A 5% reduction in voltage will roughly double a bulb’s lifetime but also make it about 16% dimmer. Some will also pass a small current through the bulb even when it is off to keep the filament warm.

Plot a graph of voltage, V on the y-axis v current, I, on the x-axis. For a filament bulb, voltage and current are not directly proportional as the graph is not a straight line through the origin.