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Context. [...] One of the most efficient methods is the measurement of theequivalent width (EW) of the Calcium II Triplet (CaT), corrected for theluminosity and temperature effects via magnitude differences from thehorizontal branch (HB). Methods. We use the Ks magnitude difference between thestar 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 differentmetellicity (met) scales: the one presented in Carretta et al. (2009), the metvalues given in Harris (2010) and a version of the former corrected viahigh-resolution spectroscopic met. Results. We calculated the calibrationrelations for the two datasets and the three met scales and found that they areapproximately equivalent, with differences almost negligible. We compared ournIR calibrations with the corresponding optical ones, and found them to beequivalent, establishing that the luminosity-corrected rEW using the Ksmagnitude is compatible with the one obtained from the V magnitude. We thenused the met obtained from the calibration to investigate the internal metdistributions of the GCs. Conclusions. We have established that the([Fe/H]:rEW) relation is independent from the mag used for the luminositycorrection and find that the calibration relations only change slightly fordifferent met scales. The CaT technique using NIR photometry is thus a powerfultool to derive met. In particular, it can be used to study the internal metspread of a GC. We confirm the presence of at least two met populations in NGC6656 and find that several other GCs present peculiar met distributions.

Deriving Metallicities from Calcium Triplet Spectroscopy in combination with Near Infrared Photometry