Detecting Indoor and Outdoor Environments Using the ActiGraph GT3X+ Light Sensor in Children

Introduction:
Experts recommend children spend more time playing outdoors. The ambient light sensor of the ActiGraph GT3X+ provides lux measurements. A lux is the International System’s unit of illumination, equivalent to one lumen per square meter. Few studies have established a lux threshold for determining whether a child is indoors or outdoors.

Purpose:
The purpose of the study was: (1) to assess the reliability of the ActiGraph GT3X+ ambient light sensor, (2) to identify a lux threshold to accurately discriminate between indoor and outdoor activities in children, and (3) to test the accuracy of the lux threshold in a free-living environment.

Methods:
In part one, a series of reliability tests were performed using 20 ActiGraph GT3X+ monitors under different environmental conditions. Cronbach’s alpha was used to determine inter-instrument reliability. In part two, 18 children performed 11 different activities (five indoors and six outdoors) for six minutes each. The optimal threshold for detecting indoor/outdoor activity was determined using ROC curve analysis. In part three, 18 children at a preschool wore the monitor during a school day. Percent accuracy was determined for all conditions.

Results:
Part one: The devices had Cronbach’s alpha values of 0.992 and 1.000 for indoor and outdoor conditions, respectively, indicating high inter-instrument reliability. Part two: The optimal lux threshold was determined to be 240 lux [sensitivity=0.92, specificity=0.88, AUC=0.96 (95% CI, 0.951–0.970)]. Part three: Results of the school-day validation demonstrated the monitor was 97.0% accurate for overall detection of indoor and outdoor conditions (outdoor=88.9%, indoor=99.1%).

Conclusions:
The results demonstrate that an ActiGraph GT3X+ lux threshold of 240 can accurately assess indoor and outdoor conditions of preschool children in a free-living environment.