An infrared sensor comprising an optical system, a pixel array arranged in the focal plane of the optical system, the optical system allowing radiation to be focused on the pixel array, said system comprising switching means of the optical system to a nominal acquisition mode in which radiation from an observed scene is imaged or to a calibration mode in which the optical system is configured to image radiation from a black body or partially reflected radiation from the pixel array, the optical system comprising a device with four mirrors and a reflective device of incident radiation, the device being such that: in nominal mode, the first mirror has a reflective surface parallel to the reflective surface of a second mirror which is perpendicular to the reflective surface of a third mirror which is parallel to the reflective surface of a fourth mirror, in calibration mode, in a first position of the optical system, the first mirror is arranged to reflect toward the second mirror the radiation from a black body derived from a black-painted screen disposed between the first mirror and the fourth mirror, or in a second position of the optical system to reflect the radiation from the reflective device arranged between the fourth mirror and the first mirror, the reflective device partially imaging radiation from the pixel array, said sensor comprising a processing unit configured in calibration mode to implement the following steps: - acquisition by the pixel array of a first sequence of images of the radiation from a black body corresponding to a scene at a first temperature; - acquisition by the pixel array of a second sequence of images of the partially reflected radiation from the pixel array corresponding to a scene at a second temperature; - processing (E5, E6, E7, E8, E9) the first and second sequences of images to update the gain table by calculating a correction representing the ageing and response of the pixels of the pixel array. in nominal mode, the first mirror has a reflective surface parallel to the reflective surface of a second mirror which is perpendicular to the reflective surface of a third mirror which is parallel to the reflective surface of a fourth mirror, in calibration mode, in a first position of the optical system, the first mirror is arranged to reflect toward the second mirror the radiation from a black body derived from a black-painted screen disposed between the first mirror and the fourth mirror, or in a second position of the optical system to reflect the radiation from the reflective device arranged between the fourth mirror and the first mirror, the reflective device partially imaging radiation from the pixel array, said sensor comprising a processing unit configured in calibration mode to implement the following steps: - acquisition by the pixel array of a first sequence of images of the radiation from a black body corresponding to a scene at a first temperature; - acquisition by the pixel array of a second sequence of images of the partially reflected radiation from the pixel array corresponding to a scene at a second temperature; - processing (E5, E6, E7, E8, E9) the first and second sequences of images to update the gain table by calculating a correction representing the ageing and response of the pixels of the pixel array. - acquisition by the pixel array of a first sequence of images of the radiation from a black body corresponding to a scene at a first temperature; - acquisition by the pixel array of a second sequence of images of the partially reflected radiation from the pixel array corresponding to a scene at a second temperature; - processing (E5, E6, E7, E8, E9) the first and second sequences of images to update the gain table by calculating a correction representing the ageing and response of the pixels of the pixel array.