TIGRE spectra of the Ca II K line of two giants (HD 29139 (LC III) and α Ori (LC I)) are shown with the line bandpasses of the different S-indices: The black dashed line outlines the triangular bandpass, sufficient in width for MS stars, and the red dashed line shows the wider trapezoidal bandpass used for giants. The magenta dashed line shows the rectangular bandpass used for the raw S_TIGRE index; see Sect. 4.2.2.