Local extrema of a spectrum of NGC 7027 before (top) and after (bottom) fringe flat correction measured on a single spaxel in each band. The red and orange points correspond to the upper and lower envelopes of the fringes, respectively. Both panels are on the same scale. Fringes are weaker in 1A-1B and 3B-3C because the AR coating is most efficient at 6 and 16 μm. A few peaks are present and are due to spectral lines.