Light exerts an important and profound influence on human physiology and behaviour. Within the last twenty years, how we think about light has fundamentally changed, moving away from the orthodoxy that cones and rods are the only photoreceptors in the human retina, to a more complete picture incorporating the melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs). In a rare case where basic science has affected practice rather quickly, the discovery that there is a special class of neurons in the retina directly sensitive to short-wavelength light has caused a brouhaha in the lighting industry as well, leading to the circus of “human-centric lighting”.
We are still only at the beginning of understanding how all photoreceptors in the human retina contribute to the various processes typically summarised as “non-image-forming” or “non-visual”, e.g. the suppression of melatonin by light at night, circadian phase shifting, pupil size modulation, and modulation of the cardiovascular system and alertness by light. Importantly, these are not the same functions, and neither are they unitary, and there is mounting evidence that melatonin suppression and circadian phase shifting are separable. Another open question is how the melanopsin-mediated and cone-mediated control of pupil size, which regulates retinal illuminance, interacts with the ipRGC-mediated suppression of melatonin, or phase shifting.
With more research on the horizon, it is most important to produce scientific work that is transparent and reproducible. Eventually, the scientific process relies partly on the independent replication of findings. It is through the totality of evidence, not single studies, that the scientific community accumulates knowledge and generates implications for policy and practice. Replication requires an accurate description of the methods used in a given study but what is the necessary and sufficient information for replicating a study?
Addressing the need to streamline and standardize reporting of light exposure and lighting conditions, a set of minimum reporting guidelines for describing the lighting conditions in laboratory experiments in chronobiology, sleep medicine and environmental psychology have been proposed in a tutorial paper entitled How to Report Light Exposure in Human Chronobiology and Sleep Research Experiments.
In addition to these minimum and necessary descriptions, a set of optional items for reporting, such as estimates of the participants’ pupil size for calculations of retinal intensity is also proposed. The next page provides the full set of these descriptions. We incorporate the recently published standard CIE S 026/E:2018 (DOI: 10.25039/S026.2018) which provides a set of calculation procedures in a retinally referenced framework into the reporting guidelines.
Guidelines are guidelines, and not the law. I hope that this tutorial provides a gentle introduction into often-intricate questions for measuring, documenting and reporting light exposure. I invite authors, reviewers and editors to follow, promote and enforce our minimum reporting guidelines.
Reporting checklists
From: Spitschan M, Stefani O, Blattner P, Gronfier C, Lockley S, Lucas RJ. How to report light exposure in human chronobiology and sleep research experiments. Clocks Sleep 2019; 1(3): 280–289.
Minimum reporting checklist
Measure and report the spectral power distribution of the acute stimulus from the observer’s point of view at a known and specified angle and distance from the source
Measure and report the spectral power distribution of the background light environment from the observer’s point of view at a known and specified angle and distance from the source
Make spectra available in tabulated form
Report α-opic (ir)radiances and illuminance
Describe the timing properties of stimulus (clock time, duration and pattern)
Describe the spatial properties of stimulus (spatial arrangement and extent)
Report measurement conditions and equipment
Optional reporting checklist
Measure or estimate pupil size of the study participant
Measure or estimate percentage eye open
Photography or sketch of laboratory arrangement
Light safety calculations >10,000 cd/m2
Radiometric vs. photon system
Calculate age-dependent effects on pre-receptoral filtering
Reporting retinal intensity