Filled circles: intensity contrast CLV at the Fe I 6173 Å line core (red) and 1700 Å (blue), at a) 50 G <  B ≤ 100 G and b) 640 G <  B ≤ 800 G, taken from Fig. 2b. Curves: result of estimating the intensity contrast at one passband from the measurements at the other, assuming the solar atmosphere radiates like a black body (solid) and the corresponding 1σ bounds from the uncertainty in the effective temperature at 1700 Å (dashed). See Sect. 4.5.