General Color Rendering Index Ra Research Articles

A single‐structured LuAG:Ce,Mn phosphor ceramics with high CRI for high‐power white LEDs

AbstractThe realization of high color rendering index (CRI) is still a great challenge for high‐power LEDs (hp‐LEDs), which is hindered by the phosphor converter. In this work, based on the strategy of Ce3+ and Mn2+ multi‐ion substitution, the single‐structured LuAG:Ce,Mn ceramics with high CRI were prepared via regulating the ratio of tri‐color (red, green, and blue) components. The effects of Mn2+‐Si4+ pairs doping content on the crystal structure, morphologies, and luminescence properties were investigated in detail. The red emission centered at 590 and 750 nm were effectively compensated by regulating Mn2+ occupancy sites, resulting in a significant improvement of CRI. Pure white light with general CRI Ra up to 91.0, special CRI R9 reaching 37.9 and LE as high as 85.07 lm/W was achieved, when the hp‐LEDs were constructed from related phosphor ceramic Ce02Mn7. These results suggest that the LuAG:Ce,Mn phosphor ceramics are highly promising color converters for hp‐LEDs application.

CIE 2017 color fidelity index Rf: a better index to predict perceived color difference?

The CIE General Color Rendering Index Ra is currently widely and internationally used to assess the ability of white light sources to render colors. But over the past decades, there has been increasing evidence of its limitations. As a result of several years of scientific work, the Commission Internationale de l'Eclairage (CIE) published in April 2017 an updated calculation method. To reduce misinterpretation by users and make a clear distinction with the other aspects of color quality, the color rendering index was renamed color fidelity index (CIE-Rf). Before proposing this index as a new international standard for color fidelity prediction, there is a need for validation. This paper presents a psychophysical experiment designed to address this aim. Twelve observers evaluated color differences of 16 color samples under eight pairs of light sources. 3000K and 4000K light-emitting diodes and fluorescent sources with CIE-Rf values ranging from 64 to 94 were investigated. Results show that the prediction of color differences is significantly better in CAM02-UCS color space than in CIE U*V*W* or CIELAB, whatever the color matching functions. A high correlation was found between perceived color differences and CIE-Rf. This psychophysical experiment did not make it possible to detect significant differences with the other tested color fidelity indices (CIE-Ra, CQS-Qf, CRI2012, CRI-CAM02UCS, and IES-Rf). However, there are some signs of an improvement of the prediction by CIE-Rf in comparison to CIE-Ra. To increase, in further works, the chance to detect potential significant differences between color fidelity indices, some modifications of the experimental protocols are suggested.

Whole high-quality light environment for humans and plants

Plants sharing a single light environment on a spaceship with a human being and bearing a decorative function should look as natural and attractive as possible. And consequently they can be illuminated only with white light with a high color rendering index. Can lighting optimized for a human eye be effective and appropriate for plants? Spectrum-based effects have been compared under artificial lighting of plants by high-pressure sodium lamps and general-purpose white LEDs. It has been shown that for the survey sample phytochrome photo-equilibria does not depend significantly on the parameters of white LED light, while the share of phytoactive blue light grows significantly as the color temperature increases. It has been revealed that yield photon flux is proportional to luminous efficacy and increases as the color temperature decreases, general color rendering index Ra and the special color rendering index R14 (green leaf) increase. General-purpose white LED lamps with a color temperature of 2700 K, Ra > 90 and luminous efficacy of 100 lm/W are as efficient as the best high-pressure sodium lamps, and at a higher luminous efficacy their yield photon flux per joule is even bigger in proportion. Here we show that demand for high color rendering white LED light is not contradictory to the agro-technical objectives.

Phosphor‐in‐fluorescent‐glasses for high color rendering white light emitting diodes

AbstractFluorescent glass frits were prepared and used to synthesize phosphor‐in‐fluorescent glass composites (PiFGs) to realize stable white light emitting diodes with high color‐rendering properties. Commercial red, green, and blue phosphors were co‐sintered and red phosphors were partially replaced by Eu3+ in glass frits. Phosphor‐in‐glass composites were placed on UV‐light emitting diodes (UV‐LEDs) to generate white light. Pure white light with a luminous efficacy=58.4 lm/W, general color rendering index Ra=87 and special color rendering index for strong red R9=73 was realized with glass frits containing 7 mol% Eu2O3 and RGB ratio of 35:20:15. Luminous efficacy, Ra and R9 increased as red phosphors were replaced by red‐fluorescent glass frits.

