Laser scanning confocal light microscopy of cable materials

Laser scanning confocal microscopy was used to study the morphology of vented water trees, bow-tie trees, and the interface between semiconductive and insulation layers of power cables. Relatively large, laboratory-prepared slab samples and cable samples (greater than 200 μm in thickness) layers examined directly using the unique capability of the confocal microscope focus in narrowly defined steps through the material. A series of such images recorded at increasing depths in the specimen were then reconstructed in a computer to produce a three-dimensional view of the morphology. The resolution in the x-y plane is better than that obtained in a conventional light microscope, and the z-resolution is on the order of 0.6 to 0.7 μm. The detailed morphology of the interface between semiconductive and insulation layers is revealed over large areas in coextruded cable samples. The true three-dimensional shape of trees is also revealed. The specific advantages of this technique compared to the conventional optical microscopy in the examination of cable materials are discussed.