Exploring circadian lighting.

Jonathan Rush

Jonathan Rush, Partner

The latest research and questions.

Part three of our five-part series exploring the latest thinking in circadian lighting – in celebration of the International Day of Light on 16 May.

Part one – Understanding our circadian rhythms
Part two – Understanding the colour spectrum

The main thing we need to be aware of when considering circadian lighting is that it’s difficult to establish for certain that it ‘works’. A number of case studies in schools, hospitals, and offices suggest they are successful, but often these are ‘before and after’ impressions of the environments.

Though good research is available it is often not definitive, and contradictory papers add to the confusion. One of the key questions is: what level of exposure to blue spectrum light is required from artificial sources in order to have a valuable impact? …And, if our reference basis is the spectral intensities of daylight, can an artificial lighting system meet similar criteria?

Quantifying the exact spectrum required for melatonin suppression has begun to be answered by scientists at the University of Manchester.

Illuminance sensitivity curve.

The University of Manchester first proposed the ‘melanopic’ illuminance curve of eye sensitivity in 2011. In 2014 this was expanded by the same team to include all of the contributing cones and rods in the eye. Within the retina of the eye, rods take in low light, while cones are responsible for us seeing in colour – each one has its own sensitivity to light spectrum and each forming part of our visual and non-visual responses to light. This is called the α-opic Illuminance sensitivity curve.

 a-opic illuminance sensitivity curve

The a-opic Illuminance sensitivity curve, showing the different illuminance sensitivities of the cones and rods of the human eye.

This curve represents a solid model for building the light spectrum appropriate for an artificial luminaire. It is the closest our industry has to quantifying the exact spectrum required for melatonin suppression. However, the CIE (International Commission on Illumination) recently released a statement regarding clarity on this thinking:

“…non-visual responses are subject to complex signal processing in the central nervous system and influenced by as-yet-unresolved interactions of photoreceptive units. The missing understanding of the input-output characteristics between light stimulus and the resulting non-visual response seems to make tailored light application for a desired lighting effect impossible.”

However, it’s important to note that other scientists and bodies are more supportive of the system:

The new WELL building standard offers melanopic illuminance values measured vertically at face height as a key to creating healthy lighting.

The German standard DIN Spec 6760 also offers metrics for ‘biologically effective’ lighting.

Ethical considerations.

These advancements, along with the inconclusive papers, require us to think carefully before designing any kind of circadian lighting system. Should we even be using lighting to try and enhance behaviours of our physiology when we still know so little about it? Probably one of the most striking quotes from Lucas et al 2014 paper should act as a guiding compass for us all:

In many ways, light can be considered a drug, having the potential for both beneficial and deleterious effects. These conflicting effects can occur concurrently, and in a single individual and context.

In the next part of this series, I ponder the human-centric future of lighting design.