code: def _wl_dist(wl_a, wl_b):
      
      if isinstance(wl_a, tuple):
          # central wavelength
          wl_a = wl_a[1]
      if isinstance(wl_b, tuple):
          wl_b = wl_b[1]
      if wl_a is None or wl_b is None:
          return 1000.
      return abs(wl_a - wl_b)
docs: Return the distance between two requested wavelengths.