A non-contact printer controls the sensitivity of sensors provided upstream of a print head, to detect the approach of an object. In a calibration operation to set the sensitivity level of the sensors before a printing operation, the printer displays instructions to guide the operator and adjusts the sensor sensitivity to find detection threshold levels for a background (object absent) condition and when the object is present. The calibration results may be stored in association with data identifying the conveyor and the type of object used in the calibration operation. If the same conveyor and/or object type is used again in a later printing operation, the sensitivity level of the sensors can be set using the stored calibration results so that a further calibration operation is not necessary.