code: def _channel_frequencies(info):
      """Return the light frequency for each channel."""
      # Only valid for fNIRS data before conversion to haemoglobin
      picks = _picks_to_idx(
          info,
          ["fnirs_cw_amplitude", "fnirs_od"],
          exclude=[],
          allow_empty=True,
      )
      freqs = list()
      for pick in picks:
          freqs.append(
              round(
                  float(
                      _S_D_F_RE.match(info["ch_names"][pick]).groups()[
                          2
                      ]
                  )
              )
          )
      return np.array(freqs, int)
  
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 3 for calling a global function with example usage: _channel_frequencies(info) and returns: np