Measuring circadian light through High Dynamic Range (HDR) photography

Thesis (Master's)--University of Washington, 2017Human ocular system functions in a dual manner. While the most well know function is to facilitate vision, a growing body of research demonstrates its role in resetting the internal body clock to synchronize with the 24 hour daily cycle. The internal body clock in human beings is close to, but not equal to, 24 hour rhythms, and it requires environmental cues, such as timed light and dark cycles, to synchronize with the local time. Before the introduction of electrical lighting, human’s patterns of light and dark exposures followed the natural diurnal cycles. In the modern days, this pattern changes drastically as 90% of adult human life is spent indoors and electric lighting prominently disturbs the nocturnal cycles. With most research on circadian rhythm performed in controlled laboratory environments, little is known about the variability of circadian light within built and natural environment. Currently, very few specialized devices measure the circadian light which are not accessible to many researchers and practitioners. Therefore, there is a need for accessible measurement devices. In this thesis, calibration and validation procedures of High Dynamic Range (HDR) photography to measure circadian light is developed and tested. Accuracy of HDR photographs to measure photopic luminance have been previously validated. However, color accuracy of camera sensors hasn’t been studied, and precise color information is required to accurately capture circadian light. In this thesis, camera color accuracy was evaluated through CIE trichromatic (XYZ) measurements; results demonstrated strong linear relationship between the camera recordings and scientific grade colorimeter. By applying simple correction, it is possible to correct color alignment, and therefore, to use HDR photographs to capture both photopic (lux and cd/m2) and circadian lighting values (Equivalent Melanopic Lux, EML or Equivalent Melanopic cd/m2). The developed technique and workflow has been used to capture outdoor and indoor scenes. Various examples illustrate the impact of architectural context, weather, view direction and spectra of light on circadian light exposure. Given data reduction in CIE XYZ measurements, full spectrum measurements were further collected to test validation of the methodology. Field and laboratory studies showed circadian light measurements from HDR Photographs corresponded to physical quantity of circadian luminance with reasonable precision and repeatability