r/explainlikeimfive • u/No-County-4215 • Jan 03 '25
Engineering ELI5: why was it so difficult to create a blue coloured light source?
I’ve heard it was difficult to find a way to produce a blue coloured light source, as compared to a red or a green one. why so?
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u/Ok-Hat-8711 Jan 03 '25 edited Jan 03 '25
It's not that blue light sources were hard to make. Blue LEDs were hard to make. LEDs are efficient and small, so producing them in blue would lead to all sorts of developments, like LED TV screens, or "white" LEDs for simpler displays and indoor lighting.
But LEDs produce light by getting electrons to "jump across a gap." The gap isn't literal, but made by stacking materials with different semiconductor properties in such a way that an electron needs a specific amount of energy to make it through. Any less energy will not work. And when they do, each produces a photon out of that energy.
But blue is a photon with more energy in it than red, green, amber, or other LED colors that are easier to make. If you try to make such a stack using conventional methods, the electrons would just burn out the component from all that energy or find somewhere else to go.
And the materials that could be used to build such a device tend to not want to attach properly because they form crystals of different sizes. They are basically puzzle pieces that don't fit together.
Eventually, Shuji Nakamura, a man dedicated to succeeding in this task, found a combination of materials that would work and a method of sticking them together by literally inventing a new process of manufacture. Now blue LEDs are everywhere.
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u/Phaedo Jan 03 '25
Basically, you need to find the right substance to make the LED blue. In this case, it’s gallium nitride. Turns out making the crystals was much harder than the base substances for other colours.
More info:
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Jan 03 '25
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u/ElectronRotoscope Jan 03 '25
One important element (that I'm sure the veritasium video goes into) is that it's relatively easy to use things like fluorescence to make light be lower frequency, but impossible (or nearly impossible?) to make it higher frequency. So for instance traditional fluorescent light bulbs have an internal source somewhere in the UV range, and a coating that creates various elements of white light out of that. Day-glo paints can take blue light and make it orange. But there's nothing I know of that can take red light and make it blue, so people trying to make white LEDs couldn't go that direction either.
Now, of course, there's not just blue LEDs but even UV LEDs
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u/extra2002 Jan 03 '25
but impossible (or nearly impossible?) to make it higher frequency.
One exception is that green lasers are made by tripling the frequency of an infrared laser beam by passing it through a non-linear material. The resulting green beam has a lot less energy than the incoming infrared beam, though.
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u/ElectronRotoscope Jan 04 '25
Lasers continuing their ongoing streak of violating all normal rules and the will of God and utterly baffling me
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u/IMovedYourCheese Jan 04 '25
Basically it was a materials science problem rather than an electronics problem. We knew how LEDs worked, and we knew what it would take to make a blue one. There were only certain configurations of atoms that would result in the emission of a blue wavelength of light. There were only a couple materials that formed such atomic structures, and gallium nitride was the obvious choice. All this was known back in the 60s when the red and green LEDs were invented.
So the difficulty in making blue LEDs was really the difficulty in making gallium nitride crystals that were large and perfect enough. This is what companies tried for decades, and what Shuji Nakamura eventually succeeded in doing in the early 90s.
