  Context. [...] One of the most efficient methods is the measurement of the
equivalent width (EW) of the Calcium II Triplet (CaT), corrected for the
luminosity and temperature effects via magnitude differences from the
horizontal branch (HB). Methods. We use the Ks magnitude difference between the
star and the reddest part of the HB (RHB) to generate reduced equivalent widths
(rEW) from the datasets presented in Saviane et al. (2012) and Rutledge et al.
(1997). Subsequently we calibrated these rEW against three different
metellicity (met) scales: the one presented in Carretta et al. (2009), the met
values given in Harris (2010) and a version of the former corrected via
high-resolution spectroscopic met. Results. We calculated the calibration
relations for the two datasets and the three met scales and found that they are
approximately equivalent, with differences almost negligible. We compared our
nIR calibrations with the corresponding optical ones, and found them to be
equivalent, establishing that the luminosity-corrected rEW using the Ks
magnitude is compatible with the one obtained from the V magnitude. We then
used the met obtained from the calibration to investigate the internal met
distributions of the GCs. Conclusions. We have established that the
([Fe/H]:rEW) relation is independent from the mag used for the luminosity
correction and find that the calibration relations only change slightly for
different met scales. The CaT technique using NIR photometry is thus a powerful
tool to derive met. In particular, it can be used to study the internal met
spread of a GC. We confirm the presence of at least two met populations in NGC
6656 and find that several other GCs present peculiar met distributions.
