This is the second of a two-part article addressing advances in light sources and light quality, authored by Matthew Blakeley, Vice President – Product and Business Development for Focal Point as an exclusive contribution to LightNOW. This article focuses on the evolution of light sources and measurement tools that can accurately describe the quality of the color rendition in a space. This second article highlights recent research, which provides a baseline definition of a quality light source that’s preferred by humans, independent of cultural background and familiarity with the habitual built environment.
Click here to read Part 1
Several recent research studies, conducted by Pacific Northwest National Laboratories (PNNL) and Penn State University (PSU) in the United States, as well as Zhejiang University in China, come to similar conclusions when attempting to define a light quality that is preferred by humans.
Interestingly, the fidelity of the light source to a blackbody radiator, as would be measured by CRI – the Color Rendering Index – was not found to be the key determining factor when assessing the pleasance of a light source to the human eye. Rather, saturation, especially in the red spectrum, was a key attribute that was universally preferred in North America as well as in Asia.
Published in 2016, the first PNNL study showed participants various light scenes, all at 3500K. Participants entered a room filled with common items such as fruits, clothing, and consumables and outfitted with a mirror so that changes in skin tones could be observed. Similar to an eye doctor test, they were exposed to various spectral power distributions and asked to rate each scene, which varied from low to high fidelity and from undersaturated to oversaturated.
When the research team ran the results against statistical models a very interesting trend emerged. Rather than fidelity being linked to preference, they found a strong correlation between saturation and preference. They further identified that saturation in red was key, which is consistent with some requirements for R9 when using CRI. This research culminated in a simple equation:
Preference = Fidelity + Red Saturation
Where fidelity should be greater than 74 and the red spectrum should be oversaturated by 2 to 16 percent while the whole system, expressed by the gamut, should be oversaturated.
PSU later performed a study also at 3500K and using a different methodology – they opted for an absolute test where participants were exposed to only one lighting scene per day. Interestingly, although the format and methodology were different, preferred light sources fell in the same range as with the PNNL study.
Follow-on research from PNNL sought to explore the effect of changes in chromaticity and exposed participants to light sources that varied between 2700K and 4300K. Similar preference results were observed, independent of chromaticity.
Lastly, in 2017, researchers at Zhejiang University in China showed a total of 164 lighting scenes in four chromaticity groups, ranging from 2800K to 6500K, and drew similar conclusions relative to preference. As the first study taking place outside of the United States, it also demonstrated that preference is not conditioned by the habitual built environment, but that it transcends borders and cultures.
All studies drew similar conclusions that fidelity was not the key determining factor of the pleasance of a light source. Fidelities slightly below the commonly accepted 80 CRI can correlate to a preferred light source when combined with an oversaturation of the gamut of the light source and of the red spectrum.
Specifying a quality light source
The most recent version of the WELL Building Standard™, WELL v2 pilot, defines parameters for Electric Light Quality under Feature L07. It specifies a CRI threshold > 90, or > 80 with R9 > 50. It also incorporates the PNNL targets for a preferable light source with a Fidelity (Rf) ≥ 78, a Gamut (Rg) ≥ 100 and an oversaturation in the red spectrum between 1 and 15 percent using the parameters of TM-30-18.
Light sources meeting those parameters offer the advantage to render colors in a manner that is preferred by humans. Skin tones appear more natural and healthy due to an increase in red content, organic materials such as wood have a warmer, more natural coloration and colors are more vibrant overall, making common objects appear more attractive.
Anthropological, historical, and sociological factors appear to explain this preference for a light source with a slight oversaturation in the red spectrum. Humans have evolved for millennia around fire, which would have shaped our perception of a preferable light source.
We appear to be at a tipping point, where new LED technology is becoming widely available, efficacy gains are translating to incrementally smaller savings year-after-year, new building standards are promoting health and wellness of occupants, and independent studies are converging to define a light that is deemed preferred by humans, independent of cultural backgrounds.
Using this information, specifiers can finally switch the paradigm to the users of a space or add that component in the selection of light sources that will not only be functional, sustainable, and enhance architecture, but also enhance the experience of those who live, work, heal, learn and play under the light sources.
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