Traditionally, airplanes have incorporated a variety of instrument gauges to indicate various parameters of flight to a pilot.  The most basic of these traditional instruments are four gauges arranged in a traditional “T”, pattern, and include airspeed, attitude, altitude, and directional indicators.  Other gauges such as climb and flight path deviation indicators are also common, and are usually positioned near the basic “T” instrument cluster. 
As mechanical gauge technology has been replaced with electronic and computerized flight indicator displays, integration of several of these traditional data elements onto one screen has become common, and displays such as liquid crystal displays (LCDs) have been configured to show a wide variety of flight data on a relatively small number of LCD screens.  These screens can vary from airplane type to airplane type, and even between individual airplanes depending on the sensors and navigational systems installed on each aircraft. 
Because the airplanes must be equipped to fly in darkness, the LCD displays are traditionally backlit with a light that shines behind the portion of the display that produces the image.  The formed image blocks a portion of the visible backlight, resulting in a formed image that is visible to the pilot and other flight crew members viewing the LCD instruments.  Because it can be critically important for safe flight to make sure that the LCD instrument display remains lit during night flying, the backlights often consist of two or more separately powered light sources, so that if one fails the other can still provide light.  The light sources themselves vary, and include cold cathode lights, fluorescent lights, electroluminescent panels, light-emitting diodes, and other such lighting technologies. 
Further, because the LCD and backlight systems will be installed in an airplane, they must meet rigorous specifications for long life, and a wide variety of environmental and vibrational conditions.  The optical specifications of each unit must also meet strict performance criteria to ensure legibility of critical flight data.  The design of the LCD displays and backlight systems must take all this into account, resulting in a robust system that is often expensive, large, and heavy. 
In configurations where multiple LCD displays are used, each display has its own backlight system and control electronics.  The control electronics power the backlight system, control the brightness of the backlight, and control the operation of redundant or backup lighting systems.  Because the cost, space, and weight required for multiple backlights and backlight control systems is not trivial in a crowded airplane cockpit, it is desirable to reduce the cost, space, and weight consumed by such systems.