Derivative analysis for in situ high dynamic range hemispherical photography and its application in forest stands

A new methodology is presented to threshold high dynamic range hemispherical color photographs of forest canopies. It is based on the second derivative of brightness profiles along circular transects over the whole azimuthal range of angles at narrow annulus zenith rings. A fast, reliable, and objective way of extracting canopy forest parameters is described and illustrated for calculation of the monodirectional gap fraction around the zenith angle of 57.3/spl deg/, which is independent on average leaf angle distribution.