Aurélien David

Carrier distribution in (0001)InGaN∕GaN multiple quantum well light-emitting diodes

We study the carrier distribution in multi quantum well (multi-QW) InGaN light-emitting diodes. Conventional wisdom would assume that a large number of QWs lead to a smaller carrier density per QW, enabling efficient carrier recombination at high currents. We use angle-resolved far-field measurements to determine the location of spontaneous emission in a series of multi-QW samples. They reveal that, no matter how many QWs are grown, only the QW nearest the p layer emits light under electrical pumping, which can limit the performances of high-power devices.

Droop in InGaN light-emitting diodes: A differential carrier lifetime analysis

To investigate the variation in internal quantum efficiency in InGaN structures, we measure the differential carrier lifetime of an InGaN/GaN double-heterostructure light-emitting diode under varying electroluminescence injection conditions. By coupling this measurement to an internal quantum efficiency measurement, we determine the carrier density and the radiative and nonradiative contributions to the lifetime without making any assumptions on recombination processes. We find that droop is caused by a shortening of the nonradiative lifetime with current. The observed shortening of both radiative and nonradiative lifetimes with current is found to be in excellent agreement with an ABC model including phase-space filling.

Influence of polarization fields on carrier lifetime and recombination rates in InGaN-based light-emitting diodes

We study differential carrier lifetimes in InGaN light-emitting diodes (LEDs) of varying wavelengths. Increase in wavelength is correlated with an increase in lifetime, due to the impact of the polarization fields on carrier overlap. This effect explains the early onset of droop in longer-wavelength LEDs.

Review of measures for light-source color rendition and considerations for a two-measure system for characterizing color rendition.

Twenty-two measures of color rendition have been reviewed and summarized. Each measure was computed for 401 illuminants comprising incandescent, light-emitting diode (LED) -phosphor, LED-mixed, fluorescent, high-intensity discharge (HID), and theoretical illuminants. A multidimensional scaling analysis (Matrix Stress = 0.0731, R(2) = 0.976) illustrates that the 22 measures cluster into three neighborhoods in a two-dimensional space, where the dimensions relate to color discrimination and color preference. When just two measures are used to characterize overall color rendition, the most information can be conveyed if one is a reference-based measure that is consistent with the concept of color fidelity or quality (e.g., Q(a)) and the other is a measure of relative gamut (e.g., Q(g)).

Development of the IES method for evaluating the color rendition of light sources.

We have developed a two-measure system for evaluating light sources color rendition that builds upon conceptual progress of numerous researchers over the last two decades. The system quantifies the color fidelity and color gamut (change in object chroma) of a light source in comparison to a reference illuminant. The calculations are based on a newly developed set of reflectance data from real samples uniformly distributed in color space (thereby fairly representing all colors) and in wavelength space (thereby precluding artificial optimization of the color rendition scores by spectral engineering). The color fidelity score R(f) is an improved version of the CIE color rendering index. The color gamut score R(g) is an improved version of the Gamut Area Index. In combination, they provide two complementary assessments to guide the optimization of future light sources. This method summarizes the findings of the Color Metric Task Group of the Illuminating Engineering Society of North America (IES). It is adopted in the upcoming IES TM-30-2015, and is proposed for consideration with the International Commission on Illumination (CIE).

Droop in III-nitrides: Comparison of bulk and injection contributions

We study mechanisms which are thought to contribute to efficiency droop in III-nitrides. We first observe droop in a photoluminescence (PL) experiment on bulk GaN, which confirms the existence of a bulk contribution to droop, unrelated to piezoelectric fields or alloy fluctuations. We then perform biased-PL on a series of InGaN light-emitting diodes to estimate the potential impact of carrier leakage on PL experiments. We conclude that carrier leakage is only significant at very low pump densities and does not contribute to droop, thus validating the use of PL to characterize droop.

