Observations of the infrared triplet lines of ionized calcium are presented for 91 stars brighter than mv=+7.0 in the spectral range F8-M4 of all luminosity classes and over a range of metallicities [Fe/H] from -0.65 to +0.60. The above spectra have been obtained at a spectral resolution of 0.4 A with a coude echelle spectrograph using the Thomson CCD as the detector. This study has been undertaken primarily to investigate the dependence of the Ca II triplet strengths over the broad range of atmospheric parameters like luminosity, temperature and metallicity. The Ca II triplet lines are a powerful diagnostic of the stellar populations in galaxies because of their sensitivity to the above parameters. Our detailed analysis indicates a strong correlation between the equivalent width of the Ca II triplet lines and surface gravity, much stronger in metal rich stars than in metal poor stars. The Ca II equivalent widths are fairly insensitive to temperature over the range of luminosity covered. However, they are found to be quite sensitive to metallicity, more conspicuously in supergiants than in giants and dwarfs. Observations are compared with recent theoretical calculations of these lines in NLTE atmospheres.