Effect of temperature domain perturbations on the Ca II line profile. Top: Planck functions as a function of frequency. Different colours correspond to different perturbations in the temperature domain (see Eq. B.1). The unperturbed model is denoted by a black line. The inlet shows the close proximity of the Ca II triplet, marked by a dashed rectangle. Bottom: background-normalised λ = 8542 Å Ca II line profiles calculated using the perturbed B_ν functions.