USE OF RADIANCE MEASUREMENTS TO ASSESS THE POSITION OF PHYTOELEMENTS IN A VEGETATION CANOPY

An indirect method is proposed to estimate the position of phytoelements in a vegetation canopy. It is based on camera measurements of the radiance reflected by the phytoelements when the canopy is illuminated with a small light source. The method requires that measurements be performed for at least two positions of the light source. It was tested under laboratory and field conditions and provided an estimate of the altitude of phytoelements with an accuracy of between 5 and 10% of canopy height. Once the altitude is estimated, the other coordinates are calculated from the position of the phytoelement on the camera image. The algorithm has been applied to small leaf surfaces delimited on to the camera image; however, it can, in principle, be applied on a pixel-to-pixel basis so as to analyse automatically the image. Combined with light interception models, this method should provide a way to obtain non-destructive field estimates of the vertical distribution of bi-directional gap fraction and of the profile of light interception within the canopy.