A first light source has a first wavelength corresponding to an excitation wavelength of a fluorophore. The excitation wavelength and an emission wavelength of the fluorophore delineate a predetermined interval. A second light source has a second wavelength offset with respect to the first wavelength so as to be outside said predetermined interval. The offset between the first and second wavelengths is comprised between 30 nm and 100 nm. A camera comprises a filter opaque to the first and second wavelengths and transparent to the emission wavelength and to wavelengths substantially higher than the higher of the first and second wavelengths. The light sources and camera are synchronized to alternately activate one of the light sources and make the camera alternately acquire a fluorescence image and a background noise image.