Fabry–Perot microcavities are analyzed in terms of their light emission characteristics. The analysis considers full output coupling, and we calculate both spontaneous and stimulated emission dependencies on cavity length, mirror design, and spectral characteristics. The cavities correspond to vertical‐cavity surface‐emitting lasers in the AlAs‐GaAs‐InGaAs material system, and a GaAs cavity with Bragg mirrors of CaF2/ZnSe. We show that considerable gain enhancement depends on the degree of coherence in the spontaneous emission, the microcavity length, and the Bragg reflector design.