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Sorry really ridiculous question but sure it sort of ties into the season, I recently noticed a set of battery operated LED Christmas lights I have on some garland seemed to be getting dim, upon closer inspection I noticed that it was specifically the red LED's that were dimmest, now given that energy of a photon is inverse of the wavelength and red has a lower frequency than blue photons can anyone explain why a red LED which requires less energy is not a bright as the blue LED which requires more energy (the lights are in parallel and operate at 4.5V on 3 standard AA batteries)

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  • $\begingroup$ It is not directly related to photon energy, but to the forward voltage of the light emitting diode. The battery voltage is larger, so the current needs to be limited, probable by a resistor, possible with different values for different diodes. So it becomes an electronics question how the led current depends on battery voltage: electrical engineering. $\endgroup$
    – user137289
    Dec 24 '16 at 18:33
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The voltage drop of an LED is related to the band gap and the color emitted. Red is typically about 1.8V and blue about 3V (and green a bit less). Some chemistries have a bit different forward voltages (in particular green LEDs used to have a lower Vf than modern ones), but that is approximately correct.

So, what the hey, you ask, if we put a simple resistor in series with each LED the red ones would dim less than other visible colors as the supply voltage drops.

I suspect that they are putting two red dice in series for each red lamp, to get a similar brightness and similar forward voltage.

Chances are there are resistors somewhere in the circuit.

From here is a graph of Vf vs current for different LED colors.

enter image description here

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  • $\begingroup$ That was also what I thought might be an explanation, two red leds in series. But folks at electrical engineering would know. $\endgroup$
    – user137289
    Dec 24 '16 at 21:15
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    $\begingroup$ @Pieter Sure, I'm visiting from there. It's fairly common to have two red LED dies (and one each blue and green) in RGB leds. $\endgroup$ Dec 24 '16 at 23:26

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