Spontaneous emission spectra and intensity measurements are made on buried heterostructure lasers with transparent windows above the active stripe. The bias voltageV, where eV is the separation of quasi‐Fermi levels, is determined from spectral measurements. The total luminescence intensity increases as exp(eV/nkT), wherenchanges from 1.0 to about 2 asVincreases from 1.3 V to the threshold voltage of 1.415 V. The change innis due primarily to saturation of the low‐energy electron states as a result of electron degeneracy. The spectral determination of bias voltage is justified by the close agreement between the threshold voltage determined by spectral analysis and the voltage determined by extrapolation of directly measured light versus voltage data. Calibration of the absolute radiative rate with optical absorption data yields a predicted radiative recombination current ofJth/d≊5.8 kA cm−2 &mgr;m−1, in reasonable agreement with empirical threshold data.