The invention relates to a method and an apparatus for measuring spectral responsivity of IR optical detectors irradiated with ionising radiation. The claimed method consists in measuring simultaneously, on two channels, the spectral responsivity for both a standard detector and a detector whose spectral responsivity is to be determined, such that all the possible perturbations influence, identically and simultaneously, the two measuring channels, in view of the common rejection of these perturbing signals. The claimed apparatus consists of a monochromator (1) provided with an aperture (2) wherethrough an optical radiation (3) emitted within the IR range by some IR optical sources (4 and 5) enters and it is transformed by a dispersive element (7) and a chopper (8) into a pulsed monochromatic IR radiation (9), which enters an integrating sphere (11) provided with two exit ports (12 and 13) to which there are coupled: a standard IR detector (14) and an IR detector (16) whose spectral responsivity is to be measured, the operation temperature of the detector (16) being monitored by means of a temperature measuring element (17) which is in thermal contact with the detector (16), between the exit port (12) and the standard IR detector (14) there being interposed an optical window (15) made of the same material as another optical window, of the IR detector (16), the attenuation of IR optical radiation (9) in the atmosphere being measured by means of a system (22) for measuring the atmospheric attenuation of IR radiation, the data coming from the temperature measuring element (17), the IR optical window (15) and the attenuation measuring system (22) being provided to a computing system (21) which corrects the measured spectral responsivity of the IR detector (16) depending on the said perturbations.