On the validity of the 630 nm Fe I nm lines for the magnetometry of the   internetwork quiet Sun
  The purpose of this work is to analyze the reliability of the magnetic field strengths inferred from the 630 nm pair of Fe I lines at internetwork quiet Sun regions. Some numerical experiments have been performed that demonstrate the inability of these lines to recover the magnetic field strength in such low flux solar regions. It is shown how different model atmospheres, with magnetic field strengths ranging from few hundred Gauss to kiloGauss, give rise to Stokes profiles that can not be distinguished. The reasons for this degeneracy are discussed. 
M. J. Martinez Gonzalez, M. Collados, B. Ruiz Cobo