Scanning imagers often comprise a plurality of detector elements which may be scanned in swathes to produce an image. To match the offsets or sensitivities of such elements the mean values of the channels adjacent each swathe boundary are compared, and an error signal produced which is fed back to equalize the values across each boundary. Proportionally smaller corrections are applied to the intermediate channels in each swathe. In this way the swathing or banding effect often associated with such imagers is much reduced.