Why do so many old displays/indicators particularly use red lights? Are/Were white diodes harder to create?
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Yes, blue and green LEDs are relatively modern. They existed for a while but were incredibly dim compared to the red ones. Only recently did they catch up and/or overtake the brightness, thus allowing strong colours and bright white lights.
However, before LEDs, literal bulbs were often used, and they could be any colour. If you’ve ever taken apart a torch (flashlight), it was that kind of bulb, or if you’ve ever taken apart an old Scalextric car with the tiny bulbs in them, that kind later on.
Blue/green LEDs, and thus RGB and white LEDs, came far later than the reds. New materials, new chemistry, even a bit of quantum physics was required to get them to work strongly enough to be cheap, small, easy to make AND bright.
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