Starlight polarization percentage for a column density N_H = 10²¹ cm⁻², as a function of the wavelength, for increasing values of the alignment parameter a_alig. Silicate grains are assumed to be aligned, while graphite grains are not. The dashed curve is the mean polarization curve observed in the diffuse ISM (A_V ~ 1), which constrains both the upper limit of the silicate size distribution and the minimal size of aligned grains a_alig. Using our model, it is best fitted in the optical and near-infrared (NIR) for a_alig = 100 nm (see Sect. 3.3 for a discussion of the poor fit in the UV).