Abstract: A method and an apparatus are provided for calibrating a spectrometer, in particular a Fourier transform spectrometer (FTS), in particular for the infrared spectral range. At least four black body radiators of different temperatures are provided, which are arranged horizontally, i.e. parallel to the surface of the earth, thereby avoiding temperature gradients due to convection. The temperature of the at least four radiators is determined by means of the calibrating method without a separate measurement. In the method according to the invention, at the same time the absorption of the atmosphere is considered as a characteristic of the spectrometer, thereby eliminating separate measurement and correction thereof. In addition, for Fourier transform spectrometers the calibration is carried out by means of complex spectra.