1. A device for obtaining a signal providing an indication of differentiation between foreground areas and background areas in images of a scene, comprising: an optical device designed to receive, at input, light rays coming from the illuminated scene and to deliver, at output, first optical rays in the visible frequencies range and second optical rays in the frequency band .DELTA.f centered on a first frequency fc, a trichromatic separator designed to separate the three spectral components, red, green and blue, of the first optical rays to constitute three primary rays, four sensors designed to receive respectively the three primary rays corresponding to the three spectral components and the optical signal of the frequency band .DELTA.f and to deliver, at output, four corresponding electrical signals, a device to compute said differentiation signal designed to receive, at input, the electrical signal corresponding to at least one of the primary rays and the electrical signal corresponding to the optical ray of the frequency band .DELTA.f, a device for the shaping and phasing of said differentiation signal.