A study is made of the range of applicability of the weak-field approximation for the inference of magnetic field vectors from the profiles of the Stokes parameters. It is shown that, for typical solar lines in the visible, this approximation may be applied with an accuracy of 20 percent for field strengths as great as 3500/gL G (where gL is the Lande factor) in the line wings. A systematic underestimate of the field strength seems to occur for strong fields - observations in several related spectral lines may allow a calibration of this effect, and so for an increase in accuracy of the inferences. The procedures have been tested using solar data obtained in two lines of Fe I, near 6300 A, with the Stokes Polarimeter at the Mees Solar Observatory of the University of Hawaii.
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Jefferies et al. (1991) studied this question.