Provides a parameter definition and test method for obtaining optical signal-to-noise ratio (OSNR) using apparatus that measure the optical spectrum at a multichannel interface. The measured noise does not include source relative intensity noise (RIN) or receiver noise. Three implementations for an OSA are discussed: a diffraction-grating-based OSA, a Michelson interferometer-based OSA, and a Fabry-Perot-based OSA. Characteristics of the OSA that affect OSNR measurement accuracy are provided.