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Organic–Inorganic Semiconductor Heterojunctions for Hybrid Light-Emitting Diodes
Published in Ye Zhou, Optoelectronic Organic–Inorganic Semiconductor Heterojunctions, 2021
The trivial approach is using two or more LEDs on separate chips, each emitting nearly monochromatic light, which when combined emit white light. When using two LEDs (dichromatic), their wavelengths have to be complementary and operate at a certain power ratio in order to be perceived as white. Dichromatic white LEDs in general have a high luminous efficacy (ratio of luminous flux to power in units of lumen per watt), but a low CRI. The CRI can be improved by adding more LEDs of different emission wavelengths (e.g., trichromatic); however, its luminous efficiency will decrease [2,5]. This is a fundamental trade-off between the color rendering capabilities and the luminous efficiency of an LED, which cannot be eliminated. Examples would be a blue and yellow LED or a combination of three LEDs emitting in the RGB spectral regions.
Photonic Nanodevices and Technologies against Light Pollution
Published in Tuan Anh Nguyen, Ram K. Gupta, Nanotechnology for Light Pollution Reduction, 2023
Elisangela Pacheco da Silva, Elizângela Hafemann Fragal, Ederson Dias Pereira Duarte, Sidney A. Lourenço, Edvani C. Muniz, Thiago Sequinel, Rafael Silva, Eduardo José de Arruda, Vanessa Hafemann Fragal
The wavelength is responsible to determine the color in visible light, which can be monochromatic – a single wavelength – or polychromatic – multiples wavelengths. Color is a fundamental perception of the human eye (our light sensor) and this perception is changed by the environment and conditions to which humans are exposed. The chromatic characteristic of light is divided into correlated color temperature (CCT – given in degrees of Kelvin) and color reproduction index (CRI). The CCT range of artificial light is from 2,000 K (warm) up to 10,000 K (cool), as shown in Figure 13.1b. Meanwhile, the CRI (%) represents the ability of a light source to accurately reproduce the colors of the object it illuminates. As higher the index as accurate is the reproduction [4].
White Light-Emitting Diode: Fundamentals, Current Status, and Future Trends
Published in Wengang (Wayne) Bi, Hao-chung (Henry) Kuo, Pei-Cheng Ku, Bo Shen, Handbook of GaN Semiconductor Materials and Devices, 2017
Bingfeng Fan, Yi Zhuo, Gang Wang
CRI is another important index of white LED color parameters. CRI indicates how well a light source renders colors, on a scale of 0–100, compared to a reference light source. The test procedure established by the International Commission on Illumination (CIE) involves measuring the extent to which a series of eight standardized color samples differ in appearance when illuminated by a given light source, relative to the reference source. The average “shift” in those eight color samples is reported as Ra or CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an “R9” score, which indicates how well the light source renders a saturated deep red color.
A common type of commercially available LED light source allows for colour discrimination performance at a level comparable to halogen lighting
Published in Ergonomics, 2019
Sara Königs, Susanne Mayr, Axel Buchner
Light sources with different CCTs can render the colour of an object differently which is why the CCT is taken into account in the calculation of the colour rendering index (CRI). The CRI, originally introduced in 1965 by the International Commission on Illumination (CIE), reflects the appearance of the colours of an object under a certain light source compared to a reference light source with a comparable CCT (CIE 1995). For light sources with a CCT below 5000 K, as is the case in most of the commercially available light sources (Guo and Houser 2004), the reference light source is a black body radiator of comparable CCT (CIE 1995). The maximum CRI score is 100, with 100 indicating complete accordance of the colour rendered by the reference light source with the colour rendered by a test light source. Values smaller than 100 indicate progressively less accordance. Eight medium-saturated test colours (R1–R8) are considered in the calculation of the CRI. Single indices are calculated for each reference colour. The arithmetic mean of these indices represents the general CRI (alternatively referred to as Ra).
LED-based light sources optimised for high colour rendition from an end users’ perspective
Published in Ergonomics, 2021
Sara Königs, Susanne Mayr, Axel Buchner
The complex information included in the spectral power distribution is commonly converged to a few single measures. Internationally used measures are the correlated colour temperature (CCT) and the colour rendering index (CRI; Commission Internationale de l’Éclairage (CIE) 1995). While the CCT is used to describe the appearance of the light itself, shifting from “warm” (yellow-red) to “cold” (blue) as the CCT increases, the CRI is used to describe the appearance of object colours under a light source. The CRI Ra compares the rendition of eight test object colours illuminated by a test light source to a reference standard of the same CCT (Commission Internationale de l’Éclairage (CIE) 1995). The higher the degree of accordance of the test object colours rendered by the test light source and the test object colours rendered by the reference standard, the higher is the CRI Ra, with a CRI Ra of 100 indicating complete accordance. Interestingly, several studies examining LED-based light sources have shown that CRI Ra is poorly correlated with visual appreciation (Baniya et al. 2015; Bodrogi et al. 2004; Dangol et al. 2013; Jost-Boissard, Fontoynont, and Blanc-Gonnet 2009; Narendran and Deng 2002; Szabó et al. 2007), colour discrimination performance (Königs, Mayr, and Buchner 2019) and subjective naturalness rankings (Dangol et al. 2013; Islam et al. 2013), suggesting that CRI Ra may not be an appropriate metric to predict end users’ preference. Nonetheless, apart from the CCT, the CRI Ra is usually the only metric end users can refer to when choosing a light source, at least if they are not lighting experts.
A Low-Cost, Wide-Range, CCT-Tunable, Variable-Illuminance LED Lighting System
Published in LEUKOS, 2020
Rajib Malik, Kalyankumar Ray, Saswati Mazumdar
A CCT-tunable light source can also be implemented by using bicolor LED lamps; that is, blending cool-white (CW) and warm-white (WW) LEDs (Chen and Tan 2015; Kim and others 2015; Lee et al. 2016). In such systems the CRI can be greatly improved, but the CCT of the produced light is limited to a range between that of CW and WW LEDs.