Performance of wearable light sensors for measuring photopic and melanopic illuminance under laboratory and free-living conditions.

Wearable sensors are commonly used to study the effects of free-living light exposure on physiological outcomes; however, rigorous validation of their performance has been limited. To address this gap, we quantified the accuracy and precision of light measurements from four commonly used wearable sensors (LYS, ActLumus, GENEActiv, and ActiGraph wGT3X-BT). Eight devices from each manufacturer (32 total) were compared to a criterion spectrometer under light-emitting diode-generated cool (peak ~460 nm) and warm (peak ~600 nm) white light at indoor intensities (2-1000 lux) and broad-spectrum sunlight (5 k-100 k lux). We found linear responses for all sensors between 50 and 20 k photopic lux, irrespective of spectrum, but detection ranges varied: ActLumus (2-100 k lux), LYS (2-60 k lux), GENEActiv (2-40 k lux), and ActiGraph (50-20 k lux). Accuracy-percent difference to criterion-also differed: ActLumus (+3.3 ± 13.4%, mean ± SD), LYS (+19.5 ± 42.0%), and below -20% for GENEActiv and ActiGraph. Interdevice variability was consistently lowest for ActLumus, and melanopic illuminance was less accurate with LYS than ActLumus, particularly in warm indoor light. Laboratory findings were compared to 1-week free-living light measurements in 21 individuals concurrently wearing the most and least accurate sensors in randomized positions on the nondominant wrist. The mean 24 h lux per participant was correlated (r = 0.91, p < .001) but lower for ActiGraph (77.3 ± 68.5 lux) than ActLumus (515.0 ± 436.0 lux; p < .001) with the greatest differences ≤100 lux (p < .001), consistent with the laboratory results. Thus, differences in illuminance range and accuracy can lead to large disparities in free-living measures across manufacturers, suggesting a need for greater technical standardization. Clinical Trial: A Natural History Study of Metabolic Sizing in Health and Disease. https://clinicaltrials.gov/study/NCT05398783 (NCT05398783). Statement of Significance Emerging evidence suggests light exposure has a greater impact on human health than previously appreciated. Wearable sensors embedded with light-measuring capabilities can play a crucial role in studying the relationship between light and physiological health. Thus, the performance of light sensors should be thoroughly evaluated. We tested the detection limits, accuracy, and variability of four commonly used wearable light sensors to indoor and outdoor intensities and spectra reflective of the real-world light environment and assessed the translatability of laboratory-based observations in a free-living pilot study. Our findings demonstrate broad variation in accuracy within and across manufacturers that emphasizes the need for rigorous sensor evaluation and highlights the challenges of interpreting illuminance measures across studies deploying different light sensors.