The physical structure of water trees grown in power cables and in laboratory specimens made of crosslinked polyethylene are studied and compared. Detailed observations are made by fluorescence microscopy. This technique, used for the first time in this type of research provides images with good contrast and high resolution. It is completed by 3-D views acquired by confocal laser scanning microscopy. In both cases the tree structure is a network of continuous submicroscopic branched and zigzag tracks. The previous published observations are critically examined. Moreover, the self-luminescence of water trees and its neighborhood is studied, and is shown to be localized in the tree tracks.
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