The value of _diffrn_detector_element.reference_center_units
              specifies the units in which the values of
              _diffrn_detector_element.reference_center_fast  and
              _diffrn_detector_element.reference_center_slow
              are presented.  The default is 'mm' for millimetres.  The
              alternatives are 'pixels' and 'bins'.  In all cases the
              centre distances are measured from the centre of the
              first pixel, i.e. in a 2x2 binning, the measuring origin
              is offset from the centres of the bins by one half pixel
              towards the first pixel.
              If 'bins' is specified, the data in
                  _array_intensities.pixel_fast_bin_size,
                  _array_intensities.pixel_slow_bin_size, and
                  _array_intensities.pixel_binning_method
              are used to define the binning scheme.