docs: def opt_to_ri(f, res, nm):
      r"""Convert the OPT object function to refractive index
  
      In :abbr:`OPT (Optical Projection Tomography)`, the object function
      is computed from the raw phase data. This method converts phase data
      to refractive index data.
  
      .. math::
  
          n(\mathbf{r})  = n_\mathrm{m} +
              \frac{f(\mathbf{r}) \cdot \lambda}{2 \pi}
  
      Parameters
      ----------
      f: n-dimensional ndarray
          The reconstructed object function :math:`f(\mathbf{r})`.
      res: float
          The size of the vacuum wave length :math:`\lambda` in pixels.
      nm: float
          The refractive index of the medium :math:`n_\mathrm{m}` that
          surrounds the object in :math:`f(\mathbf{r})`.
  
      Returns
      -------
      ri: n-dimensional ndarray
          The complex refractive index :math:`n(\mathbf{r})`.
  
      Notes
      -----
      This function is not meant to be used with diffraction tomography
      data. For ODT, use :py:func:`odt_to_ri` instead.
      """
      ri = nm + f / (2 * np.pi) * res
      return ri