Sputter occurs when the energy level is very low, below 15% of the explosion energy, or when the pulse duration is very long. Sputter is the vaporization of metal from the cathode, which is redeposited on the walls of the lamp, blocking the light output. Because the cathode is more emissive than the anode, the flashtube is polarized, and connecting the lamp to the power source incorrectly will quickly ruin it. However, even if connected properly, the degree of sputter may vary considerably from lamp to lamp. Therefore, it is impossible to predict the lifetime accurately at low energy-levels.