Perceptual responses to LED illumination with colour rendering indices of 85 and 97

A light source’s ability to render colours and whites is one aspect of lighting quality. Since better colour rendition generally comes with a penalty in luminous efficacy, it is reasonable to consider the balance: Should colour and whiteness rendition be sacrificed to achieve higher efficacy? Here, we investigate perceptual responses under illumination from two sources, a blue-pumped LED with a colour rendering index of 85 (BLED85), and a violet-pumped LED with a colour rendering index of 97 (VLED97). Forty-eight participants completed three experiments. Neutral, red, and pink were preferred under VLED97. Skin rendition was preferred by Caucasians under VLED97; Asians had no preference. Teeth and a white shirt were whiter and more preferred under VLED97. We conclude that the colour and whiteness rendition must be considered on a par with luminous efficacy.

Color Fidelity of Light Sources Evaluated over Large Sets of Reflectance Samples

Abstract We study the use of large reflectance sample sets to assess the color fidelity of light sources. We introduce three data sets covering a large fraction of the color space and a method to compute color fidelity over these sets. By applying this calculation to various source spectra, we show that some sources have significantly lower fidelity than would be predicted using the eight samples of the color rendering index. Structured and truncated source spectra are especially prone to this pitfall, whereas broad and smooth spectra are more resilient.

Whiteness Perception under LED Illumination

ABSTRACT Visual perception of many manufactured white objects is driven by the concentration of fluorescent whitening agents (FWAs) that they contain. FWAs serve to increase overall whiteness perception by enhancing luminance and providing a chromatic blue shift. White objects with FWAs have been engineered for many decades to interact with conventional illuminants such as daylight and incandescent light in order to provide desirable whiteness perception. Whiteness perception has not previously been investigated under light emitting diodes (LEDs). In this study, three psychophysical experiments—forced choice, selection, and sorting—were conducted to investigate the whiteness perception of a series of whiteness standards containing predetermined amounts of FWAs, illuminated by five sources: a typical blue-pumped LED (BLED), a filtered halogen lamp, and violet-pumped LEDs with three violet emission levels. Thirty-nine participants with normal color vision completed the experiments. Results from the sorting experiment can be explained by FWA excitation: the BLED induced no fluorescence and standards could not be ordered properly, whereas for other sources the perceived whiteness increased with the amount of FWAs. Results from the other two experiments can be explained by FWA excitation together with shifts in source chromaticity. Overall, all results are compatible with the known trend for blue shifts to induce whiteness perception. Adaptations of the CIE whiteness formula are also shown to agree well with the experimental results. The results indicate that engineering of an LED source’s spectrum is necessary for an accurate rendering of whiteness.

Why Color Space Uniformity and Sample Set Spectral Uniformity Are Essential for Color Rendering Measures

ABSTRACT Recently, a group including the present authors developed a new color rendering (fidelity) measure, approved by the IES and henceforth referred to as IES Rf, that has two major updates with respect to the general color rendering index Ra of the CIE. First, it proposes an update to the more perceptually uniform CAM02-UCS color space. Secondly, instead of using only a small number (eight) of moderately saturated samples to determine a general color fidelity index, the IES method proposes using a set of 99 samples uniformly distributed in color space. In addition to the latter, the sample set has one other important property: spectral uniformity. This ensures that the sample set is not wavelength biased: all wavelengths contribute equivalently to the general color fidelity score. This article explores the importance of these two updates for color fidelity evaluation. It shows how the color space update results in a substantial spread of the new color fidelity scores relative to the old CIE Ra values and how the sample set update results in an overall reduction of color fidelity scores for most light source spectra with high CIE Ra (≥80) and high luminous efficacy of radiation (LER) values, especially those with narrowband or spiked spectral features. It is shown that calculated color fidelity scores are affected by the degree of sample set spectral uniformity and that, in the absence of sample set spectral uniformity, light source spectra can be tuned to yield anomalously high CIE Ra values without necessarily yielding an increase in actual perceived color fidelity.