[en] An optical device, like a microscope phase-contrast, is described in this paper. It is theoretically explained that the properties regarding a perceptible transformation of a transparent image. The approximate analytic formula of these properties are compared with the experimental data in which a transparent image to a perceptible image using a correct spatial filter at montage de double diffraction. This optical device is able to compute the neutron fluences concerning etch pits which was originated by ²⁵²Cf neutron beams' exposure, because the output illuminance of the etch pits due to this optical device were directly proportional to the concerned neutron fluence. (author)