Abstract: In a double grating monochromator, the scanner comprises a programming device specifying the angular positions of the diffraction gratings, as the radiation spectrum is scanned, so that the angles of radiation incident thereon are related by ##EQU1## where .sigma..sub.1, .alpha..sub.1, and .beta..sub.1 are: constant of one of the diffraction gratings, angle of radiation incident on this grating, and angle of radiation diffracted from this grating, respectively; .sigma..sub.2, .alpha..sub.2, and .beta..sub.2 are: constant of the other diffraction grating, angle of radiation incident on this grating, and angle of radiation diffracted from this grating, respectively; and "K" is spectral order of radiation diffracted from one of the diffraction gratings and extracted by this slit positioned immediately after this grating, the value of "K" being determined by the working portion of the monochromator spectrum.