A Wrist-Worn Internet of Things Sensor Node for Wearable Equivalent Daylight Illuminance Monitoring

Light exposure is a vital regulator of physiology and behaviour in humans. However, monitoring of light exposure is not included in current wearable Internet-of-Things (IoT) devices, and only recently have international standards defined α-opic equivalent daylight illuminance measures for how the eye responds to light. This paper reports a wearable light sensor node that can be incorporated into the IoT to provide monitoring of equivalent daylight illuminance exposure in real-world settings. We present the system design, electronic performance testing, and accuracy of equivalent daylight illuminance measurements when compared to a calibrated spectral source. This includes consideration of the directional response of the sensor, and a comparison of performance when placed on different parts of the body, and a demonstration of practical use over 7 days. Our device operates for 3.5 days between charges, with a sampling period of 30 s. It has 10 channels of measurement, over the range 415–910 nm, balancing accuracy and cost considerations. Measured α-opic equivalent daylight illuminance results for 13 devices show a mean absolute error of less than 0.07 log lx, and a minimum between device correlation of 0.99. These findings demonstrate that accurate light sensing is feasible, including at wrist worn locations. We provide an experimental platform for use in future investigations in real-world light exposure monitoring and IoT based lighting control.