By locating the emitters around the first and second antinode of the metal electrode,the escaped and trapped emission of small molecule based bottom emission organic light-emitting diodes is investigated by using an integrating sphere,a fiber spectrometer and a glass hemisphere.It is found that the exteal coupling ratio by locating the emitters at the second antinode (at a distance of 220 nm from the cathode) is 70％,which is higher than that of an emitter at the first antinode (60nm from the cathode) in theory and experiment.Extending the "half-space" dipole model by taking the dipole radiation patte into account,we also calculate the optical coupling efficiency for the emitter at both the first and second antinode.Our experimental and theoretical results will benefit the optimization of device structures for the higher out-coupling efficiency.