A calibration method for adjusting hemispherical photographs to appropriate black-and-white images

In order to quantify the light conditions of a deciduous broad-leaved forest, we proposed a method to determine the threshold level of the brightness of a hemispherical photograph when converting it into a one-bit screen image. The coefficient of determination was examined for the regression between the measured ISF (indirect site factor) using the quantum sensors and the estimated ISF from the photographs, by changing the threshold level for the photographs. In the range of threshold levels between 60 and 180, the coefficients of determination (r ²) were all significant, andr ² had a single maximum value at the threshold level of 115. This method was found to be effective for determining a single threshold level without subjectivity at any season.