The coating is the non-glass protective material applied to the fiber. It is most commonly a dual layer acrylate, with a soft mechanically isolating inner layer and a tougher outer layer.
The inner coating, also called primary coating, is the first layer of coating material applied to the glass. It is essential for the inner coating to offer good mechanical protection to the fiber whilst yet showing good bonding properties to the glass and the outer coating, so delamination in between the layers is avoided.
Related Terms: Coating, Coating Concentricity