Back light

In LCD displays, the screen does not produce light by itself; it needs a back light bulb to make the display visible. The back light bulb sits behind the display, and when powered on shines brightly, making the display visible.
A common example is a digital watch; you can read the display in the sunlight, but in a dark room, you can not. When you press the glow, or light button, then the watch face lights up so you can read it. The light you are seeing is being produced from a back light.

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