Minchen Wei

Review of measures for light-source color rendition and considerations for a two-measure system for characterizing color rendition.

Twenty-two measures of color rendition have been reviewed and summarized. Each measure was computed for 401 illuminants comprising incandescent, light-emitting diode (LED) -phosphor, LED-mixed, fluorescent, high-intensity discharge (HID), and theoretical illuminants. A multidimensional scaling analysis (Matrix Stress = 0.0731, R(2) = 0.976) illustrates that the 22 measures cluster into three neighborhoods in a two-dimensional space, where the dimensions relate to color discrimination and color preference. When just two measures are used to characterize overall color rendition, the most information can be conveyed if one is a reference-based measure that is consistent with the concept of color fidelity or quality (e.g., Q(a)) and the other is a measure of relative gamut (e.g., Q(g)).

Color Preference under LEDs with Diminished Yellow Emission

Abstract A psychophysical experiment was conducted to investigate brightness perception and color preference under illumination from typical 3000 K blue-pumped light emitting diode (LED) A19 lamps (BP-LED) and 3000 K blue-pumped LED A19 lamps with diminished yellow emission (YD-LED). Eighty-seven participants between 19 and 25 years completed brightness matching and preference comparisons between two full-size side-by-side rooms that contained chromatic objects and still life arrangements, with one lamp type in each room. Participants had higher overall preference to the room illuminated by YD-LED. Specifically, red, green, orange, and wood objects were preferred under YD-LED; no preference was found for neutral, yellow, and blue objects between the two lamp types. The words saturated, vivid, vibrant, pleasant, inviting, colorful, stand-out, distinct, appealing, and comfortable were reported as reasons for the color preference under YD-LED; dull, dead, and fade were used by participants to describe the lower color preference under BP-LED. Skin tone preference under the illumination of these two lamp types was significantly different between Asian (n = 32) and Caucasian (n = 52) participants. Caucasians evaluated their own skin tone more favorably under YD-LED, with the average assessment that skin appeared healthy and colorful under YD-LED but grey and pale under BP-LED. No preference between the two lamps was found among Asians for their skin tone, though some Asians thought YD-LED rendered their skin too red and odd. Many existing measures of color preference and gamut were able to predict the higher overall preference to YD-LED, but they could not predict the preference of specific colors or for ethnic groups. The higher saturation and preference for red and green colors under YD-LED illumination is consistent with the higher red–green opponent signal provided by YD-LED. Coupled with several past studies, the spectral region around 570–580 nm appears to be deleterious to color and brightness perception.

Perceptual responses to LED illumination with colour rendering indices of 85 and 97

A light source’s ability to render colours and whites is one aspect of lighting quality. Since better colour rendition generally comes with a penalty in luminous efficacy, it is reasonable to consider the balance: Should colour and whiteness rendition be sacrificed to achieve higher efficacy? Here, we investigate perceptual responses under illumination from two sources, a blue-pumped LED with a colour rendering index of 85 (BLED85), and a violet-pumped LED with a colour rendering index of 97 (VLED97). Forty-eight participants completed three experiments. Neutral, red, and pink were preferred under VLED97. Skin rendition was preferred by Caucasians under VLED97; Asians had no preference. Teeth and a white shirt were whiter and more preferred under VLED97. We conclude that the colour and whiteness rendition must be considered on a par with luminous efficacy.

Status of Solid-State Lighting Based on Entries to the 2010 US DOE Next Generation Luminaire Competition

This paper analyzes the performance of 138 LED luminaires comprised of 95 indoor and 43 outdoor, all of which were within the judging phase of the 2010 U.S. Department of Energy Next Generation LuminaireTM (NGL) competition. The analyses focus on three areas derived from LM-79 and LM-80 test reports that were supplied by the manufacturers: energy efficiency, color, and long-term performance. Indoor recessed downlights and street lighting luminaires had the most entries. All luminaires employed phosphor-converted LED (PC-LED) packages or remote phosphor technology, reflecting the fact that these technologies now have better performance than color-mixed light emitting diodes (LEDs) for white-light applications. Comparisons of luminaire efficacy and luminous efficacy of radiation (LER) were made among the NGL 2010 submissions, luminaires with conventional light sources, and the expected performance of LED luminaires in the near future. 0–10V and reverse-phase control were the prominent dimming methods among the luminaires studied. High correlation (r 0.98) was found between the CIE color rendering index (CRI) and NIST’s Color Quality Scale (CQS). Compared to the luminaires recognized in 2009, a better balance was achieved between efficacy and CRI in 2010. The need to standardize the methods for evaluating LED products is discussed, including dimming performance, color-shift, long-term performance, and luminaire rated life. In person evaluation and mockups are recommended prior to specifying LED products. Keywords—Solid-state lighting, white light light-emitting diode (LED), energy efficiency, colorimetry, long-term performance. Department of Architectural Engineering, The Pennsylvania State University, USA *Corresponding author: Kevin W. Houser, E-mail: khouser@engr.psu.edu L E U K O S V O L 8 N O 4 A P R I L 2 0 1 2 P A G E S 2 3 7 – 2 5 9 ©2012 The Illuminating Engineering Society of North America doi: 10.1582/LEUKOS.2012.08.04.001

