The signal of voltage, current and light required to ignite the common and superfine lead azide (LA) were studied by semiconductor bridge (SCB). Results show that the appearance time of light is shorter than the end time of the second peak, but for the common LA, it is opposite. With increasing the ignition voltage, the duration time of the second peak and appearance time of light for two kinds of samples are short. The ignition data of the two kinds of LA obtained by D-optimization show that the minimum firing voltage of superfine LA is about 50% less than that of the common LA.