As illustrated in the typical Current (If) Vs. Forward Voltage (Vf) graph above, a slight change in Vf will cause a large change in If. This can result in a possible over-drive condition, thus reducing the life and/or reliability of the LED. Even if over-drive conditions are not reached, large variations in If will result in significant luminous intensity variation. The preferred method of operating from a voltage source is via a series resistor to limit the current flow through the LED.