 Once this working point is reached, it is simpler to keep it as error signals read in real time provide a measurement of the deviation between the actual state of the interferometer and its optimal condition.From the measured differences, mechanical corrections are applied on the various mirrors to bring the system closer to its best working point.The optimal working point of an interferometric detector of gravitational waves is slightly detuned from the "dark fringe", a configuration in which the two laser beams recombined on the beam splitter interfere in a destructive way