Variation in the fringe-like pattern for extracted flat-field spectra where the median lamp spectrum has been removed. Each row represents one fibre and wavelength increases from left to right. A subset of the wavelength range, covering pixels 2100 to 3900 and thus corresponding to about 5130 Å, to 5675 Å, is shown. The example spectrum shown in Fig. 1 is taken from quadrant Q3, where only one fibre module shows a significant fringe-like pattern.