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.