As far as I understand, you need to be able to mix red, green and blue light to get white light and replace lightbulbs. But the earliest example I can find of white LEDs used a blue LED and a yellow (apparently the result of mixing red and green light) phosphor coating to scatter the light across the visible spectrum. Why couldn’t something similar be done with a red LED and a cyan (mixing blue and green) coating/cover to produce white light instead?
In: Engineering
Blue is special because it is the highest frequency of visible light and has the most energy per photon. You can remove energy by absorbing and re-radiating but you can’t add energy the same way. Hence, you can use phophors to turn blue into other colors but you can’t use the same method to go “uphill” and turn other visible colors into blue.
We use LEDs for stuff like photolithography. Shorter wavelengths we can use the better for various reasons. Blue or violet is the shorter end of the visible spectrum. We want as short as possible for photolithography.
The phosphor discovery happened slightly afterwards; it wasn’t known at the time (I don’t think). We needed the blue at the time to mix with red and green (as you mention).
Slight aside, but the blue one was so important that the dude who figured it out got a Nobel.
Due to the way LEDs actually create light, longer wavelengths of light (like red and green) are much easier to create. That’s why red LEDs were invented first, followed by green. Blue, and violet were impossible until researchers made a breakthrough in how to actually build the structure of the LED on a microscopic level. Even then, blue LEDs were a whole lot dimmer for quite a while than the red and green ones we’re used to. Commercially viable blue LEDs weren’t available until the late 1980s or early 1990s.
For a much more in-depth (yet still mostly approachable) explanation, [check out this video from Veritasium](https://www.youtube.com/watch?v=AF8d72mA41M).
EDIT: Interestingly enough, many commercial white LEDs are actually blue LEDs with a yellow phosphor coating on them.
The phosphor’s light doesn’t mix with the blue. The phosphor needs blue or UV light to reach its excited state and radiate white (broad spectrum) light. Ideally almost none of the actual LED’s light gets out.
There were plenty of “fake” blue LEDs before the real thing was invented. But they were too dim and inefficient to be useful, let alone light up a room.
LEDs are very efficient, yes, but if you need 10x the intensity to get usable light through a filtering material, heat becomes a huge issue. Super bright LED arrays used for some industrial applications need active cooling to not burn out, for example.
Phosphors can only downconvert wavelengths, not upconvert them. Fluorescent tubes downconvert UV light, and white LEDs downconvert blue light.
There are some fancy frequency doubling crystals that can go the other way, used in green laser pointers and such, but I believe they only work with laser light.
With just red and green light, we can make anything in between. Even when mixing them, we can get oranges and yellows, but we can’t get white. We need blue to round out the spectrum to make white light.
The first white LEDs used this method, but it was a bit harsh on the eyes. Modern white LEDs light up phosphors on the inside of the bulb to make a wider range of wavelengths. The phosphors can only emit light in a longer wavelength than what activates them, so the green can’t fill in the gap above it, but the blue can fill in the gap below it.
It’s all a careful balancing act to make it easy on the eyes and look natural. It wasn’t until a few years ago that soft white LEDs could reach the point that they don’t bother me, and now I won’t use anything else between energy efficiency and the calmness of the light.
Our eyes specifically respond to 3 different colours. A particular blue, a particular green, and a particular red. If a colour comes in that isn’t exactly one of those colours, it’ll stimulate a combination of the rods in our eyes, and our brain will interpret it as that specific colour between A and B.
We can use that to trick our eyes. If we send the combination of red green and blue to your eyes, and they’re individually small enough points, our eyes can’t really distinguish between 3 specific lights of those colours, or one colour that is some proportion of all 3.
Once we got blue LED’s, we could use that to create panels of lights that use that trick to make the full colour spectrum available on a dynamic display.
Not and ELI5, but there’s a [good layman’s explanation of the invention of blue LEDs on Veritasium YouTube channel](https://youtu.be/AF8d72mA41M?si=ClFUxIXJ5BbY5c8q).
I though it was pretty interesting to hear the story behind it, and it was relatively easy to follow, but not for a five-year-old.
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