Circuit work process is: When the momentary switch S between, since the voltage across the capacitor can not change suddenly, the Pi C zero voltage across the SCR vs no trigger
voltage cut-off stop. In this case the current flowing through the lamp H is through diode VD2 half-wave rectified current, the preheating dark light bulb, the impact of current is very small. If the capacitor C via VDI and Rl charge, after a delay of about 0.6s, C rises to reach the voltage across the controllable silicon vs opening level, vs opened, namely normal lamp H lights up. Since the bulbs from the cold state to the normal light room with 0.6s buffering delay time, so the lamp life has been extended.