Raout-in: Color rendering of objects in a daylit room viewed from outdoors

This paper addresses the definition of a new criterion Raout-in to evaluate the color rendering of objects in a room viewed by an observer who is outdoors. This is not only relevant when outdoor observers are viewing persons in the room. It is also frequently taken into account when the color of internal solar-control or glare protection systems is selected so that they have a pleasing appearance for both indoor and outdoor observers. The relation of the new criterion to the general color rendering index Ra as applied for daylight transmitted through glazing is discussed and several solar control systems are assessed with respect to Ra and Raout-in. Both color rendering indices are based on an evaluation of the color rendering of eight moderately saturated test colors illuminated with the reference illuminant D65 and the illuminant under consideration (e.g. the same illuminant transmitted through a spectrally selective solar-control glazing). A discussion about an extension of the range of test colors by addition of strongly saturated test colors is also included in the paper.

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

ABSTRACTRecently, 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.

Highly efficient all‐nitride phosphor‐converted white light emitting diode

AbstractThe development and demonstration of a highly efficient warm‐white all‐nitride phosphor‐converted light emitting diode (pc‐LED) is presented utilizing a GaN based quantum well blue LED and two novel nitrogen containing luminescent materials, both of which are doped with Eu2+. For color conversion of the primary blue the nitridosilicates M2Si5N8 (orange‐red) and MSi2O2N2 (yellow‐green), with M = alkaline earth, were employed, thus achieving a high luminous efficiency (25 lumen/W at 1 W input), excellent color quality (correlated color temperature CCT = 3200 K, general color rendering index Ra > 90) and the highest proven color stability of any pc‐LED obtained so far. Thus, these novel all‐nitride LEDs are superior to both incandescent and fluorescent lamps and may therefore become the next generation of general lighting sources. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Possibility of Daylight Simulators Composed of Several Monochromatic Radiations

The possibility of realizing daylight simulators satisfying the conditions specified in JIS Z8720-1983 and Publication CIE No. 51, with line spectrum sources composed of several monochromatic radiations (lines), was examined by a nonlinear programming technique. It was found possible to realize simulators satisfying: (1) the condition of a visible range metamerism index Mlvis ≤ 0.5 (CIELAB10) with 5 spectral lines, (2) the condition of an ultraviolet range metamerism index Mluv ≤ 1 (CIELAB10) with 3 lines, (3) both Mlvis ≤ 0.5 and Mluv ≤ 1,while the general color rendering index Ra is not sufficiently high, with 6 lines, (4) Ra ≥ 95 (CIE 2nd Edition), while Mlvis is not sufficiently good, with 5 lines, and (5) Mlvis ≤ 0.5,Mluv ≤ 1 and Ra ≥ 95 with 7 lines. (6) Six lines are necessary to realize both Mlvis ≤ 0.5 and Ra ≥ 95, in contrast with 5 lines in cases (1) and (4). It is suggested that daylight simulators in either case (5) or (6) are insufficient to reproduce the color appearance of chromatic fluorescent objects excited by visible range radiation under a natural daylight illuminant.

Characteristics of Metal Halide Lamp with Aluminium Tri-iodide

This paper describes the characteristics of a new metal halide lamp with aluminium tri-iodide emitting a continuous spectrum. Many substances are metal halide lamp to give high efficiency and good color rendering Here, aluminium tri-iodide is taken, and experiments are carried out with special discharge tube to achieve calor rendering index Ra highr than 90.With the vapor pressure of aluminium tri-iodide increased in the discharge tube, lamp voltage, light output, spectral energy distribution, CIE general color rendering index Ra, special color rendering index Ri and color temperature are' measured for various values of lamp current.The result indicates that the lamp shows a continuous spectrum at the bulb wall temperature higher than 300°C. Specially, optimum spectral energy distribution is obtained near 400°C, then CIE general color rendering index Ra of the lamp becomes 95 at color temperature of 5700°K with high efficiency. It is confirmed that these data are available for the designing of high pressure and extra high pressure mercury lamp.