The amount of power loading the glass can handle is the major mechanical limit. Even if the amount of energy (joules) that is used remains constant, electrical power (wattage) will increase in inverse proportion to a decrease in discharge time. Therefore, energy must be decreased along with the pulse duration, to keep the pulsed power levels from rising too high. Quartz glass (1 millimeter thick per 1 second discharge) can usually withstand a maximum of 160 watts per square centimeter of internal surface-area. Other glasses have a much lower threshold. Extremely fast systems, with inductance below critical damping (0.8 microhenries), usually require a shunt diode across the capacitor, to prevent current reversal (ringing) from destroying the lamp. If the pulse is allowed to ring through the lamp it will lengthen the flash, so the diode traps the ringing, allowing the lamp shut down at the correct time.