Identification of the ridges in red giants observed by Kepler. Targets were chosen with oscillation spectra exhibiting very different visibility coefficients (see Sect. 7). Left side: high visibility of ℓ = 0 to 2 from top to bottom. Right side: low visibility of ℓ = 0 to 2 from top to bottom. The spectra are corrected for the background contribution (negative values being omitted), and plotted as a function of the reduced frequency. The color code, derived from Eqs. (7) and (8), presents radial modes in red, ℓ = 1 in blue, ℓ = 2 in green, and ℓ = 3 in light blue.