This paper provides recommendations for optimum LED light spectra in shop environments. The main aspects of the research were to elaborate the optimal LED spectral power distribution for the lighting of different colour textiles, fruits and vegetables, meat and bakery products. The spectrum was tailored towards different colour quality metrics such as the colour rendering index and the colour quality scale. Small scale investigations with eye-tracker studies were carried out by Osram Opto Semiconductors at the University of Regensburg and full scale experiments were conducted at the University of Pannonia during the project in order to determine which metric correlates best to the preference of the observers. Results of the psychophysical have been evaluated with the help of analytic hierarchy process and a modified Thurstone method. As a result of the investigation, optimal spectral power distributions for shop environment are described.
The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously. Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency
In previous user-acceptance studies conducted at Aalto University, it was found that the preferred light-emitting diode spectral power distributions (SPDs) were not characterised by a high-CIE colour-rendering index but by a high-colour quality index (CQS) colour preference scale (Qp) and a high-CQS gamut area scale (Qg). In these studies, the SPDs were realised with a 12-channel LED spectra simulator. It is, however, foreseen that LED light sources consisting of 12 different types of LEDs will not be commercially exploitable due to complexity. The objective of this work was to investigate the possibility of generating simplified LED SPDs having CQS Qp and CQS Qg values similar to those of the preferred complex LED SPDs found in the previous user-acceptance studies. Useracceptance studies were carried out in lighting booths to investigate people's preferences for the lit environment under both the complex and simplified LED spectra. The results suggest that the preferred complex LED SPDs can be optimized both for efficiency and cost without sacrificing the colour quality of the light.
We study the lateral diffusion of photogenerated carriers within InGaN/GaN quantum wells by scanning in the focal plane of our confocal microscope. The photoluminescence (PL) images are analyzed at varying excitation densities in the temperature range between 4 K and room temperature. The results of a blue and a green emitting sample are discussed in context of reported diffusion lengths for InGaN/GaN quantum wells.1 Motivation To further enhance the efficiency of InGaN-based LEDs, it is necessary to get a good knowledge of the processes in the active layer. We study the lateral diffusion of photogenerated carriers within the quantum well by analyzing the PL image of InGaN/GaN quantum well test layers.When looking at the PL image of InGaN/GaN-heterostructures one can see a luminescent area which is much larger than the exciting laser spot. This could be explained by lateral diffusion of photogenerated carriers out of the excited region. Kaneta et al. analyzed carrier dynamics by PL measurements taken with a scanning near-field optical microscope . The large diffusion length is explained by acceleration of carriers due to fluctuations of the piezolectric field, which are induced by fluctuations of the Indium content within the quantum well. Huang et al. also observed fast 2D lateral diffusion within the quantum well [4], increasing with increasing quantum well width. Due to the strong built-in piezoelectric fields within the quantum well the charge carriers are spatially separated perpendicular to the quantum well. The attractive Coulomb interaction between electrons and holes is reduced and can no longer compensate the repulsion between carriers of the same type. Due to this, the ambipolar lateral diffusion within the quantum well increases with increasing separation [5].Xu et al. observed the luminescent images of InGaN/GaN quantum ells [6]. They see light emitting areas which are several tens of micrometers larger than the exciting laser spot, which they take as a strong hint for enhanced lateral diffusion. Within the exciting laser spot, the density of photogenerated carriers is sufficiently large to at least partially screen the built-in piezoelectric fields by the photogenerated carriers. Due to the gradient of the carrier density, the effective piezoelectric field shows a lateral gradient within the quantum well. The resulting electrical fields push electrons and holes laterally away from the excited region, which results in an enhanced lateral diffusion.
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