Provides procedures for calibrating an optical spectrum analyzer designed to measure the power distribution of an optical spectrum; this analyzer is equiped with an input port for use with a fibre-optic connector. This document applies to optical spectrum analyzers developed for use in fibre-optic communications and is limited to equipment that can directly measure the optical spectrum output from an optical fibre, where the optical fibre is connected to an input port installed in the optical spectrum analyzer through a fibre-optic connector.