The ISAAC SW infrared detector bias is a function of the detector integration time (DIT) and the detector illumination. It also varies in time, most notably at the rows where the reading of the detector starts, that is rows 1, 2, 3, ... and rows 513, 514, 515, ...
It is therefore common to observe bias variations from one image to the next. This is particularly the case for the first image in a sequence (template) of images. These bias variations are non-uniform across the array, but are uniform along most rows. These variations are usually not a serious problem, and do not prevent one from using all the images, but they may require some special treatment, e.g. fitting all lines with rejection of positive signal so as to derive the vertical pattern of the bias (or of its variation between images). The effect is strongest in DCR mode and is more easily seen in low background conditions.