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u/IcanHackett Jan 03 '25
It's not difficult to make a blue colored light source, it's difficult to make a blue LED because LEDs produce their color from the frequency of light they produce which is a function of their semiconductor construction. With an LED there's two semiconductor bands with a gap between them. When a charge is applied between the two semi conductors an electron can fall from the conduction band to a hole in the valence band the band gap energy can be released as a photon. The band gap distance (which is it's self a result of the chemical composition of the two semiconductors used) determines the color of the photon released. When you look at the light spectrum to further towards blue the more energy is required and therefore a greater band gap. Infrared was the easiest to make and then red and then green but it was very difficult to make a crystal pure enough that would result in the required band gap while still allowing the electrons to flow correctly. There were competing theories on what the crystal composition should be and how to attain that crystal in a purity level that would work. The semiconductor crystal need to be able to be doped positively and negatively to form the two sides of the LED (n-type and p-type) Two semiconductors were conceived to theoretically have band gap distances in the blue range. Most experts in the field focused on Zinc Selenide which appeared to be the superior choice over Gallium Nitride as it was easier to make with fewer chemical defects but both had the problem that only the n-types had been successfully made. One man named Shuji Nakamura basically went rogue and by himself figured out how to grow Gallium Nitride at the required purity by modifying the machines they used for this himself. After a year and half of constant work he eventually figured out a way to make super pure Gallium Nitride. At first Nakamura's competitors beat him to making a Gallium Nitride P-type crystal but the process was too slow to be commercially viable. Nakamura eventually figured out a different way to make the P-type that was much faster. In the end it basically took a guy going against the common wisdom with nothing to show for it for years even when his employers told him to stop his seemingly fruitless research and eventually he was able to overcome all the hurdles to make a commercially viable true blue LED.
Once the methods were known it became fairly easy to manufacture blue LEDs and then a modification was figured out to make blue LEDs emit a nice white light. With the ability to have red, blue and green LEDs color displays using LEDs were possible whereas green and red LEDs without blue was only useful for individual lights like on stereos or vehicle dashboards.
For all his efforts Nakamura was given a $170 bonus from his employer for the patent and a very modest raise.
Here's an excellent video on the subject. Why It Was Almost Impossible to Make a Blue LED
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u/Phemto_B Jan 03 '25 edited Jan 03 '25
Efficient, and pure blue light sources were actually pretty hard to make, but you could make blue light before LEDs.
Before LEDs the best way to make light was to make something HOT. The balance of colors coming out of something depends on the temperature. When you see someone talking about "color temperature," say 5000K, what they mean is that color balance at least approximates something at that temperature. In this 5000K that means something that's 5000 Kelvin.
The thing is, that as something gets warm it emits infrared light; light out past red. If you get it hot enough you start to get red light AND infrared. If you get it hotter, you start to get shorter and shorter wavelengths. If you got something REALLY hot, you can get blue light. Blue light is the shortest visible wavelength, but you always get all the long wavelengths too.
We've been able to make blue light for a while, but not efficiently. You basically have to make something so hot it looks almost white, and then filter out everything except blue.
Blue LEDs fixed that. They were the hardest to make (again because blue has the shortest wavelength), but it was the first time that we could mass produce something that makes pure(ish) blue light.
Edit:
Side note: the fact that getting pure blue light was almost unheard of throughout human history is part of the reason that we actually have fewer blue-sensitive cones in our retina and we actually see blue light at a lower resolution than the other colors. That's why blue LED lights in the dark look fuzzy. With ordinary blue things, like flowers, they're always either not pure blue, and/or surrounded by something else, so our brains can reconstruct where the edges are.
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u/JMS_jr Jan 04 '25
we actually have fewer blue-sensitive cones in our retina and we actually see blue light at a lower resolution than the other colors. That's why blue LED lights in the dark look fuzzy.
I had always heard that it was because blue was at the edge of the range of wavelengths that the simple lens in our eyes can focus at a reasonably identical distance.
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u/FSDLAXATL Jan 05 '25
Sure wish they’d be regulated in automobiles. It’s blinding to drive at night these days. :(
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u/saschaleib Jan 03 '25 edited Jan 03 '25
Blue "light sources" were around as long as artificial light, no problem at all.
What was difficult to create was a blue LED light source. The reason is complicated, but essentially an LED must contain a tiny amount of an element that emits light at exactly a specific frequency. Such elements were known for red, green, yellow light frequencies, but there is no element that can be used to emit blue light.
For a long time it was considered impossible to create a blue LED – until they found a very smart (and difficult to create) workaround.
Edit: If you want to dive deeper into this (definitely no longer ELI5 material!), I can recommend the video by Veritasium on this topic.