Lighting plays an important functional and aesthetic role in architecture, contributing to user safety and enhancing the built environment. Today, lighting upgrades in commercial buildings continue to focus on reducing energy usage, with a lesser emphasis on light quality.
The proliferation of energy efficient commercial building products has allowed the sector to improve its energy efficiency. According to the US Energy Information Administration (EIA), in 2011 lighting was responsible for 38 percent of all electricity consumed in the commercial sector, by 2017 that number had been reduced to 11 percent. Considering the long lifetimes of LEDs, the environmental impact of lighting as it pertains to maintenance and replacement has been significantly reduced. As the efficacy of LED light sources is reaching a plateau, an argument can be made to re-focus the discourse on lighting quality and how it can improve the experience of those who inhabit the spaces.
Recent research outlines the parameters of a light quality that humans prefer, one that supports the creation of more human-centric environments. The Illuminating Engineering Society (IES) TM-30-18 technical memorandum provides a method for measuring the color rendition of a light source and its parameters are used by the WELL Building Standard™ (WELL) under Feature L07 – Electric Light Quality of WELL v2 pilot.
Tools used to assess light quality
Used since the 1960s, CRI, an abbreviation for Color Rendering Index, is an average color rendering score used to measure light quality. Generally, a score above 80 is desired and most LED light sources installed in commercial buildings have a CRI of 80 or above. Higher quality light sources with a CRI of 90, which theoretically translates into more naturally rendered colors, is also commonly available from luminaire manufacturers but specified less often. 90 CRI is typically found in healthcare or other environments where accurate color perception is paramount, such as when evaluating the condition of a patient.
While CRI is an easy-to-understand rule of thumb, it also has limitations. It is a measure of fidelity only: it describes how a light source differs from a reference illuminant, a blackbody radiator, without describing the direction of the change in either chroma or hue. For example, consider a light source with a 20 percent over-saturation of yellow content and another with a 20 percent over-saturation of blue content. These two light sources with very different color saturations, assuming the other colors are the same, would both have a similar Color Rendering Index value. To compensate for this, a red metric called R9, which specifically measures red fidelity, is often used along with CRI.
TM-30-18, a technical memorandum from the Illuminating Engineering Society (IES), addresses the evaluation of a light source’s color rendition. TM-30 measures not only the color fidelity of a light source to a blackbody radiator with the Fidelity Index (Rf), but also measures saturation with the Gamut Index (Rg). Additionally, TM-30 divides the color spectrum into 16 hue bins and provides measures of fidelity, hue, and chroma shift for each hue bin. As a result, it spells out not only if a light source is oversaturated, but also in what color spectrum the over-saturation occurs. Going back to the above example, we would see the over-saturation in yellow or blue, or any of the 16 hue bin colors.
Though quality of light is far from being a new topic for the lighting design industry, new initiatives have recently emerged surrounding lighting for human preference. These initiatives include white tuning, circadian lighting, warm dimming, and human preference lighting. In addition, new building standards have also emerged, such as the WELL Building Standard (WELL), contributing to an increased awareness of the importance of balancing human health and well-being with efficiency and sustainability.