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tokens: (some works on this topic are Uitenbroek 1989 ; Uitenbroek Bruls 1992 ; Belluzzi Trujillo Bueno 2014 ; del Pino Alemán, Casini Manso Sainz 2016 ). However, in the case of the K i lines it seems that this effect is very weak even at extremes heliocentric angles (see the results for the K i D 1 line presented in Uitenbroek Bruls 1992 ). Still, we study the PRD effects for both K i lines using the FALC model [in the previous work the VALC model (Vernazza, Avrett Loeser 1981 ) was used instead]. In this regard, we plot in Fig. 3 the results for the K i D 1 (left) and D 2 (right) lines. For the latter case, we also included the neighbouring photospheric Fe i line (see label 4 in Table 1 ) as it is relatively close. We can see that, for all the selected heliocentric angles, the differences between the computed PRD and CRD profiles are negligible (see the bottom part of each plot). This is in agreement with the results of Uitenbroek Bruls ( 1992 ) and indicates that PRD effects are very small for these lines, at least for the intensity spectrum. Figure 3. PRD (solid) and CRD (squares) profiles for the K i D 1 D 2 lines (we also included the neighbour Fe i line for the latter case). Colours indicate the selected heliocentric angle (larger μ corresponds to an LOS closer to the disc centre and to higher continuum intensity). In addition, we show the difference between PRD and CRD profiles in the bottom of each panel. 3.2 NLTE effects In previous works related to the alkali lines as in Bruls et al. ( 1992 ) it has been demonstrated that the K i D 1 line, in particular its line core, is sensitive to NLTE effects. In other words, the LTE assumption is unable to reproduce the observed line depth. In this section, following the steps of the previous one, we check if the same occurs for the K i D 2 line and also revisit the studies of the K i D 1 line with the FALC model. In addition, as the results of the previous section revealed that PRD effects are negligible for the K i lines, we also compute the differences between NLTE and LTE for the polarization spectrum. We show in Fig. 4 the comparison between an NLTE and an LTE computation for the K i lines (the Fe i is always computed in LTE) assuming a heliocentric angle μ = 1. If we look at the first column, we can see that the line core in case of the NLTE computation (pink) is much deeper than for the LTE case (black). This happens for both potassium lines; the K i D 2 line is very deep with a line core intensity (in NLTE) of almost 0.1 of I c . Moreover, if we examine the rest of the columns, we also find differences between the NLTE and LTE polarimetric signals, the latter always with less amplitude. This is an interesting result that points out the necessity of an NLTE treatment for both the intensity and the polarization spectrum. Finally, we can see that these differences also affect additional spectral features as the Stokes V asymmetries. We plot in Fig. 5 the Stokes V LTE and NLTE profiles for both K