In the previous article, “Circadian principles require a new light language,” we discussed how the current lighting language is concerned exclusively with the visual system, lacking the vocabulary to describe light’s impact on the circadian system. While the visual system is concerned with nearly instantaneous environmental cues, the circadian system is accumulating photons throughout the 24-hour day, registering a lot of light as day and an absence of light as night. Remember that without a robust 24-hour cycle of bright light during the day and darkness at night, the circadian system loses track of when to sleep, when to eat, and when to work.

The visual cortex and biological clock are located in different parts of the brain, and have differing sensitivities to light. Using the current vocabulary of footcandles and correlated color temperature (CCT) as metrics can describe the visual impact; however, these terms are a language that is