Light emitting diodes (LEDs) are on the verge of a breakthrough in general lighting, due to their rapidly improving efficiency. Currently, white LEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. This Review first describes how to quantify the quality of a light source, discussing the color rendering index (CRI) and alternative color quality indices. Then, six main criteria are identified and discussed, which should be fulfilled by a phosphor candidate to be considered for actual application in LEDs. These criteria deal with the shape and position of the emission and the excitation spectra, the thermal quenching behavior, the quantum efficiency, the chemical and thermal stability and finally with the occurrence of saturation effects. Based on these criteria, the most common dopant ions (broad-band emitting Eu 2+ , Ce 3+ and Mn 2+ , line-emitting rare earth ions,...) and host compounds (garnets, sulfides, (oxy)nitrides,...) are evaluated. Although many phosphor materials have been proposed in literature in recent years, the number of phosphors effectively fulfilling all six requirements is relatively small.

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Selecting Conversion Phosphors for White Light-Emitting Diodes

This paper surveys current technology and research in the area of digital color imaging. In order to establish the background and lay down terminology, fundamental concepts of color perception and measurement are first presented using vector-space notation and terminology. Present-day color recording and reproduction systems are reviewed along with the common mathematical models used for representing these devices. Algorithms for processing color images for display and communication are surveyed, and a forecast of research trends is attempted. An extensive bibliography is provided.

Digital color imaging

The color rendering index (CRI) has been shown to have deficiencies when applied to white light-emitting-diode-based sources. Furthermore, evidence suggests that the restricted scope of the CRI unnecessarily penalizes some light sources with desirable color qualities. To solve the problems of the CRI and include other dimensions of color quality, the color quality scale (CQS) has been developed. Although the CQS uses many of elements of the CRI, there are a number of fundamental differences. Like the CRI, the CQS is a test-samples method that compares the appearance of a set of reflective samples when illuminated by the test lamp to their appearance under a reference illuminant. The CQS uses a larger set of reflective samples, all of high chroma, and combines the color differences of the samples with a root mean square. Additionally, the CQS does not penalize light sources for causing increases in the chroma of object colors but does penalize sources with smaller rendered color gamut areas. The scale of the CQS is converted to span 0-100, and the uniform object color space and chromatic adaptation transform used in the calculations are updated. Supplementary scales have also been developed for expert users.

Color quality scale

White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multichip and phosphor-type white LED models were analyzed by simulation of their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting. Color rendering characteristics were evaluated based on the CIE Color Rendering Index CRI, examining not only Ra but also the special color rendering indices Ri, as well as on the CIELAB color difference Eab for the 14 color samples defined in CIE 13.3. Several models of three-chip and four-chip white LEDs as well as phosphor-type LEDs are optimized for various parameters, and some guidance is given for designing these white LEDs. The simulation analysis also demonstrated several problems with the current CRI, and the need for improvements is discussed. © 2005

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Spectral design considerations for white LED color rendering

In computer games, there is typically a display that represents a view into a virtual world of some type. There is also typically a user in a real world environment that surrounds the display screen interacting with the virtual world. Disclosed herein are systems and methods for using lighting systems, in particular LED based lighting systems, to allow a user to receive information from the virtual world in coordination with, in addition to, and/or instead of the information received from the display.

Systems and methods for digital entertainment

“Visual clarity” or “feeling of contrast” of object colors under illumination is affected significantly by changing color-rendering property of light source used. Though the feeling of contrast is considered one of the most important characteristics on color-rendering properties of light sources, it cannot be estimated adequately by using the present Ra method. The new index FCI is proposed for estimating the effect of feeling of contrast quantitatively under any light sources. The FCI is derived using a simple transformation of the gamut area, which is constituted by a specially selected four-color combination in CIE LAB color space. The FCI correlates well with the illuminance ratio for equal feeling of contrast (or equal visual clarity) on various light sources reported so far. Using the FCI together with the present CIE Ra, the color-rendering capability of a light source can be well clarified. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 361–371, 2007

New method for specifying color‐rendering properties of light sources based on feeling of contrast

In an illumination control method of the present invention, illumination of an appreciation space is controlled in association with an image displayed on an image display device so that a realism of the image displayed on a screen of the image display device can be enhanced, where an appreciator appreciating the image is in the appreciation space. More specifically, one or more light sources provided in the appreciation space is controlled so that at least one parameter of a level, a light color, a luminous intensity distribution, and a direction of illumination to the appreciation space is made substantially coincident with a corresponding parameter of a virtual image space imaginarily created from the image displayed on the image display device.

Illumination control method and illumination device

A nonlinear color-appearance model was extended to apply to white and light-gray background. Based on this extended model, two kinds of chromatic-adaptation transforms were derived, which correspond to lightness-chroma match and brightness-colorfulness match. The chromatic-adaptation transform for lightness-chroma match is also an extension of the transform proposed by CIE for further testing. The differences between the two transforms were confirmed by visual observations. The usefulness of a combination of a color-appearance model and its corresponding chromatic adaptation transform is discussed. In addition, the practical importance of brightness and colorfulness is discussed.

Color‐appearance model and chromatic‐adaptation transform

The visual clarity of a lighting environment is significantly affected by changing the general color rendering index (Ra) of its illumination. This effect has been studied by a number of researchers, but the cause of this effect has not been thoroughly studied. In order to clarify this, the mutual relations between visual clarity, lightness perception, and feeling of contrast are analyzed by using object colors under illuminations with various Ra values. the results obtained are as follows. (1) the visual clarity of a lighting environment is different from the lightness perception of the object colors in the environment. (2) the change in visual clarity of a lighting environment is highly correlated to a feeling of contrast between object colors under the illumination. (3) the effect of visual clarity is estimated effectively by assessing the feeling of contrast using a specially selected four-color combination. (4) the illuminance for equal visual clarity for any illumination is predicted by the equality of feeling of contrast under the same illumination specified by the gamut area made by the component colors of the four-color combination in a brightness and colorfulness space. (5) the effect of visual clarity under various illuminations cannot be predicted by using their Ra values.

Visual clarity and feeling of contrast

The nonlinear color-appearance model of Nayatani et al. is reformulated by changing the fundamentals from the Pitt to the Estevez-Hunt-Pointer primaries. The predictions of the reformulated model correlate with the Munsell and the NCS schemes better than those of the original. Formulas for various color-appearance metrics are proposed, such as colorfulness, chroma, saturation, brightness (including a chromatic-component contribution), and others. In the present model, the normalizing illuminant D65 is used in the transformation from the CIE tristimulus values to those in the fundamental-primary system, and the normalizing illuminance is kept at 3,000 lx. The effects of changing the normalizing illuminant to D50 and the normalizing illuminance to 1,000 lx are studied. The results, however, show insignificant differences in the predictions of color appearance.

Kenjiro Hashimoto

Papers

New method for specifying color‐rendering properties of light sources based on feeling of contrast

Illumination control method and illumination device

Color‐appearance model and chromatic‐adaptation transform

Visual clarity and feeling of contrast

A nonlinear color-appearance model using estévez-hunt-pointer primaries