code: def _vis_calibrate(self, data):
          
          solar_irradiance = self['esun']
          esd = self["earth_sun_distance_anomaly_in_AU"].astype(float)
  
          factor = np.pi * esd * esd / solar_irradiance
  
          res = data * factor
          res.attrs = data.attrs
          res.attrs['units'] = '1'
          res.attrs['standard_name'] = 'toa_bidirectional_reflectance'
          return res
docs: Calibrate visible channels to reflectance.