An acceptor device for sheet objects such as banknotes, comprises a sensor to derive data corresponding to a spatial array of data samples from a face of a sheet object, said data being configured in a sampling frame that lies within a range of positional relationships to a reference frame. The acceptor device also comprises a processor operable to process the data to determine the relationship between the reference frame and the sampling frame for the sensed data, and being operable to transform pre-selected regions of the sensed data from the sampling frame so as to correspond to data in the reference frame, and to make a comparison of the transformed data with reference data corresponding to the pre-selected regions in the reference frame and to the sheet object depending on the outcome of the comparison.