. The rings are displayed
as yellow lines 
on the
picture.The transmission of the light through a canopy is a function of Θ (theta),
the angle relatively to the zenith. A hemispherical photograph is therefore
analysed by classifying its light or dark pixels into classes of Θ,
that's to say into rings of a defined width. In Hemisfer, the user can
choose both the width (in degrees) of the rings and their number, with a total not
exceeding the maximum zenith angle of the lens. This is done
either by entering numbers directly or by clicking on up and down arrows
. The rings are displayed
as yellow lines on the
picture.
The number of rings should not be too small for a precise estimation of the leaf area index and leaf angle. On the other hand, narrow rings are more likely to be entirely black, especially close to the horizon. In this case (transmission T = 0), the LAI cannot be calculated and the software will use a transmission corresponding to half a pixel. To avoid errors from this approximation, the rings should thus not be too narrow. Five to ten rings usually give good results.
The total angle analysed should be chosen as to cover most of the relevant part of the picture. Close to the horizon, the picture may show parts of the canopy which are already outside of the area of interest in the field. They may also show mountains. Too large zenith angles should thus be excluded from the analysis by setting the product of ring number and ring width to less than 90° (or the viewing angle of the lens).
A segmentation in azimuth sectors is necessary for the implemented
analyses according to Lang &
Xiang (1986) and to Walter & Torquebiau (2000).
As for the rings, this is done either by entering numbers directly or by clicking
on up and down arrows .
The sectors are also displayed as yellow lines
on the picture.
The number and width of the rings is saved when exiting Hemisfer. These settings are used again when the software is launched the next time.