I-band model spectrum of supergiant stars obtained by fitting at each wavelength the flux-normalized (at 2.20 μm) sample spectra along SpTs. The model spectrum for different SpTs is offset for displaying purposes and the SpT is given. The green regions mark the bandpasses of the newly defined indices and, the red and grey regions, mark their adjacent continuum as defined in this paper and in literature respectively (see Table 2). Some relevant absorption features are marked.