Light-at-night validation: Correlation of satellite imagery with daysimeter photometry in the Athens, Georgia area

Background: Studies have shown an association between higher levels of artificial light at night (LAN) and increased risk of breast cancer (BC), mediated through circadian rhythm disruption. Satellite imagery may inaccurately classify LAN by measuring wavelengths of light not detrimental to circadian function. This study aims to describe the relationship between circadian-calibrated photometry at ground level and satellite LAN in the local area. Methods: Fifteen sampling points of 1-km radius were randomly selected from GIS, five each from low (4-23 Raster value), medium (26-44), and high (48-62) relative LAN according to DMSP-OSL satellite images. Readings of outdoor circadian light (CL) were obtained by driving through each sampling area for 5 minutes, recording every 30 seconds using calibrated photometry. CL medians were determined per site and by aggregate. Pearson correlation for log means versus satellite LAN values was calculated. Circadian Stimulus Activity (CSA) was recorded for all sites. Results and Conclusion: CL in low-brightness areas clustered at lower limits of detection (median 0.02 CL, range 0-0.05). Medium sites demonstrated a median of 0.16 CL (range 0.01 to 6.76). Median for high LAN sites was 0.44 CL (range 0.03-3.87). Pearson correlation coefficient for log means was 0.77 (p=0.001). CSA remained low (0.12 or less) regardless of LAN value. Results show generally low circadian light levels and stimulus activity outdoors. However, CL positively correlates with satellite imagery, provides an estimate of misclassification and will facilitate studies to further quantify the risk that artificial LAN may pose.