The present invention relates to an infrared imager suitable for detecting an infrared flux in a first and a second spectral band, the imager comprising an enclosure and comprising: a separator separating an incident infrared flux into two fluxes, a single detector suitable for detecting a flux in the two spectral bands, the detector being cooled to a temperature lower than the temperature of the enclosure, a switching optic movable between a first position in which the first flux is sent to the detector and a second position in which the second flux is sent to the detector, and a reflector having a bottom positioned in order that the image of the bottom in the first spectral band by the switching optic in the second position is situated on detector.