IEC 62129-1:2016 specifies procedures for calibrating an optical spectrum analyzer that is developed for use in fibre-optic communications and designed to measure the power distribution of an optical spectrum. It does not apply to an optical wavelength meter that measures only centre wavelengths, a Fabry-Perot interferometer or a monochromator that has no display unit. This first edition of IEC 62129-1 cancels and replaces the first edition of IEC 62129, published in 2006. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - update of term and definitions; - update of calibration conditions; - calculation change of uncertainties related to wavelength temperature dependence, power linearity, power level temperature dependence; - move of Annex E to the bibliography. Keywords: calibrating an optical spectrum analyser