The value of _diffrn_detector_element.reference_center_fast is the fast index axis beam centre position relative to the detector element face in the units specified in the data item _diffrn_detector_element.reference_center_units along the fast axis of the detector from the centre of the first pixel to the point at which the Z-axis (which should be collinear with the beam) intersects the face of the detector, if in fact it does. At the time of the measurement all settings of the detector positioner should be at their reference settings. If more than one reference setting has been used the value given should be representative of the beam centre as determined from the ensemble of settings. It is important to note that for measurements in mm, the sense of the axis is used, rather than the sign of the pixel-to-pixel increments.