Whiteness Perception under LED Illumination

ABSTRACT Visual perception of many manufactured white objects is driven by the concentration of fluorescent whitening agents (FWAs) that they contain. FWAs serve to increase overall whiteness perception by enhancing luminance and providing a chromatic blue shift. White objects with FWAs have been engineered for many decades to interact with conventional illuminants such as daylight and incandescent light in order to provide desirable whiteness perception. Whiteness perception has not previously been investigated under light emitting diodes (LEDs). In this study, three psychophysical experiments—forced choice, selection, and sorting—were conducted to investigate the whiteness perception of a series of whiteness standards containing predetermined amounts of FWAs, illuminated by five sources: a typical blue-pumped LED (BLED), a filtered halogen lamp, and violet-pumped LEDs with three violet emission levels. Thirty-nine participants with normal color vision completed the experiments. Results from the sorting experiment can be explained by FWA excitation: the BLED induced no fluorescence and standards could not be ordered properly, whereas for other sources the perceived whiteness increased with the amount of FWAs. Results from the other two experiments can be explained by FWA excitation together with shifts in source chromaticity. Overall, all results are compatible with the known trend for blue shifts to induce whiteness perception. Adaptations of the CIE whiteness formula are also shown to agree well with the experimental results. The results indicate that engineering of an LED source’s spectrum is necessary for an accurate rendering of whiteness.

Human perceptions of colour rendition vary with average fidelity, average gamut, and gamut shape

An experiment was conducted to evaluate how subjective impressions of a light source’s colour quality depend upon the details of the shifts it causes in the colour appearance of illuminated objects. Twenty-eight participants each evaluated 26 lighting conditions in a 3.1 m by 3.7 m room filled with objects selected to cover a range of hue, saturation, and lightness. IES TM-30-15 Fidelity Index (Rf) values ranged from 64 to 93, IES TM-30 Gamut Index (Rg) values ranging from 79 to 117, and IES TM-30 red chroma shift (Rcs,h1) values ranging from −19% to 26%. All lighting conditions had the same nominal illuminance and chromaticity. Participants were asked to rate each condition on eight point scales for saturated–dull, normal–shifted, and like–dislike, as well as classifying the condition as one of saturated, dull, normal, or shifted. The findings suggest that gamut shape is more important than average gamut area for modeling human preference, with red playing a more important role than other hues. Average fidelity alone is a weak predictor of human perception, especially CIE Ra. Nine of the top 12 rated products had a CIE Ra value of 73 or less, which indicates that the criteria of CIE Ra ≥ 80 may be excluding many preferred light sources.

Colour preference varies with lighting application

Numerous efforts have been made to investigate how to characterize the effect of light sources on colour preference. This paper describes a psychophysical experiment to compare the colour preference evaluation among three sources for different lighting applications. Three lighting applications – restaurant, retail display, and supermarket – were created in two side-by-side full-scaled rooms. It was found that the colour preference varied with the lighting applications. Although none of the existing single-value measures can predict the preference for all three applications, a single measure is necessary for general consumer use given a careful definition of general lighting. In addition, lighting application specific measures could be useful for lighting professionals and experts. To provide detailed information, a colour distortion icon, with improved test colour samples will be useful.

What Is the Cause of Apparent Preference for Sources with Chromaticity below the Blackbody Locus

ABSTRACT Two recent articles reported that light stimuli with chromaticity below the blackbody locus were preferred to those on or above the blackbody locus. Neither article offered an explanation for why that occurred. In this article we suggest that chromaticity may not be the sole cause of preference. The preference to the light stimuli reported in these two articles may be due to the combined effects of color rendition and chromaticity. Through simulation, we show that illuminants with chromaticity below the blackbody locus (that is, negative Duv) are more likely to have higher scores for relative gamut (for example, Qg) than illuminants on or above the blackbody locus, while maintaining high scores for fidelity (for example, Qf). Future psychophysical studies that separate the effects of chromaticity and color rendition will be necessary to more fully understand preference for light stimuli.

Colour gamut size and shape influence colour preference

Two psychophysical experiments were conducted to investigate the effect of colour gamut size and shape on colour preference. In the first, two side-by-side booths were designed to resemble a retail setting with clothing; in the second, a single booth was designed to resemble a restaurant setting, but also included a mirror to permit the observers evaluation of skin tone. These settings were illuminated with two sets of sources, where, compared to a fixed reference, one set created modest chroma enhancement and the other set created greater increases in object chroma. Within each set, gamut shape varied, meaning different hues were saturated, even though, on average, the spectra created the same average increase in chroma. When objects were unfamiliar, as with the fabrics, all chroma-enhancing spectra were preferred to the fixed reference regardless of the gamut shapes. When familiar objects were present, such as food, observers were more discerning about changes in chroma and hue. We conclude that a graphic of gamut shape is an important adjunct to average measures of colour fidelity and gamut.

The Role of Presented Objects in Deriving Color Preference Criteria from Psychophysical Studies

ABSTRACT  Although it is a critical component of any measure of color rendition, a standardized set of color samples can seldom perfectly match a real space or a real set of observed objects. This means that there will always be some level of mismatch between predicted and observed color shifts. This article explores how the color distortions of three object sets that could be used in experiments compare to the color distortions predicted using the color evaluation samples of IES TM-30-15 (TM-30). The experimental object sets include those from a recent experiment [Royer and colleagues 2016], a set of produce (10 fruits and vegetables), and the X-Rite ColorChecker Classic. This numerical analysis focuses on the range of differences between viewed and characterized color shifts—using the TM-30 fidelity index (Rf), the TM-30 gamut index (Rg), and an alternative to Rg based on ΔC in CIECAM02—over a set of 344 spectral power distributions. The differences depended on the average chroma and spectral features of the sample set. The substantial range of differences shown for the produce and the ColorChecker means that design criteria for color rendition derived using these sample sets are less reliable. Specifiers should carefully consider how average measures of color rendition are applied to real spaces, and experimenters should carefully select experimental objects to avoid mischaracterizations.