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In today's screens, each individual pixel is created by the combination of three transistor as Red Green Blue. But, colours are already just different frequency waves. Why aren't pixels made by just one single component to create different colour values? Is it cost or capability of electronic components?

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For each pixel, manufacturers are putting three different colour generating transistors/materials, and by the combination of those colours, we can have millions of different colours. My thought was that if colour is a wave as a photon, and colour of photon changes according to the frequency of it, we could use just one transistor/material for each pixel and change its frequency to have any colour we want. But as explained in comments, colours are very high frequency waves (3-digit Terahertz values), and technologically we are not able to generate those frequencies yet, which explains why it is not done in that way.

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I think you might need to explain this a little more. –  Andy aka Jan 2 at 17:37
    
Different physical materials are needed to produce various wavelengths. I'm not an LED expert - you might be able to make a hypothetical "single component" Red/Green from Gallium(III) phosphide (GaP) but you wouldn't be able to make blue from it, which would need an entirely different material. en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials –  dext0rb Jan 2 at 17:59
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4 Answers

up vote 9 down vote accepted

I'm taking a guess about what you are misunderstanding, but here goes...

colours are already just different frequency waves.

This is true, but the frequencies involved are very very high.

\$f = \frac{c}{\lambda}\$.

So for a wavelength of 640 nm (red), you're looking at a frequency of about 470 THz. That's 470 x 1012 Hz. We don't have any technology that can route signals at these frequencies over wires, or emit them using the kind of antennas used for RF.

Instead we rely on the natural resonances of certain materials (the bandgaps of different compound semiconductors or transition energies of phosphors) to generate these frequencies. And different wavelengths require different materials, which is why pixels in a display require 3 separate LEDs or 3 separate phosphors, for example.

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+1 for inferring what tack wanted to ask! (and answering it, of course) –  Wouter van Ooijen Jan 2 at 18:23
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The answer is that the LEDs have fixed materials and construction for each color.

LED colors come from a combination of their physical construction and the materials used. The thickness of layers of material and exotic materials determining the energy needed for electrons to move from one layer to another. In an LED, when these electron go from higher voltage to lower they emit light and the light matches the energy lost in the movement.

For example, you can look at the light from a red LED with a spectroscope to find its wavelength. The wavelength has an energy associated with it. If you calculate the energy in electron volts, it will match the voltage drop in volts. The voltage drop on a typical red LED is about 1.8 volts. And 1.8 electron volts (eV) is equal to the energy of light with wavelength 700nm, which is a very red red.

There are also displays that use micro emitters to excite phosphors to glow. These also have fixed colors based on the mix of phosphorescent materials.

The calculation for wavelength from voltage drop in an LED is L = hc/E with h= Plank's constant, c is the speed of light, and E is energy in eV. The result L is in nm.

The "exotic materials" are gallium arsenide, gallium nitride, and even YAG which was previously found in the highest power research lasers and now is on many green laser pointers.

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Silicon, being an indirect-gap semiconductor, is not an efficient light emitter and is not used to make commercial LEDs (although light-emitting Si structures have been demonstrated in the lab). LEDs are generally made from compound semiconductors like GaP or GaN. –  The Photon Jan 2 at 19:34
    
Thanks @ThePhoton. I didn't want to get into Gallium Arsenides and I used an explanation more like a typical diode with the difference in band gap being the important point. I'll edit for accuracy. –  C. Towne Springer Jan 2 at 20:54
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Current technology uses red, green and blue sources of light whether or not it is the old tv tube or LEDs. If you wanted a particular shade of yellow you'd set the intensity of RGB to produce that hue. If you wanted it to be darker or lighter you'd increase or decrease the RGB signals appropriately. How could you do this with a single element picsel - it's not only the colour that is needed but the luminance too.

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This is a question about transducers. So up to now we can modulate electric and magnetic energy and then to trigger a chemical compound to produce a disired visible light. Same analogy with amplitude modulated electrical signal and to trigger a moving coil acting as a piston to produce air presure in different frequencies. That you want I think. But eye respods to higher frequencies than ear, and up to now there is not a single compound able to change its emission wavelegth so rapidly to produce all visible spectrum without afterglow acceptable by human eyes.

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