Relative radiance measurements and zenith angle dependent segmentation in hemispherical photography

When analysing hemispherical photographs taken from forest stands the grey value of the smallest picture unit — so-called ‘pixel’ — is taken as a measure of transmissivity. This derived information of transmission is based on the grey value of unobscured pixels which serves as a reference value. However, a particular grey value of unobscured pixels is actually not constant for all pixels of a photograph; but is influenced by (i) sky luminance heterogeneity and (ii) vignetting properties of lenses. A method to assess the two factors mentioned is proposed. This method takes into account the film-specific relation between received relative radiation energy and the films grey value. The relation is constant for a particular film and can be determined by photographing an optical density wedge on each film. The method is applied in vignetting measurements on two custom ‘fisheye’ lenses which reveal significant differences in the properties of the lenses as well as for particular aperture openings. Consecutive relative radiance measurements on photographs taken under overcast skies gave consistent sky luminance distributions with this method compared to standard distributions from literature. Moreover, the method gives reasonable relative radiance values from photographs of increasing degree of over-exposure taken under identical skies. An instruction for deriving zenith angle dependent reference values for unobscured sky within one single photograph taken on an overcast day is given which can also be applied to digital camera device.