Por que os LEDs de três componentes em um LED RGB são tão desequilibrados?

Recentemente, eu estava especificando alguns LEDs RGB para um projeto, quando notei que as classificações de milicandelas nas três cores raramente são próximas do mesmo número. (ou seja, 710mcd vermelho, 1250mcd verde e 240mcd azul).

Isso cancela de alguma forma ou significa que o LED sempre ficará amarelado?

Além disso, por que os fabricantes fabricam esses LEDs desequilibrados? Não faria mais sentido emparelhar 3 LEDs com aproximadamente o mesmo brilho?

Exemplo: CLY6D-FKC-CK1N1D1BB7D3D3 fabricado por Cree

Parece correto. Para obter branco (6500K) usando fósforos NTSC (TV em cores), as intensidades relativas são G = 0,59, R = 0,3, B = 0,11 - a maior parte da energia está no verde, menos no azul. (números ligeiramente arredondados de forma diferente na Wikipedia ) Com a mesma intensidade, o azul pareceria mais brilhante. Os números reais serão diferentes aqui (LEDs não fósforo), mas as intensidades relativas são realmente mais semelhantes do que eu esperava.

O interessante comentário de Spehro explica de alguma forma o porquê. Candela é uma definição de intensidade luminosa ponderada de modo que 100mcd de luz vermelha, verde ou azul sejam percebidos como igualmente brilhantes.

Now as I understand the colour space conversion process - it doesn't follow from that, that mixing equal perceived intensities of R, G, B will result in what we see as white!

Indeed how can it? Our eyes are most sensitive to green. So the actual intensity of the green light is reduced in the definition of the Candela to give the same perceived intensity as red, blue (Nitpick : I believe the other intensities are increased instead). Then, to mix the three and make white, we need to increase the perceived intensity of green light to restore the correct intensity in the mixed light. (That is why the measured intensity must be greatest at the wavelength where our eyes are most sensitive. That wouldn't make sense otherwise!)

Em outras palavras, 100mcd cada um de vermelho, verde e azul contêm muito menos energia real no canal verde, enquanto a verdadeira luz branca conteria energia aproximadamente igual em cada canal - daí a definição de "ruído branco" na eletrônica.

EDIT: Um artigo interessante coloca as eficiências quânticas dos LEDs vermelho e azul na região de 70 a 80%, bem acima da dos LEDs verdes (anteriores a 2008) (é um argumento de vendas, afinal!). Isso torna provável que, seja qual for o motivo da baixa intensidade dos LEDs azuis, não é que eles sejam difíceis de fazer.

Portanto, as intensidades relativas dos três LEDs em questão são a tentativa do fabricante de desfazer essa ponderação e combinar os LEDs, de modo que a luz gerada fique aproximadamente branca na corrente nominal.

Ilustração (fonte da imagem) Para meus olhos, pelo menos, na ilustração acima, G é de longe a principal mais brilhante, com R segundo e B mais escuros, mas quando misturados, eles produzem um branco muito bom.

I do not claim the other answers are wrong, but they miss two important points. One of them that I consider the most relevant.

RGB-LEDs are not meant to produce white light. They are meant to reach a certain gamut Wikipedia on gamut, i.e. the colour space that can be displayed by the LED. And they do. If the three channels are driven with an 8-bit resolution probably only less than 1% of all possible settings will yield a light mixture on the Planckian locus. Wikipedia on planckian locus, where white light can be found. So one can guess, white light is not the primary objective for a RGB LED.

The gamut is an outcome of the use case analysis a manufacturer is performing. In most cases, the use case demands high output for signal colours like red, green and yellow but only limited power when producing white light.

Even if the use case is covering the omnipresent RGB LED-strips it is neither necessary nor possible to hit the Planckian locus when driving all LED at 100%. The human eye tolerates many MacAdam-ellipses away from the Planckian locus when it has no good light source to compare and even more when the owner of the eye got the LEDs at a bargain price.

As I wrote in my comment, the die size of the three colours is usually equal which leads to a nearly equal electrical and thermal power rating for all three chips. This and the limited bandwidth of the current available epitaxial process finally prevents manufacturers to "please everybody". Hence it is extremely unlikely to get a RGB-device which hits the Planckian locus when driven at 100%. On top of that, even if there was a RGB-chip with that property, it would fail to produce the same result at an ambient temperature just 20° higher.

There's one more fact to consider if white light is desired at 100% current for all LEDs. Colour LED each produce a narrow spectrum around their so called dominant wavelength $λ_{dom}$. For them to imitate a white spectrum together they either have to have adjacent spectral humps or produce more light, if their dominant wavelength is far from the adjacient LEDs. For RGB, the green one is in fact in a long gap between R and B. So the output power must be increased to generate the same tristimulus as daylight. This means the green LED bears the main load in providing the flux for a light that appears white. The eye thanks to its metameric properties is rather forgiving concerning the actual "form" of the spectrum.

The outrageously abysmal colour rendering of RGB-generated white is another story.

LEDs of different colors are made with quite different materials and processes and designs. There's no guarantee that they'll turn out to be the same brightness. It makes more sense to put more efficient LEDs in there when they are available rather than degrade the more efficient ones in order to match the least efficient color. Sure they will have to run at different currents (or duty cycles) to get a white balance, but that's not a big deal.

If you pay close attention to the specs, you will notice that the mcd ratings are given with approximately equal power (30mw) applied to each LED. assuming that our eye will see "white" when the three colors have the same luminosity, one way to achieve this would be to reduce the brightness of the red and green LEDs and increase the brightness of the blue LED. Assuming that the brightness is proportional to the current, I would reduce the green LED current to 5ma, the red LED to 8.8ma, and the blue would be increased to 26ma. This would make each LED provide approximately 625 mcd. Of course, this assumes the blue LED can handle 26 ma, if not, the currents would have to be proportionally reduced based on the max current the blue LED can handle.

The answer your main question, is simply the manufacturing and price constraints. For your second question... no, it does not have to look yellowish, it just depends on the accuracy with which you balance the currents to the LEDs (and background brightness). For the third question, the answer is similar to first case, optimizing the manufacturing process dictates equal die size, deposition process, etc.