Erbium implanted sodalime silicate glass was covered with a range of transparent liquids. The spontaneous emission rate of Er3+ around 1. 54 µm shows clear changes depending on the liquid refractive index and Er depth. A rigorous quantum mechanical calculation shows that the effect of the interface on the spontaneous emission rate is fully described by the classical density of optical modes as a function of distance from the interface. The data provide the first direct determination of the radiative transition rate (45 s-1) at 1.54 µm of Er3+ which is widely used in optical amplifiers.