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After some studies I've found LEDs having 3 colors RGB (Red, Green and Blue), I was very interested in this topic so this question is up. The Diagram will tell the colors in LEDs.Red,Green and Blue color in LEDs 

> Red, Green and Blue color in LEDs

Why only RGB is needed?

Why can't any other color be replaced?

Why MCY Cannot be used?

lad
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  • You can use CYM but your color space will be much narrower. Or you can use 4 or 5 colors to give you a even larger color space than 3 color. – user3528438 Aug 15 '17 at 14:32

3 Answers3

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LEDS actually come in all colours.

However, what we as humans call colour is dictated by the fact that the cones in the human eye are only actually sensitive to three primary groups of wavelengths which we call red, green, and blue.

enter image description here enter image description here

Different LED technologies produce light in a similar but narrower bell curves around a particular wavelength. When we look at them the cones are excited by the wavelengths present and the brain interprets the mixture of excitement as the intermediate colours. The colours that match the cone frequencies are obviously the brightest LEDS.

enter image description here

Notice normal red LEDs are actually off quite a bit from the cone. As such red LEDS are actually a lot brighter than they appear to be.

What that means is that having more LEDs of other colours gives you a limited return since all you need to do is excite the cones in the eye using the appropriate amounts of the three "primary colours" it is sensitive to.

Interestingly, to other sighted animals, colours look quite different. Even you and I will have genetic differences in how we perceive each colour.

EDIT

As others have mentioned in the comments the above description which is widely used is in fact a little too simplified.

Because of the wide bell curve shape of the cone sensitivity the pure RGB excitation method does not work quite as described. For example, notice the "red" cones actually peak out somewhere around yellow-orange. The brain actually interprets red as some excitation on the red cone and no excitation on the green cone. Green on the other hand has equal excitation of blue and red.

In other words, the primary colour model is just not quite that simple.

Adding more colours will in fact produce more detectable hues to the brain. This is why RGBY produces quite a noticeable difference. If you look at the second colour chart you will notice a bigger gap between the red and green than the green and blue. A yellow, or amber LED, fills that gap and produces many more detectable hues with no blue component.

Ultimately, a LED that can produce every wavelength that the subject was illuminated with would be required to produce a 100% natural image. However the costs, both of the display LEDS and the capture and transmission mechanisms becomes an issue.

Fortunately, the colours and image quality that can be produced using a simple mixture of RGB does cover a sufficiently wide gambit of human detectible colours to be satisfactory for most cases.

