A protective coating around the core fibre of an optical-fibre cable.
The primary coating is made from acrylate and is applied directly to the glass core.
The thickness of the primary coating depends on whether the cable is designed for indoor or outdoor use and on the operating temperature range.
The secondary coating is applied on top of the primary coating. When optical fibre cables are made up, both these coatings are stripped and only the optical fibres themselves are inserted into the connectors.