The LED is similar to the ordinary signal or rectifying diode in that it is polarity sensitive. In reverse bias the device behaves as an open and prevents current flow. In forward bias, the device allows current flow once its forward barrier potential is reached. This potential is significantly higher than that of ordinary diodes and depends on the material used, and hence, the color that is displayed. Generally, luminous intensity is a function of the forward current. That is, the greater the current, the brighter the output. In operation, a series limiting resistor or other control device must be used to limit the forward current and prevent damage that could occur to the LED from excessive current. Different technologies are used in the design and production of LEDs and there are many variations including full spectrum (white) and high brightness versions. The cathode of an LED is typically denoted by a flat spot on the plastic casing and/or by the shorter of the two leads.