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LEDs consist of pn-junctions, so why can blue LEDs be used for generating white light, but red LEDs cannot

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    $\begingroup$ Because the blue/UV light can be transformed with phosphors into much of the visible spectrum, since they are (energetically) at the top end of the visible range. Up-converting red photons into the yellow/green/blue is really hard and inefficient. $\endgroup$ – Jon Custer Jan 15 at 13:55
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    $\begingroup$ @Jon That looks like an answer... :) $\endgroup$ – PM 2Ring Jan 15 at 14:10
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"White" light consists of a mixture of at least three colors that should be blue(ish), green (ish) and red(dish).

The most common way to get white from basically monochromatic LED light is to use fluorescent material.

This is what a spectrum of a blue LED looks like:

Spectrum of a blue LED with one single peak at about 480 nm.

This gets converted in the LED to something like this:

Spectrum of a white LED with the narrow blue peak (450 nm) and a wide second peak (500-600 nm).

In fluorescent material, an electron is excited by a single incident photon and then relaxes over intermediate energy levels. This means that photons emitted during this relaxation can only be of lower energy (shifted to the red end of the spectrum) compared to the exciting photon.

Jablonski diagram of a typical fluorescence.

So it's easily possible to get red from blue but not the other way round.

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The blue led has wavelength of about 450nm and has more energy than red photons at about 600nm wavelength. To create white light phosphors were discovered a long time ago, phosphors are used in fluorescent bulbs (convert UV to blue, green, red) and in old CRT TVs that converted electrons into colours of light. The phosphor atom takes in a higher energy photon and then produces a lower energy photon ( color) and heat. There are many different phosphor chemicals (1000s to 10000s) that absorb UV or blue or other and make different colours of light.

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    $\begingroup$ This does not get to the point why blue is necessary. $\endgroup$ – Jasper Jan 15 at 15:02
  • $\begingroup$ Its just economics, more expensive white led lights (highest CRI) actually used UV and converted to to B,G,R. Also some white LEDs had red leds for red and blue led and green phosphor etc.... over the years many combinations have been tried. $\endgroup$ – PhysicsDave Jan 15 at 16:33
  • $\begingroup$ That doesn't explain why blue or UV is necessary then. The necessary bit is that phosphor converts from higher-frequency to lower - blue to yellow or red for instance - not the other way around, red to blue for instance. That said, I do have a sliver of material that does just that... $\endgroup$ – Maury Markowitz Jan 15 at 20:31

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