Panel a: as a function of B, specific contrast, C_max/B (black, left-hand axis) and the position of C_max, μ_max (grey, right-hand axis) at the continuum of the Fe I 6173 Å line. The plot points correspond to the values derived from the CLV profiles (Fig. 2a) and the curves to the estimate from the empirical intensity contrast models (Eq. (2) and Table 1). Panel b: specific contrast at the Fe I 6173 Å line core (red) and 1700 Å (blue). To facilitate the comparison, we scaled the Fe I 6173 Å line core values by a factor of 11.9 to match the 1700 Å values at the first plot point. At these two passbands, μ_max = 1 at all levels of B, hence the omission from the figure.