Effect of settings of digital fisheye photography to estimate relative illuminance within forest under low light conditions

In order to evaluate the effects of settings of digital fisheye photography on estimates of relative illuminance (RI) within a forest under low light conditions, we assessed the effectiveness of some estimators, i. e., canopy openness (CO), sky factor (SF), and diffuse transmittance (DIF). The estimates were calculated from digital hemispherical photographs taken with two different exposure settings, i. e., the reference exposure setting and the auto-exposure setting. We also presented a new estimator (%Exposure), which does not require image analysis, and compared it with other estimators calculated from a digital hemispherical photograph using image analysis. The CO, SF, and DIF calculated from digital hemispherical photographs taken with auto-exposure setting were not significantly correlated with RI. However, those calculated from digital hemispherical photographs taken with reference exposure settings were significantly correlated with RI. On the other hand, %Exposure showed a slightly higher correlation with RI compared with the other estimators.