Human Centric Lighting and Semantic Drift

I’m at once intrigued and frustrated by the phrase human centric lighting. If considered from a strictly linguistic point-of-view, the phrase implies the use of light for the benefit of people. But, since human centric lighting has no formal definition, the intended meaning may vary with the user, and is ripe for misuse and prone to miscommunication. Knowing how this phrase evolved, and what it could means when used today, will help support clearer communication when human centric lighting is a topic of discussion.

In the 1990s and 2000s (before the age of LEDs!) there was debate, often heated, about the potential benefits of light sources with higher ratios of scotopic to photopic lumens (i.e., higher S/P ratios). Practically, the controversy was related to the practice of retrofitting fluorescent lighting system that employed lamps with a CCT of 3500 K or lower, with lamps of lower wattage but higher CCT, generally 5000 K or higher. Those in favor of higher CCT lighting cited improvements in visual acuity and brightness perception, and reductions in energy use, in comparison to lower CCT lighting. Others were more cautious, believing the evidence in favor of high CCT lighting at the photopic levels consistent with IES recommended practices was tenuous, and noting that decisions about CCT should be based on numerous design factors, including preference, mood, and aesthetics. Thus, in the age of fluorescent lighting, and before variable CCT products became common, lighting described as “high CCT” or “high S/P ratio” was viewed as positive by some, negative by others, or most rationally, as an objective attribute of light spectrum that the design team could specify in support of user needs.

In response to negative connotations associated with high CCT lighting, the phrase human centric lighting was adopted as a euphemism for “high CCT.” Ostensibly, it would be easier to sell human centric lighting than to explain CCT, S/P ratio, visual acuity, and the disparate views about high CCT lighting that were being expressed in lighting periodicals. In that original context, human centric lighting might be viewed as an effort to rebrand a debatable practice with feel-good language.

Perhaps because the phrase is benign and appealing, human centric lighting has been coopted by others and is now in popular use among the lighting community. I’ve heard the phrase used to mean many different things. As a rough chronology, after entering the lighting lexicon as a euphemism for high CCT lighting, it was adopted by others as a near equivalent to lighting quality, but with a focus on visibility. As interest in circadian photobiology grew, the phrase drifted to also encompass human health. It’s now used as a catchall phrase that can be broadly defined as:

Human Centric Lighting (HCL): Evidence-based lighting solutions optimized for vision, performance, concentration, alertness, mood, and general human health and well-being. HCL balances visual, emotional, and biological benefits of lighting for humans, recognizing the role of light on human vision, psychology, and physiology.

To achieve such broad aims, it would be necessary to have deep scientific understanding of how people are affected by the quantity of optical radiation, its spectral and spatial distributions, the duration of exposure, and the timing of exposure. Also required is knowledge of the observer, including photic history, visual health, and age. Thus, while the above definition is an appealing way to think about light and lighting, the simultaneous achievement of all objectives is more aspirational than achievable. Vision science and lighting application experience does not provide evidence to support all outcomes for all users in all contexts.

While there has been semantic drift with the use of human centric lighting, the current situation can be more aptly described as semantic anarchy. Different companies, organizations, and people use the phrase to mean different things. Unless the people using the phrase have agreed on an operational definition, miscommunications are likely. Moving forward, an institutional authority such as IES or CIE could adopt a formal definition for human centric lighting. Doing so would add structure and clarity to the evolving language of light and lighting. Until then, it would be prudent to seek clarification about what is meant when the phrase is used. If human centric lighting is part of a designer’s scope, or a manufacturer’s claims, or an owner’s wants, it is prudent for all involved to seek and develop a common understanding of what human centric lighting means for their project.