"The first commercially available LEDs started to appear in the late/mid 1960s. These LEDs early LEDs used a semiconductor made using gallium, arsenic and phosphorus - GaAsP. This produced a red light, and although the efficiency of the devices was low (typically around 1 - 10 mcd at 20mA) they started to be widely used as indicators on equipment."
it was originally to show its an emergency light and also could be maintained or non maintained, or even sustained, they even now put an led on which will show if the lamp is good or dud.
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