Yoshihiro Ohno

Color quality scale

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.

Spectral design considerations for white LED color rendering

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.

Color Rendering and Luminous Efficacy of White LED Spectra

White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multi-chip and phosphor-type white LED models were analyzed by simulation on 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), using not only Ra but also the special color rendering indices Ri as well as the CIELAB color difference ΔE*ab for the 14 color samples defined in CIE 13.3. Several models of 3-chip and 4-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 CIE Color Rendering Index (CRI), and the need for improvements is discussed.

Four-color laser white illuminant demonstrating high color-rendering quality

Solid-state lighting is currently based on light-emitting diodes (LEDs) and phosphors. Solid-state lighting based on lasers would offer significant advantages including high potential efficiencies at high current densities. Light emitted from lasers, however, has a much narrower spectral linewidth than light emitted from LEDs or phosphors. Therefore it is a common belief that white light produced by a set of lasers of different colors would not be of high enough quality for general illumination. We tested this belief experimentally, and found the opposite to be true. This result paves the way for the use of lasers in solid-state lighting.

Toward an improved color rendering metric

Several aspects of the Color Rendering Index (CRI) are flawed, limiting its usefulness in assessing the color rendering capabilities of LEDs for general illumination. At NIST, we are developing recommendations to modify the CRI that would overcome these problems. The current CRI is based on only eight reflective samples, all of which are low to medium chromatic saturation. These colors do not adequately span the range of normal object colors. Some lights that are able to accurately render colors of low saturation perform poorly with highly saturated colors. This is particularly prominent with light sources with peaked spectral distributions as realized by solid-state lighting. We have assembled 15 Munsell samples that overcome these problems and have performed analysis to show the improvement. Additionally, the CRI penalizes lamps for showing increases in object chromatic saturation compared to reference lights, which is actually desirable for most applications. We suggest a new computation scheme for determining the color rendering score that differentiates between hue and saturation shifts and takes their directions into account. The uniform color space used in the CRI is outdated and a replacement will be recommended. The CRI matches the CCT of the reference to that of the test light. This can be problematic when lights are substantially bluish or reddish. Lights of extreme CCTs are frequently poor color renderers, though they can score very high on the current CRI. An improved chromatic adaptation correction calculation would eliminate the need to match CCT and an updated correction is being considered.

LED-based Spectrally Tunable Source for Radiometric, Photometric, and Colorimetric Applications

A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology. The source is designed to have a capability of producing any spectral distribution, mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as a photometric and colorimetric standard. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of display colorimeters. The results indicate that the errors can be reduced by an order of magnitude when the tunable source is used to calibrate the colorimeters, compared with measurement errors when the colorimeters are calibrated against Illuminant A. The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.

Practical Use and Calculation of CCT and Duv

Abstract Correlated color temperature (CCT) is often used to represent chromaticity of white light sources, but chromaticity is two-dimensional, and another dimension, the distance from the Planckian locus, is often missing. Duv is defined in ANSI C78.377 for this purpose but is not yet widely used. In this article, the use of a combination of CCT and Duv is proposed as an intuitive expression of chromaticity of white light sources for general lighting. In addition, this article presents practical calculation methods to calculate CCT and Duv, having sufficient accuracy, within an error of 1 K, in a wide range of chromaticity, from 1000 to 20,000 K in CCT and −0.03 to 0.03 in Duv.

Integrating sphere simulation: application to total flux scale realization

A method is proposed for realizing the total flux scale of light sources by use of an integrating sphere with an opening to introduce a known amount of flux from a luminous intensity standard or a spectral irradiance standard lamp placed outside the sphere. Computer simulations were made on several models of an integrating sphere, designed to compare the total flux of a test lamp inside the sphere with the flux introduced from an external source. I describe the theory and algorithm of the simulation, present the results of the simulation for varying conditions of sphere geometry such as size and location of the baffles, internal source, and wall reflectance, and predict that one of the models has sufficient accuracy to calibrate lamps for total flux.

Detector-based luminous-flux calibration using the Absolute Integrating-Sphere Method

The Absolute Integrating-Sphere Method was developed and introduced by the NIST for the realization of the luminous-flux unit using an integrating sphere rather than a goniophotometer. The total luminous flux of a lamp inside the sphere is calibrated against the known amount of flux introduced into the sphere from the external source through a calibrated aperture. The key element of this method is the correction for the spatial nonuniformity of the integrating sphere, which has been made possible by a technique using a scanning-beam source. This method is to be applied directly to routine calibration measurements of luminous flux. A new luminous-flux calibration facility with a 2.5 m integrating sphere has been recently completed at the NIST and will allow the calibration of test lamps with no need for luminous-flux standard lamps. Calibrations will be performed based on the illuminance measurement of the external source by standard photometers. This method makes the luminous-flux calibration a detector-based procedure, thereby eliminating the uncertainties associated with the use of standard lamps while achieving lower uncertainties by shortening the calibration chain. The characteristics of the new integrating sphere obtained by computer simulation and measurements are reported.

THE NIST DETECTOR-BASED LUMINOUS INTENSITY SCALE

The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.