Data items in the ARRAY_INTENSITIES category record the
              information required to recover the intensity data from
              the set of data values stored in the ARRAY_DATA category.

              The detector may have a complex relationship
              between the raw intensity values and the number of
              incident photons.  In most cases, the number stored
              in the final array will have a simple linear relationship
              to the actual number of incident photons, given by
              _array_intensities.gain.  If raw, uncorrected values
              are presented (e.g. for calibration experiments), the
              value of _array_intensities.linearity will be 'raw'
              and _array_intensities.gain will not be used.