An optronic observation device including a detector having a photocathode and a sensor arranged to receive an incident light beam. The device also including a switch to position an optical element opposite the detector in the pathway of the incident beam or to retract the optical element and a focusing element to focus an incident ray on the photocathode when the optical element is in a retracted position. When the optical element is positioned opposite the detector, it is able to focus the beam on the sensor and to filter the beam spectrally to block all or part of the wavelengths for which the responsiveness of the photocathode is greater than a given threshold.