Reliability and validity of the Actiwatch and Clouclip for measuring illumination in real-world conditions.

These data illustrate that different Clouclip devices produce comparable measures of viewing distance and illumination in real-world settings. Both Actiwatch-2 and Clouclip underestimate illumination in the field compared to gold standard photometer measures. The disparity increases at higher levels of illumination and the discrepancy was greater for Actiwatch-2 measures. For researchers interested in categorising light exposure, Clouclip classifies illumination levels >2 lux more accurately than Actiwatch-2 but cannot discriminate between scotopic and low mesopic light.