 0.2 nm and 0.6% for centroid wavelength and irradiance, respectively, with other reference instruments.

Switched integration amplifier-based photocurrent meter for accurate spectral responsivity measurement of photometers.

This work introduces a switched integration amplifier (SIA)-based photocurrent meter for femtoampere (fA)-level current measurement, which enables us to measure a 107 dynamic range of spectral responsivity of photometers even with a common lamp-based monochromatic light source. We described design considerations and practices about operational amplifiers (op-amps), switches, readout methods, etc., to compose a stable SIA of low offset current in terms of leakage current and gain peaking in detail. According to the design, we made six SIAs of different integration capacitance and different op-amps and evaluated their offset currents. They showed an offset current of (1.5-85) fA with a slow variation of (0.5-10) fA for an hour under opened input. Applying a detector to the SIA input, the offset current and its variation were increased and the SIA readout became noisier due to finite shunt resistance and nonzero shunt capacitance of the detector. One of the SIAs with 10 pF nominal capacitance was calibrated using a calibrated current source at the current level of 10 nA to 1 fA and at the integration time of 2 to 65,536 ms. As a result, we obtained a calibration formula for integration capacitance as a function of integration time rather than a single capacitance value because the SIA readout showed a distinct dependence on integration time at a given current level. Finally, we applied it to spectral responsivity measurement of a photometer. It is demonstrated that the home-made SIA of 10 pF was capable of measuring a 107 dynamic range of spectral responsivity of a photometer.

Biologically inspired LED lens from cuticular nanostructures of firefly lantern.

Cuticular nanostructures found in insects effectively manage light for light polarization, structural color, or optical index matching within an ultrathin natural scale. These nanostructures are mainly dedicated to manage incoming light and recently inspired many imaging and display applications. A bioluminescent organ, such as a firefly lantern, helps to out-couple light from the body in a highly efficient fashion for delivering strong optical signals in sexual communication. However, the cuticular nanostructures, except the light-producing reactions, have not been well investigated for physical principles and engineering biomimetics. Here we report a unique observation of high-transmission nanostructures on a firefly lantern and its biological inspiration for highly efficient LED illumination. Both numerical and experimental results clearly reveal high transmission through the nanostructures inspired from the lantern cuticle. The nanostructures on an LED lens surface were fabricated by using a large-area nanotemplating and reconfigurable nanomolding with heat-induced shear thinning. The biologically inspired LED lens, distinct from a smooth surface lens, substantially increases light transmission over visible ranges, comparable to conventional antireflection coating. This biological inspiration can offer new opportunities for increasing the light extraction efficiency of high-power LED packages.

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.

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.

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.

Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay.

InGaN-based micro-light-emitting diodes have a strong potential as a crucial building block for next-generation displays. However, small-size pixels suffer from efficiency degradations, which increase the power consumption of the display. We demonstrate strategies for epitaxial structure engineering carefully considering the quantum barrier layer and electron blocking layer to alleviate efficiency degradations in low current injection regime by reducing the lateral diffusion of injected carriers via reducing the tunneling rate of electrons through the barrier layer and balanced carrier injection. As a result, the fabricated micro-light-emitting diodes show a high external quantum efficiency of 3.00% at 0.1 A/cm2 for the pixel size of 10 × 10 µm2 and a negligible Jmax EQE shift during size reduction, which is challenging due to the non-radiative recombination at the sidewall. Furthermore, we verify that our epitaxy strategies can result in the relaxation of self-heating of the micro-light-emitting diodes, where the average pixel temperature was effectively reduced.

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.

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.

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).

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.