Research
May/June 2021
A MEASURE OF THE UNSEEN The 2020 Jean Heap Research Bursary recipient, Manuel Spitschan, outlines his research subject, luox, a novel open-access platform for quantifying the non-visual effects of light
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he quality and quantity of light in our environment profoundly affect how we feel, how and when we sleep, and how we appreciate a given space. Over the past few years biomedical research has found that different pathways connect the eye to the brain and influence different functions. We have, of course, known about cones and rods for a long time. While the cones allow us to see colour, motion and spatial detail under daytime lighting conditions (photopic vision), the rods give us rudimentary vision under lighting typically dimmer than natural twilight (scotopic vision). As has now become more widely known, around 20 years ago a new type of neuron was identified: the intrinsically photosensitive retinal ganglion cells, or ipRGCs, which are sensitive to light independent of the cones and rods. They respond to light due to the photopigment melanopsin. Melanopsin is most sensitive to short-wavelength, or blue light, with a peak sensitivity near 480nm. The ipRGCs are involved in a range of effects important for our health and wellbeing. Most prominently, they signal the intensity of environmental illumination and thereby track the light-dark signal. Signals from the ipRGCs are sent to the suprachiasmatic nucleus (SCN), an area buried deep in the brain about the size of a grain of rice. The SCN is our circadian pacemaker, telling our brain and body whether it is day or night – biologically speaking. Through this brain pathway, rhythms in our physiology and behaviour
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become synchronised to the light-dark cycle in our environment. For example, the body starts producing the 'sleep hormone' melatonin a few hours before our usual bedtime. Exposure to light in the evening and at night can disturb melatonin production and shift our circadian rhythms. Exposing yourself to light at the wrong time can lead to circadian disruption: the misalignment of our biological clock with solar time, which has negative consequences for mental and physical health in the long run.
Biomedical research on these non-visual effects of light is very much ongoing. We now hold several key pieces of the puzzle, including the primary role of melanopsin in driving these effects, and a developing understanding of how much light is necessary to support optimal health and performance.1 Over the past decade or so, there has also been considerable interest in optimising lighting solutions for the built environment. 'Human-centric lighting' is one of the terms that is often used, and the HCL market is large and growing.
UNDERSTANDING THE NONVISUAL EFFECTS OF LIGHT Recent meta-analyses have shown that melanopsin best explains our current evidence base for the non-visual effects of light,2 simplifying the daunting and challenging task of quantifying the impact a given spectrum of light has. However, the spectral sensitivity of melanopsin and the photopic luminosity curve (Vλ), which is used to calculate (il)luminance, are different. Vλ reflects a weighted combination of the L and M cones in the retina. Consequently, the non-visual effects of light cannot generally be predicted by (il)luminance, requiring metrics reflecting the melanopsin-weighted (or melanopic) signal.
p Fig 1: Workflow for using luox platform
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