Low dose data
In XDS, the background of a data frame is used for scaling adjacent frames relative to each other in the INTEGRATE step.
This seems to work well down to an average of 0.05 counts/pixel. If, however, the crystal is exposed so shortly/weakly that the average drops significantly below this value, scaling becomes impossible - the ratio of averages which are around zero is obviously undefined. The keyword DATA_RANGE_FIXED_SCALE_FACTOR has been introduced to handle this situation.
See also: Difficult datasets