A backlight is used behind the LCD glass to allow the LCM to be read in dark conditions. The vast majority of backlights are now LED. Historically there were also EL (electroluminescent) and CCFL (Cold Cathode Fluorescent Light) backlights, but as LED efficiency has increased and cost has decreased, EL and CCFL backlights have all but disappeared.
LED backlights for LCM are typically edge-lit. LEDs are placed at the edge of a light pipe and light escapes from the light pipe at strategically placed imperfections in the surface. By placing more of these imperfections farther from the LED, and fewer imperfections closer to the LED, the entire area of the LCD glass can be evenly illuminated.
There are also some array style LED backlights, which are truly things of beauty. Array backlights are becoming less popular as higher efficiency LEDs make the large number of LEDs that were used in an array backlight unnecessary.
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