Jeanne Pindar
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Trevor_G
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    Good answer, skipping usual topics in the subject. As additional and interesting info: not all humans are trichromatic, someones are tetra or di. – pasaba por aqui Aug 15 '17 at 14:55
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    Yellow/orange (AKA amber) LEDs look like they would be quite a valuable asset - and indeed, they do exist. I'm just not sure if they package them into four-color LED packages like they do with RGB. Why is the blue LED off-peak, though? – John Dvorak Aug 15 '17 at 15:02
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    @JohnDvorak yes indeed, single LEDs exist that are not primary colours and have their uses. Having those in multi-color LEDs intended to produce the full gambit of colors though is pointless. As for the blue one, the actual color emitted is dependent on the chemistry of the LED. Blue LEDs are relatively new and have a rather exotic chemistry. No idea when that image was created, there may be closer varieties available now. However, it does not matter much. Since the blue sensitivity is rather offset from the red and green cones, the blue LED can also be off by quite a bit more. – Trevor_G Aug 15 '17 at 15:06
  • Full gamut of colors isn't pointless. RGBY TVs sell quite well - and at a rather high base price that the customers are willing to pay anyways. Of course those are LCD panels, not arrays of LEDs, but I'm wondering if some posh stadium might want to pay extra bucks for the perfect yellow. [Even though it doesn't actually work in the TVs.](https://en.wikipedia.org/wiki/Quattron#Scientific_analysis) – John Dvorak Aug 15 '17 at 15:17
  • @JohnDvorak if you are exciting the RED and Green cone 50% using the red and green LEDS the eye has no way to know if it is from two LEDs or one yellow one assuming they are close enough together. As such it becomes pointless to add more complexity. However, that being said, keeping everything linear is not easy, having an extra yellow LED may buy you something, though I suspect it's more a gimmick than a feature and could easily be mimicked by adjusting the colour translation profile of the simple RGB version. – Trevor_G Aug 15 '17 at 15:24
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    Notice how the green LED excites the "blue" (short) cone cells but the amber LED doesn't? You can't fix that by shining red and green even brighter, and we don't know how to inhibit the short cones with light either. And _that's_ the point of RGBY. – John Dvorak Aug 15 '17 at 15:32
  • @JohnDvorak yes I do see your point, and yes the model is not as simple as widely acknowledged. It should also be mentioned though, how well the display works must also be tempered with how well whatever is recording the live image works and how the data is being transferred. An RBGY display with an RGB camera/transmission introduces a whole new can of worms. – Trevor_G Aug 15 '17 at 15:56
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    As someone who owns RGBA pars (stage lighting for the band I used to be in), I can say that the reason I bought them is the "warmth" of the light. Pure white light mixed from RGB looks cold, because evolution and environment tell us to expect the colour temperature we get from the Sun. It's actually rather hard to get a good "natural" yellow-white light purely from RGB; whereas adding an amber LED to the mix gives you that instantly with no messing about. The result was stage lighting which was simply more pleasant. – Graham Aug 15 '17 at 16:06
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    @Trevor: The RGB color model works well when light isn't reflected off or shone though colored objects. It's possible for objects which look identical when viewed with some combinations of wavelengths to look very different with others. – supercat Aug 15 '17 at 17:00
  • @Graham: Have stage-paint and stage-makeup companies responded to the popularity of LED lighting with products designed for them? Make-up designed for incandescent lamps or natural light may look bland under LED lighting, but it should be possible to adjust the make-up so it would look good under the LED lights (that might make it look bad under incandescent or natural light, but except for outdoor productions which start before sunset I wouldn't think that would be a problem). – supercat Aug 15 '17 at 17:04
  • @supercat While RGB works for additive color mixing, once you bounce that light off a physical object, you're dealing with the combined effects of additive and subtractive mixing. RGB with its 3 narrow peaks is not a good starting point for subtractive color mixing, so while the light itself might look fine, the objects that the light bounces off will not. For illumination, the peaks should be wide and/or many; You want to cover the greatest possible portion of the visible spectrum. Compensating for bad CRI by repainting people and objects seems like a bodge. – Dampmaskin Oct 07 '20 at 15:53
  • @Dampmaskin: People and props have "painted" for stage productions for centuries before the invention of the electric light. Use of narrow-band LEDs creates opportunities as well as problems, especially if assemblies combine RGB with a separate white. If, for example, one has a paint formulated so it reflects most colors in the white, but absorbs the narrow-band red LED, but most objects would appear identical under white-light or RGB illumination, such a design could be made to appear or disappear by transitioning between the two kinds of LED. – supercat Oct 07 '20 at 16:20
  • Yes, for special effects that would be way cool. I had to google it. Voila. https://www.mikewoodconsulting.com/articles/Protocol%20Spring%202010%20-%20Color%20Rendering.pdf – Dampmaskin Oct 07 '20 at 16:43
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Single LEDs come in orange, yellow, amber, infrared, ultraviolet, cold white, warm white and many other colors/hues. Therefore I assume you mean to ask why multicolor LEDs are typically red, green and blue.

The answer lies in how colors are mixed.

Additive color is color created by mixing a number of different light colors, with shades of red, green, and blue being the most common primary colors used in additive color system.

https://en.wikipedia.org/wiki/Additive_color

This stands in contrast to subtractive color mixing, where cyan, magenta yellow and optionally black - CMY(K) - is commonly used in printers etc.

A subtractive color model explains the mixing of a limited set of dyes, inks, paint pigments or natural colorants to create a wider range of colors, each the result of partially or completely subtracting (that is, absorbing) some wavelengths of light and not others. The color that a surface displays depends on which parts of the visible spectrum are not absorbed and therefore remain visible.

https://en.wikipedia.org/wiki/Subtractive_color

Naturally, LEDs work with light, so additive color mixing is the most effective and the prefered method.

With RGB you can mix a lot of different colors using only three LEDs in one package or device.

It is of course possible to use other color combinations than RGB, but that will most likely increase the cost, and/or decrease the gamut and therefore the general usefulness of the light.

Dampmaskin
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It's not true to say these are the only LED colours available. However. I suspect you are talking about colour mixing. By altering the intensity of the three [RGB] LEDs you can produce any colour under the sun. This is commonly done by driving LEDs with PWM signals.

CMY(K) can be used to make any colour too, but, you can't get [pure] cyan and magenta LEDs and these colours are used for subtractive rather than additive colour mixing.

LEDs such as white are made with an blue LED and a phosphorous coating. Cyan and magenta by having multiple die within a package, so you're already colour mixing here.

DiBosco
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