With the aim of providing a thorough description of the optical properties of Er3+-doped endohedral fullerenes, we studied their characteristics in the light of those of well-known Er3+-doped β-PbF2 single-crystals. Various Er3+-doped endohedral fullerenes were considered: Er2C2@C82, where the Er2C2 group is encapsulated inside a cage of 82 carbon atoms and the Er3-xScxN@C80 (x = 0, 1 and 2) family, where the Er3N, Er2ScN and ErSc2N clusters are trapped in a 80 carbon atom cage. In this article, we discuss the absorption and photoluminescence of trivalent erbium ions in fullerenes and in β-PbF2 crystals. The extinction coefficient of Er3N@C80 was found to be 4.8 (±0.5) × 10^3 mol/l-1 cm-1 at 540 nm, due to the C cage absorbance. Even in a saturated fullerene solution, the absorption of Er3+ encapsulated inside a C cage cannot be observed at room temperature. We suggest that this is due to an insufficient number of Er3+ ions in the solution and their low absorption cross-section. Low temperature photoluminescence measurements show that the line width of Er3+ in a carbon cage, dissolved in a polycrystalline solvent, is similar to the one of Er3+ in β-PbF2 single-crystals. The quantum efficiency of Er3+ at 1.5 μm in fullerenes is four orders of magnitude lower than that for Er3+ in crystals, due to very efficient non-radiative decay processes. Molecular vibrations of the cage might be responsible for those rapid non-radiative de-excitations.