Variation of the L band instrument + atmosphere visibility, averaged between 3.1 and 3.8 μm, as a function of seeing in arcsecond (bottom) and of the atmospheric coherence time measured in millisecond in the visible (top). The colour code indicates the seeing ranges and the coherence time. The observations were carried out with a DIT of 111 ms on calibrators with L band fluxes between 1 and 100 Jy.