Estimation of Relative Iilluminance using Digital Hemispherical Photography

概要

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The sky factor, which is an estimate of relative illuminance used in the field of architecture, measured from digital hemispherical photography was compared with relative illuminance measured directly by illuminance meters. At fifty sample points selected randomly in ten stands, digital hemispherical photography was taken using a digital camera (Coolpix 900, Nikon) with an exclusive fish-eye lens (Fish-eye converter FC-E8, Nikon), and relative illuminance was simultaneously measured using illuminance maters (T-10, Minolta). The sky factor was significantly higher than relative illuminance (p<0.001), with the average and maximum differences being 17.191% and 24.995%, respectively. This suggested that the sky factor measured from digital hemispherical photography could not be directly used as an estimate of relative illuminance. However, there was a high correlation between sky factor (SF) and relative illuminance (RI) (r=0.971, p<0.001), and the regression line of SF against RI was SF=1.091RI+15.354 (r^2=0.953, p<0.001). The regression line between sky factor and relative illuminance could therefore be used in correcting the overestimation. In conclusion, although digital hemispherical photography would be a convenient means for estimating light environment, we should pay attention to the characteristics presented in this study to avoid the overestimation.
森林計画学会の論文