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.