code: def _channel_chromophore(info):
      """Return the chromophore of each channel."""
      # Only valid for fNIRS data after conversion to haemoglobin
      picks = _picks_to_idx(
          info, ["hbo", "hbr"], exclude=[], allow_empty=True
      )
      chroma = []
      for ii in picks:
          chroma.append(info["ch_names"][ii].split(" ")[1])
      return chroma
  
file_path: mne-python/mne/preprocessing/nirs/nirs.py
imports: import re
  import numpy
  from ..._fiff.pick import _picks_to_idx, pick_types
  from ...utils import _check_option, _validate_type, fill_doc
label_desc: Use image node_id 4 for calling a global function with example usage: _channel_chromophore(info) and returns: chroma