A method for measuring a spectrum of an optical sensor, advantageously in the infrared region, in which a light beam impinges on an optical sensor in contact with a medium to be measured, wherein the optical sensor transmits a measurement beam changed by the medium to be measured and the measurement beam is fed to a pyrodetector, which issues output signals corresponding to the spectrum. The intensity of the measurement signal is modulated before impinging on a pyrodetector. In order to provide a cost effective, vibration free measuring apparatus, which has a long lifetime, intensity modulation of measurement beam occurs by tuning-in wavelengths contained in the optical spectrum of measuring beam.