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Abstract

1. Motivation, specific objective: Light is a crucial driver of human physiology and behaviour, affecting the biological clock, hormone production and sleep. Consequently, light can be used as an intervention and a clinical target. These non-visual effects of light are due to a set of cells in the eye's retina called the intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin. The ipRGCs are quite unlike the cones and rods, which underlie vision and visual perception. They have a spectral sensitivity with a maximum of around 490 nm. The spectral sensitivities of the ipRGCs were standardized in 2018 by the International Commission on Illumination (CIE) in the International Standard CIE S 026/E:2018. Most of what we know about these non-visual effects of light comes from very well-controlled laboratory studies, in which the light conditions are quite unlike those in the real world, which vary in intensity (several orders of magnitude), time (time-varying conditions in illumination), space (different scenes depending on the presence of light sources) and spectrum (mixtures of electric, display and daylight). To characterize light exposure of human participants living under daily conditions and build personalized predictions and interventions, it is necessary to measure light exposure. The measurement plane to determine the physiologically relevant light exposure is the corneal plane, requiring measurements near the eye. Here, we developed a novel light logger for capturing light exposure in the corneal plane. 2. Methods: We developed a novel light logger, capturing light using 18 channels in the visible and near IR range (410-940 nm) and two channels in the UV (330 nm and 365 nm) range. The sensors are commercially available. In addition to the light sensors, we have also included a 6-axis accelerometer (linear acceleration and angular velocity) to measure movement to extract information when the light logger is worn. The light logger is connected to a microcontroller for recording and storing the data. The microcontroller is connected to a computer for recording setup and data retrieval. Integration time is controlled using an adaptive mechanism for accounting for large differences in environmental illumination spanning multiple orders of magnitude. Software for analysis of the light exposure data is in preparation. 3. Results: Our preliminary calibration data show the viability of the multi-sensor approach underlying the light logger. In early tests, we tested the feasibility of long-term light exposure measurements. We are working on further miniaturization, making the light logger more robust and weather- proofing it for long-term use. 4. Conclusions: Our early tests show that our novel light logger can be used in field studies. We are expanding our test suite to develop novel ways of using the acquired light exposure data in behavioural interventions to modify light exposure to optimally support sleep, circadian rhythms and alertness. A key future functionality is the ability to transfer the light exposure data via Bluetooth to a smartphone for integration in a mobile application.