code: def _get_lengthunit_scaling(length_unit):
      """MNE expects distance in m, return required scaling."""
      scalings = {"m": 1, "cm": 100, "mm": 1000}
      if length_unit in scalings:
          return scalings[length_unit]
      else:
          raise RuntimeError(
              f"The length unit {length_unit} is not supported "
              "by MNE. Please report this error as a GitHub "
              "issue to inform the developers."
          )
  
imports: import datetime
  import re
  import numpy
  from ..._fiff._digitization import _make_dig_points
  from ..._fiff.constants import FIFF
  from ..._fiff.meas_info import _format_dig_points, create_info
  from ..._fiff.utils import _mult_cal_one
  from ..._freesurfer import get_mni_fiducials
  from ...annotations import Annotations
  from ...transforms import _frame_to_str, apply_trans
  from ...utils import _check_fname, _import_h5py, fill_doc, logger, verbose, warn
  from ..base import BaseRaw
  from ..nirx.nirx import _convert_fnirs_to_head
label_desc: Use image node_id 5 for calling a global function with example usage: _get_lengthunit_scaling(length_unit) and returns: scalings