Wendy L. Davis

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.

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.

Approaches to color rendering measurement

Color rendering refers to a light sources ability to make the colors of illuminated objects appear natural or accurate. The color rendering index (CRI) is currently the only internationally-standardized way to assess a light sources color rendering abilities. The CRI has shortcomings in application, however, and its problems are pronounced when applied to newer lighting technologies, such as light-emitting diodes (LEDs). Since the introduction of the CRI to the present day, alternative methods have been proposed and studied. Some methods are based on the shape of the spectral output of the source, considering broadband sources to have better color rendering than sources with spectral peaks or valleys. Several proposals share the basic method of CRI, with modifications to improve performance. Still other ideas are based on measures of the gamut area of rendered object colors. The International Commission on Illumination (CIE) is in the process of developing and recommending a new metric of color rendition.

Optimising light source spectrum for object reflectance.

Much light in architectural spaces is absorbed by objects and never perceived by occupants. The colour appearance of objects illuminated by light spectra that minimized this absorption was determined. Colour differences were calculated for 15 reflective samples when illuminated by various coloured test light sources and reference white illuminants. Coloured test spectral power distributionos (SPDs) can reduce energy consumption by up to 44% while maintaining identical colour appearance of illuminated objects. Energy consumption can be reduced further, but with noticeable colour shifts. Results quantify the trade-off between colour fidelity and energy consumption with this approach.

Development and evaluation of colour control interfaces for LED lighting

To capitalise on the colour tuning capabilities of LED lighting, a model for converting device-specific control signals to chromaticity coordinates was used in a psychophysical experiment evaluating the usability of three colour control interfaces based on RGB (red, green, blue), HSB (hue, saturation, brightness) and opponent colour mixing systems. Although common and well accepted, the RGB interface had lowest usability based on both psychophysical results and subjective ratings. The usability of HSB and opponent colour interfaces was not significantly different. These findings can guide the development of useful and efficient colour control interfaces for tunable LED lighting systems.

The Effect of Control Resolution on the Usability of Color-Tunable Lighting Systems

ABSTRACT Color-tunable lighting systems with inappropriate resolutions may confuse end-users and prevent them from easily specifying their desired colors. To improve the design of color-tunable systems, a psychophysical experiment explored the independent and interacting effects of hue, saturation, and luminance control resolutions on usability. Three criteria—efficiency, effectiveness, and satisfaction—were used to evaluate these effects. Results showed that combinations of middle-range hue resolution and saturation resolution facilitated the greatest usability. When luminance control was independent of hue and saturation control, luminance resolution did not significantly impact usability.

The Effect of Control Resolution on the Usability of Color-tunable LED Lighting Systems

A psychophysical experiment explored the independent and interacting effects of hue, saturation and luminance control resolutions on the usability of color-tunable LED lighting systems. Three usability criteria were used to evaluate these effects.

Dimming Curve Based on the Detectability and Acceptability of Illuminance Differences

A new approach to step-dimming leverages knowledge of the detectability and acceptability of illuminance differences to reduce lighting energy consumption. Computations show that this method can reduce lighting energy use more than continuous dimming