Digital LED displays involve single crystal phosphor materials, which distinguishes them from the polycrystal electroluminescent displays. Light emitting diodes (LED) are PN junction devices that give off light radiation when biased in the forward direction. Most light emitting diodes function in the near infrared and visible ranges, though there are now UV LEDs. Light emitting diodes are a reliable means of indication compared to light sources such as incandescent and neon lamps. LEDs are solid-state devices requiring little power and generating little heat. Because their heat generation is low and because they do not rely on a deteriorating material to generate light, LEDs have long operating lifetimes. One of the alternatives, incandescent bulbs, consumes much more power, generate a great deal of heat, and rely on a filament that deteriorates in use. Neon bulbs, on the other hand, rely on excited plasma, which, along with its electrodes, can deteriorate over time.  Digital LED displays are highly versatile and well suited to a variety of measurement applications.  Display types can be segmented or dot matrix.  A seven-segment display can display numbers only, a sixteen-segment display can display numbers and letters.  A 4x7 dot matrix display can display numbers only, a 5x7 dot matrix display can display numbers and letters.  Digital LED displays can be numeric or alpha numeric.  Numeric displays display numbers only, a seven-segment display or 4x7 dot matrix display.  An alphanumeric display can display both numbers and letters, a sixteen-segment display or 5x7 dot matrix display.