An anisotropic photonic crystal light source has been realized by impregnation of a thin film latex opal with brightly luminescent CdTe nanocrystals. Its photoluminescence along a given direction has been studied as a function of excitation power. An increase of the photoluminescence saturation threshold in the directional photonic band gap has been observed. This result is interpreted as a stimulation of emission coupled to specific eigenmodes of a directional band gap. A theoretical model exploiting the low group velocity eigenmodes has been elaborated to explain the resonance character of the emission with band gap frequencies.