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LEDs that have 3-in-1 RGB capability where each is driven independantly of the other usually contain three seperate LEDs in a single package. Each of them is made using different material to get different light color. I see that this has two implications:

  1. The forward voltage for each color LED inside may be different.
  2. On the LEDs I have seen on the RS website, they always seem to have different luminousity per color. I assume this means that with same forward current per color, each color will have different luminousity.

The result is that using the same circuit for each color one will get different light intensity for each color, this will make color mixing more complex. Why are the LEDs not made to have same light intensity per LED and (always) with same forward voltage per LED inside the package?

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    \$\begingroup\$ " Why are the LEDs not made to have same light intensity per LED and (always) with same forward voltage per LED inside the package?" Because of cost, and the material used to make each colour can't really do this easily, without doing some kind of crazy stack to get the least common denominator. Otherwise your assumptions are correct. \$\endgroup\$ – KyranF Oct 15 '15 at 22:57
  • \$\begingroup\$ Because apparently nobody is willing to pay for that what it would take. \$\endgroup\$ – Fizz Oct 15 '15 at 23:03
  • \$\begingroup\$ The forward voltage of a LED depends on its colour - it takes less energy to generate red light than green, for some deep physics reason, that I don't want to understand. :-) \$\endgroup\$ – Peter Bennett Oct 15 '15 at 23:05
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    \$\begingroup\$ Also keep in mind that the intensity emitted by the LED and what's perceived by human eye vary significantly with the color/frequency \$\endgroup\$ – Fizz Oct 15 '15 at 23:07
  • \$\begingroup\$ This is true @RespawnedFluff, humans have photopic vision, centered around highest energy sensitivity being in the "green" wavelength \$\endgroup\$ – KyranF Oct 15 '15 at 23:48
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Simple. Different colors correspond to different wavelengths of light, shorter wavelengths of light require more energy, producing photons of higher energy requires a larger bandgap in the semiconductor, and a larger bandgap produces a larger voltage drop. All of this means that Vf of a blue LED is the largest, green is in the middle, and red is the smallest. The quantum efficiency (current to photons) can also vary. Our eyes also have different sensitivities to different colors.

Whenever you need to do something precisely, there is also a tradeoff between designing things to inherently be 'perfect' vs. compensating for non-idealities with some sort of external calibration. In this case, you don't just have the difference in inherent bandgaps and efficiencies, you will also have variation between LEDs in the same batch. Unless you are going to bin your LEDs based on efficiency and voltage drop and hope that they age at the same rate, the only realistic solution is to get them roughly in the ballpark and then compensate in the drive circuitry/software.

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  • \$\begingroup\$ Very nice, +1 for you. :) \$\endgroup\$ – EM Fields Oct 16 '15 at 3:51

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