def detector_distance(event):
    """
    Gets the detector distance for a data event retrieved from the filesystem.

    Files written by detectors don't usually contain detector distance information.
    This function takes it from the configuration file, specifically from the
    'fallback_detector_distance_in_mm' entry in the 'DataRetrievalLayer' parameter
    group.

    Arguments:

        event (:class:`~onda.utils.data_event.DataEvent`): an object storing the event
            data.

    Returns:

        float: the detector distance in mm.
    """
    return event.framework_info['detector_distance']
