Why does production of white light using a LED require combining a short wavelength LED such as blue or UV, and a yellow phosphor coating?

Why can't a single LED produce pure white light?

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    $\begingroup$ They can, if they are coated with the appropriate sized quantum dots: en.wikipedia.org/wiki/Quantum_dot $\endgroup$ – alexsaintx Feb 17 '14 at 17:38
  • $\begingroup$ That sounds like it's the package that's producing white light, not the diode itself. $\endgroup$ – David Richerby Feb 17 '14 at 18:56

White light does not have a single frequency, it is a mixture of all of the colors. Similarly pink does not have a single frequency it is a mixture of red with white i.e. white with extra red.

If the color does not appear in the color spectrum (rainbow) then it is not a single frequency but a mixture of different frequencies.

In an LED light is produced by an electron dropping from an excited state to a relaxed energy state which produces a light of a given energy (frequency) however all of the light is going to be of the same color since the bandgap (energy difference) is going to be the same for all of the electrons.

It is possible (see here) to put three different LEDs on one chip corresponding to Red Green and Blue. This produces the effect of a white light. But is not truly white since it only contains Red Green and Blue and no Orange Yellow or Violet / purple.

Your computer screen cannot produce true whites either for the same reason. If you see yellow on your computer screen, there is not light of the frequency yellow only the right amount of Red Green and Blue to convince your eyes nerves to fire the same as if it were a true yellow frequency.

  • $\begingroup$ very good for beginner level like me.Thankyou very much $\endgroup$ – violet kiwi Feb 17 '14 at 14:39
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    $\begingroup$ Colors are described using the way our eyes and brains interpret them. To call red+green+blue in proportions that look white to humans "not truly white" is inaccurate or at best misleading, I think. All white is made up of a mix of colors; whether this is from a black body source, or red green and blue, or some other wavelengths is rarely relevant. $\endgroup$ – Tim S. Feb 17 '14 at 18:59
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    $\begingroup$ @TimS.: If light is directly stimulating a person's eyes, any combinations of wavelengths which appear "white" may be considered indistinguishable. If, however, the light is shining on or through other objects, the set of wavelengths comprising it may affect the appearance of those objects. For example, a red object illuminated with a mixture of red, green, and blue light will appear red; such an object illuminated with a mixture of monochromatic yellow and blue light would appear dull yellow. $\endgroup$ – supercat Feb 17 '14 at 19:47

At its heart, an LED is a diode fabricated out of semiconducting materials. There are fixed bandgaps for a given material (or mixture of materials at a given ratio), so only certain energy states are available. This means only certain wavelengths of light can be generated, since wavelength is a function of photon energy.
In theory, if you could design some diode which had three different bandgaps, you could produce, perhaps, Red, Green, and Blue output to achieve apparent white light. In practice, I doubt this is achievable. You could build close-packed clusters of RGB diodes -- think LED television sets-- but I believe the color-temperature and efficiency of a LED + phosphor design is much better for general lighting purposes.


Actually, there is a type of LED that does directly produce white light. It was developed in the early days of blue LED development (mid 1980s) and consisted of a diode in zener breakdown and a blue lens. A diode in breakdown produces, for lack of a better word, light noise (white noise = white light). I doubt that these type of white LEDs are available today simply because they are too expensive and they consume too much power. When on, they have a forward voltage of around 7 volts (breakdown voltage) and have a "reverse" breakdown of about 0.6 volts (forward biased diode).

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    $\begingroup$ +1. I don't know about this, but I was going to comment that it's not that fundamental that LEDs must be monochromatic. (Indeed even normal ones aren't monochromatic in the sense you'd apply to lasers, lacking a resonator that stabilises the coherence.) $\endgroup$ – leftaroundabout Feb 18 '14 at 16:47

If we define "pure white light" as the combination of all the frequencies if the visible spectrum, then the possibility of one diode generating it, is practically nil. If we define "white light" as the combination of 3 frequencies (red, green, & blue), then we can generate it with 3 small and "closely spaced" LEDs with the appropriate color and amount. I am going to call this method the "parallel" method of generation. In theory, it should be possible to create a diode with 3 different(but appropriate) breakdown regions, on "top" of each other (serial method), to achieve the same thing. But regardless of the method used, the minimum number of different frequencies required to produce "white light," is three.

So, to sum up, white light requires a minimum of 3 different frequencies, a given diode breakdown region can only produce one frequency, therefore white light can not be produced by a single breakdown region (single LED)!


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