those of the array of bolometers the magnification between the entry pupil 101 and the array of micromirrors provided by the convex mirror 102 is substantially 2 and the magnification between the array 104 of micromirrors and the array 105 of bolometers provided by the convex mirrors 103 is substantially u2212 2 during frame scanning the micromirrors are oriented to form an image of the entry pupil 101 on the array 105 of bolometers along the geometrical path 106 according to the invention when the mirrors are oriented to deflect the light beam away from the array 105 of bolometers they form on the array 105 an image of a black body 107 placed in such a manner in this example between the convex mirrors 102 and 103 and just in front of the array 104 of micromirrors that the image of a black body is formed on the surface of the array 105 along the geometrical path 108 the black body is of the passive type and consists for example of a material whose emissive characteristics are well known and very close to those of an ideal black body the material is chosen to have a high conductivity so as to remain at a very stable temperature it includes measuring means for example an ntc resistor so that its temperature is known very accurately and thus likewise its emission characteristics the detector can therefore be calibrated continuously and thereby achieve a very high radiometric resolution if necessary a plurality of separate black bodies can be used addressable individually by modifying the orientation of the micromirrors to have separate calibration points appropriately distributed over the response curve of the microbolometers finally in the event of saturation or underexposure of the detector the cyclic.