Metameric Circadian Lighting: A Technology Analysis
The absence of natural light causes serious health problems in humans. Although known since ancient times it was only after the recent discovery of the so-called intrinsically photosensitive retinal ganglion cells (ipRGCs) that specialists started to understand the responsible biological mechanisms. This discovery boosted the research and development efforts on lighting solutions that exploit non-visual effects to promote human health (broadly known as Human Centric Lighting, HCL). In particular, being the most important “Zeitgeber” for circadian rhythms of our inner clock, light is a powerful tool to trigger effects such as alertness, sleepiness and cognitive performance and consequently, a large deal of research and development has been committed to the development of color tunable lighting solutions that match daylight variable spectral power distribution, SPD.
However, the preferred white color coordinates aka white tone depends strongly on the application (e.g. office, retail, home, etc.), the illuminated objects (office tables, cloths, food, etc.) and the brightness and not surprisingly white tone variation is not desired for many applications. Fortunately, since ipRGCs do not participate in vision, different circadian effects can be produced by visual stimuli of the same color coordinates (metameric stimuli).
Metameric circadian SPDs for display applications are a subject of extensive investigation. However, the widespread adoption of circadian lighting solutions relies not only on the non-visual effects but also on their ability to deliver excellent visual experiences which ultimately demands for the optimization of different performance metrics including the ability to render the real colors, a strong overlap with the eye sensitivity function and on-locus white tone.
Today, several different LED technologies can be combined to realize metameric circadian lighting solutions which include broadband phosphor-converted white LEDs (blue and UV pumped) as well as narrow-band colored LEDs and QD-converted LEDs. Here we provide design rules to best exploit these technologies for the realization of circadian metameric light sources with excellent visual characteristics.